Android的ViewDragHelper源码解析

其实我想看的是DrawerLayout, 但发现DrawerLayout里面是使用了ViewDragHelper去实现.谷歌比较早就放出这个类了,但ViewDragHelper是开发中很少用到一个类.顾名思义这是一个和拖曳触摸有关的类.本着追根溯源的想法, 加上ViewDragHelper的源码也不算多,就决定将ViewDragHelper的源码看一遍.对实现原理了解下.

代码一千多行,看完还是需要点时间的. 因此不会逐一讲完, 当然下面也会放出该类源码的解析,注释中也有一些个人理解的点写在里面. 有兴趣可以看看.所以这里直接讲下概括的东西,也足以对ViewDragHelper有个了解, 起码不至于陌生.

ViewDragHelper的原理和几个点:

原理:

ViewDragHelper定义大量的常量状态值( 比如设定边缘触发拖曳事件的临界值,边缘大小, 可拖曳的边缘标记等等 )用于监听整个拖曳的开始到结束的过程. 这个过程主要分为三个步骤: 闲置状态( IDLE ), 运动( DRAG 或者 SETTLE ) , 闲置状态( IDLE ).

要知道,要令父view中的子view发生位移, 除了手动拖曳令子view同步跟着手势运动, 还可以通过设定结束位置的X,Y坐标和LEFT,TOP值来直接使子view移动到该指定位置. 前者是拖曳( DRAG ),拖曳的过程是有过渡动画效果的. 而后者就是SETTLE (这个词不好翻译, 直译是"安置"), 没有过渡动画, 就辣么直接出现在指定的位置. 

但无论是前者还是后者, 整个拖曳或settle的过程, 一开始时子view是静止的, 叫闲置态( IDLE ), 然后手指在屏幕触摸拖曳, 当手势运动超过触发拖曳的临界值,子view就会被拖曳或者被settle, 当移动的过程结束, 又会变成闲置态(IDLE). 

说说几个点:

1, ViewDragHelper的拖曳,比如drag,settle这些操作,是通过Scroller去实现的. (Scroller 是滑动事件的辅助类,非常有用, 写过类似上下拉等滑动效果的童鞋应该用过.)

2,  ViewDragHelper的是有记录历史功能的, 事件的过程会将触摸的X,Y坐标存在数组中, 然后通过本地的System.arraycopy(Object src, int srcPos, Object dst, int dstPos, int length)方法,传递旧的的触摸坐标值到数组中保存.这使得拖曳过程是连贯的.

3, 有历史记录,当然也有清空记录的功能. ViewDragHelper提供了cancel()方法,类似onTouch的ACTION_UP事件. 当拖曳结束,或settle的过程结束, 可能系统还认为过程还在, 因此就需要提供的cancel()或abort()方法去终止这个过程,同时也自动调用clearMotionHistory()方法,置空历史记录.确保下次触摸拖曳事件是"新的开始".

4, 要理解触摸的点( pointer ),触摸事件和子view,父view在拖曳或settle中的相互关系. 

手指触摸屏幕,刚接触屏幕肯定有一个点是先碰到的, 然后深按下去,手指的一定面积触摸到屏幕, 就意味着触摸动作是由一定数量的点( pointer )组成.这些pointer都有自己的X,Y坐标. 而能够响应拖曳或settle触摸事件的父view中如果子view所处的位置保证这些pointer是落在子view内的, 那这个子view才能被捕获到( be catured ). 子view被捕获到, 才能发生我们所看到的拖曳或settle过程. 当然,因为ViewDragHelper给view的边缘设定了大小, 也设定了触发view拖曳事件的临界值, 只有手指在屏幕边缘移动大于这临界值,才能使子view运动.(具体过程可参考源码)

5, ViewDragHelper的触摸拖曳事件,是针对父view最顶层的子view才会响应该事件.这个应该不难理解吧. 所以该类提供了public View findTopChildUnder(int x, int y)方法捕获父view中最顶层的子view对象.

6, ViewDragHelper 提供了强大的通讯接口 - Callback. 这是一个静态抽象类,定义了相当齐全的监听拖曳或settle过程各种状态的接口方法:

void onViewDragStateChanged(int state);//当拖曳状态变更时回调该方法

void onViewPositionChanged(View changedView, int left, int top, int dx, int dy);    //当捕获view由于拖曳或者设定而发生位置变更时回调

void onViewCaptured(View capturedChild, int activePointerId);   //当子view被由于拖曳或被settle, 而被捕获时回调的方法.

void onViewReleased(View releasedChild, float xvel, float yvel);     //当子view不再被拖曳时调用.如果有需要,fling的速度也会被提供.速度值会介于系统最小化和最大值之间.

void onEdgeTouched(int edgeFlags, int pointerId);//当父view其中一个被标记可拖曳的边缘被用户触摸, 同时父view里没有子view被捕获响应时回调该方法.

boolean onEdgeLock(int edgeFlags); //当原来可以拖曳的边缘被锁定不可拖曳时回调

void onEdgeDragStarted(int edgeFlags, int pointerId);//当用户开始从父view中"订阅的"(之前约定允许拖曳的)屏幕边缘拖曳,并且父view中没有子view响应时调用.

等等.

Callback是理解ViewDragHelper的关键,因为它反映了开始移动到结束的监听过程.对了解ViewDrag事件有很大帮助.

7, 因为移动的子view肯定是View的子类, 因此ViewDragHelper还有延迟移动的功能,借助View的post()方法, Runnable接口实现.

8, ViewDragHelper通过EDGE_LEFT, EDGE_RIGHT, EDGE_TOP和EDGE_BOTTOM 来标记哪个边缘可被拖曳或settle. 从可拖曳或settle标记为不可拖曳或settle, 这个过程叫做被锁了( locked ). 因为CallBack提供的接口方法有监听这种状态变更的方法.

类源码

Then 几个点说到这,下面贴上源码解析. 有点长... 晚了, 具体的应用找个时间放在后面的文章再接着写.

/* * Copyright (C) 2013 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package android.support.v4.widget;import android.content.Context;import android.support.v4.view.MotionEventCompat;import android.support.v4.view.VelocityTrackerCompat;import android.support.v4.view.ViewCompat;import android.view.MotionEvent;import android.view.VelocityTracker;import android.view.View;import android.view.ViewConfiguration;import android.view.ViewGroup;import android.view.animation.Interpolator;import java.util.Arrays;/** * @tranlator AlexTam * ViewDragHelper是自定义ViewGroup时的实用类.它提供大量有用的操作和状态,来追踪用户在父View内的 * 拖曳子view和重新定位子view.(看到这里,估计就会想,要同步监听拖曳事件,肯定少不了在onTouch事件中随处用到的 * MotionEvent这个对象.是的,下面的确有它.) *  * ViewDragHelper is a utility class for writing custom ViewGroups. It offers a number * of useful operations and state tracking for allowing a user to drag and reposition * views within their parent ViewGroup. */public class ViewDragHelper {    private static final String TAG = "ViewDragHelper";    /**     * 空/无效的pointer ID     * A null/invalid pointer ID.     */    public static final int INVALID_POINTER = -1;    /**     * 状态量:(IDLE是闲置的 意思)表示 view当前没有被拖曳或者运行的动画结束     * A view is not currently being dragged or animating as a result of a fling/snap.     */    public static final int STATE_IDLE = 0;    /**     * 状态量: view当前正被拖曳.根据用户的输入或者模拟用户的输入,view当前位置发生改变.(表示,     * 用户怎么拖曳移动,view就根据拖曳的动作而发生位置改变.)     * A view is currently being dragged. The position is currently changing as a result     * of user input or simulated user input.     */    public static final int STATE_DRAGGING = 1;    /**     * 状态量: 由于fling动作,或者预定的无交互的运动,view被"安置"到一个结束的地方.(可以想象,一个view被用户快速拖曳     * 并甩动,从而view被甩到某个结束的位置,的过程.)     * A view is currently settling into place as a result of a fling or     * predefined non-interactive motion.     */    public static final int STATE_SETTLING = 2;    /**     * 标记可从左边缘拖曳.     * Edge flag indicating that the left edge should be affected.     */    public static final int EDGE_LEFT = 1 << 0;    /**     * 标记可从右边缘拖曳.     * Edge flag indicating that the right edge should be affected.     */    public static final int EDGE_RIGHT = 1 << 1;    /**     * 标记可从顶部拖曳.     * Edge flag indicating that the top edge should be affected.     */    public static final int EDGE_TOP = 1 << 2;    /**     * 标记可从底部拖曳.     * Edge flag indicating that the bottom edge should be affected.     */    public static final int EDGE_BOTTOM = 1 << 3;    /**     * 标记所有地方(边缘的上下左右)都能被拖曳.     * Edge flag set indicating all edges should be affected.     */    public static final int EDGE_ALL = EDGE_LEFT | EDGE_TOP | EDGE_RIGHT | EDGE_BOTTOM;    /**     * 指引值:     *      * 表示a check(指引) 应该沿着水平轴发生.     * Indicates that a check should occur along the horizontal axis     */    public static final int DIRECTION_HORIZONTAL = 1 << 0;    /**     * 表示a check 应该沿着垂直轴发生.     * Indicates that a check should occur along the vertical axis     */    public static final int DIRECTION_VERTICAL = 1 << 1;    /**     * 表示a check可水平可垂直的发生.     * Indicates that a check should occur along all axes     */    public static final int DIRECTION_ALL = DIRECTION_HORIZONTAL | DIRECTION_VERTICAL;    //将边缘大小定位20dp    private static final int EDGE_SIZE = 20; // dp    //时间值    private static final int BASE_SETTLE_DURATION = 256; // ms    private static final int MAX_SETTLE_DURATION = 600; // ms    // 当前的拖曳状态,值为idle, dragging or settling.    // Current drag state; idle, dragging or settling    private int mDragState;    // 在拖曳开始前的滑动位移.(可以这么理解,触发拖曳的最大临界值.)    // Distance to travel before a drag may begin    private int mTouchSlop;        // 上一次的位置或点    // Last known position/pointer tracking    private int mActivePointerId = INVALID_POINTER;    //初始化的X坐标    private float[] mInitialMotionX;    //初始化的Y坐标    private float[] mInitialMotionY;    //下面这些变量不写了噻,看名字也能知道.    private float[] mLastMotionX;    private float[] mLastMotionY;    private int[] mInitialEdgesTouched;    private int[] mEdgeDragsInProgress;    private int[] mEdgeDragsLocked;    private int mPointersDown;    private VelocityTracker mVelocityTracker;    private float mMaxVelocity;    private float mMinVelocity;    //边缘的大小,单位px    private int mEdgeSize;    private int mTrackingEdges;    //兼容新API所提供的Scroller    private ScrollerCompat mScroller;        //内部抽象类,提供一些规范的接口方法    private final Callback mCallback;    private View mCapturedView;    private boolean mReleaseInProgress;    private final ViewGroup mParentView;    /**     * 这个Callback是作为通信接口,当ViewDragHelper返回父view时使用."on"为首的方法是重要事件的回调方法,几个     * 接口方法用于提供更多关于请求父view的状态的信息给ViewDragHelper.这个抽象类同时提供子view拖曳的一些细节信息.     *      * A Callback is used as a communication channel with the ViewDragHelper back to the     * parent view using it. <code>on*</code>methods are invoked on siginficant events and several     * accessor methods are expected to provide the ViewDragHelper with more information     * about the state of the parent view upon request. The callback also makes decisions     * governing the range and draggability of child views.     */    public static abstract class Callback {        /**         * 当拖曳状态变更时回调该方法.可看"STATE_"为首的常量了解更多信息.         * Called when the drag state changes. See the <code>STATE_*</code> constants         * for more information.         *         * @param state The new drag state         *         * @see #STATE_IDLE         * @see #STATE_DRAGGING         * @see #STATE_SETTLING         */        public void onViewDragStateChanged(int state) {}        /**         * 当捕获view由于拖曳或者设定而发生位置变更时回调..         * Called when the captured view's position changes as the result of a drag or settle.         *         * @param changedView View whose position changed - 发生位置变更的view         * @param left New X coordinate of the left edge of the view - 新的左边缘X坐标         * @param top New Y coordinate of the top edge of the view- 新的顶部边缘Y坐标         * @param dx Change in X position from the last call- 从旧到新位置发生的X偏移值         * @param dy Change in Y position from the last call- 从旧到新位置发生的Y偏移值         */        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {}        /**         * 当子view被由于拖曳或设置(settle有点难翻译)而被捕获时回调的方法.提供拖曳的pointer的ID.         * 如果activePointerId被标记为{@link #INVALID_POINTER},它会代替没有初始化的pointer.         *          * Called when a child view is captured for dragging or settling. The ID of the pointer         * currently dragging the captured view is supplied. If activePointerId is         * identified as {@link #INVALID_POINTER} the capture is programmatic instead of         * pointer-initiated.         *         * @param capturedChild Child view that was captured         * @param activePointerId Pointer id tracking the child capture         */        public void onViewCaptured(View capturedChild, int activePointerId) {}        /**         * 当子view不再被拖曳时调用.如果有需要,fling的速度也会被提供.速度值会介于系统最小化和最大值之间.         *          * Called when the child view is no longer being actively dragged.         * The fling velocity is also supplied, if relevant. The velocity values may         * be clamped to system minimums or maximums.         *         * <p>Calling code may decide to fling or otherwise release the view to let it         * settle into place. It should do so using {@link #settleCapturedViewAt(int, int)}         * or {@link #flingCapturedView(int, int, int, int)}. If the Callback invokes         * one of these methods, the ViewDragHelper will enter {@link #STATE_SETTLING}         * and the view capture will not fully end until it comes to a complete stop.         * If neither of these methods is invoked before <code>onViewReleased</code> returns,         * the view will stop in place and the ViewDragHelper will return to         * {@link #STATE_IDLE}.</p>         *         * @param releasedChild The captured child view now being released         * - 被捕获到的要释放的子view         * @param xvel X velocity of the pointer as it left the screen in pixels per second.         * - pointer离开屏幕X轴方向每秒运动的速率,单位是px.         * @param yvel Y velocity of the pointer as it left the screen in pixels per second.         * - pointer离开屏幕Y轴方向每秒运动的速率,单位是px.         */        public void onViewReleased(View releasedChild, float xvel, float yvel) {}        /**         * 当父view其中一个被标记可拖曳的边缘被用户触摸, 同时父view里没有子view被捕获响应时回调该方法.         * Called when one of the subscribed edges in the parent view has been touched         * by the user while no child view is currently captured.         *         * @param edgeFlags A combination of edge flags describing the edge(s) currently touched         * - 描述所当前所触摸的位置的边缘标记, 如EDGE_LEFT,EDGE_RIGHT等等.         * @param pointerId ID of the pointer touching the described edge(s)         * - 触摸的点的ID.         *          * @see #EDGE_LEFT         * @see #EDGE_TOP         * @see #EDGE_RIGHT         * @see #EDGE_BOTTOM         */        public void onEdgeTouched(int edgeFlags, int pointerId) {}        /**         * 该方法当原来可以拖曳的边缘被锁定不可拖曳时回调.如果边缘在初始化开始拖曳前被拒绝拖曳,就会发生前面说的这种情况.         * 但这个方法会在{@link #onEdgeTouched(int, int)}之后才会被回调.这个方法会返回true来锁定该边缘.或者         * 返回false来释放解锁该屏幕.默认的行为是后者(返回false来释放解锁该屏幕).         *          * Called when the given edge may become locked. This can happen if an edge drag         * was preliminarily rejected before beginning, but after {@link #onEdgeTouched(int, int)}         * was called. This method should return true to lock this edge or false to leave it         * unlocked. The default behavior is to leave edges unlocked.         *         * @param edgeFlags A combination of edge flags describing the edge(s) locked         * - 描述被锁定的边缘的边缘标记,如EDGE_LEFT等.         * @return true to lock the edge, false to leave it unlocked         * - 返回true来锁定该边缘.或者 返回false来释放解锁该屏幕.         */        public boolean onEdgeLock(int edgeFlags) {            return false;        }        /**         * 当用户开始从父view中"订阅的"(之前约定允许拖曳的)屏幕边缘拖曳,并且父view中没有子view响应时调用.         *          * Called when the user has started a deliberate drag away from one         * of the subscribed edges in the parent view while no child view is currently captured.         *         * @param edgeFlags A combination of edge flags describing the edge(s) dragged         * - 描述该边缘的边缘标记,如EDGE_LEFT等.         * @param pointerId ID of the pointer touching the described edge(s)         * - pointer的ID.         * @see #EDGE_LEFT         * @see #EDGE_TOP         * @see #EDGE_RIGHT         * @see #EDGE_BOTTOM         */        public void onEdgeDragStarted(int edgeFlags, int pointerId) {}        /**         * 调用设置子view z轴次序的参数.         * Called to determine the Z-order of child views.         *         * @param index the ordered position to query for         * @return index of the view that should be ordered at position <code>index</code>         */        public int getOrderedChildIndex(int index) {            return index;        }        /**         * 返回拖曳的子view水平移动范围的值,单位为px.这个方法如果返回0,那么该view则不能水平移动.         * Return the magnitude of a draggable child view's horizontal range of motion in pixels.         * This method should return 0 for views that cannot move horizontally.         *         * @param child Child view to check - 目标子view         * @return range of horizontal motion in pixels- 水平拖曳的值,单位为px.         */        public int getViewHorizontalDragRange(View child) {            return 0;        }        /**         * 返回拖曳的子view垂直移动范围的值,单位为px.这个方法如果返回0,那么该view则不能垂直移动.         * Return the magnitude of a draggable child view's vertical range of motion in pixels.         * This method should return 0 for views that cannot move vertically.         *         * @param child Child view to check         * @return range of vertical motion in pixels         */        public int getViewVerticalDragRange(View child) {            return 0;        }        /**         * 当用户通过pointerId 输入特定值令目标子view移动时回调该方法.callback接口如果返回true,则表示用户         * 允许通过用于引导的pointer来拖曳该子view.         * Called when the user's input indicates that they want to capture the given child view         * with the pointer indicated by pointerId. The callback should return true if the user         * is permitted to drag the given view with the indicated pointer.         *         * 如果该子view已经被捕获, ViewDragHelper可能多次重复的调用该方法.多次的调用会导致新的pointer尝试去控制这个view.         * <p>ViewDragHelper may call this method multiple times for the same view even if         * the view is already captured; this indicates that a new pointer is trying to take         * control of the view.</p>         *         * 如果该方法返回true,并且当成功捕获到该子view时,方法{@link #onViewCaptured(android.view.View, int)}会随即被调用.         * <p>If this method returns true, a call to {@link #onViewCaptured(android.view.View, int)}         * will follow if the capture is successful.</p>         *         * @param child Child the user is attempting to capture- 用户视图捕获的子view         * @param pointerId ID of the pointer attempting the capture- 捕获该子view的pointerID.         * @return true if capture should be allowed, false otherwise- 如果允许并且捕获成功应该返回true.否则返回false.         */        public abstract boolean tryCaptureView(View child, int pointerId);        /**         * 该方法用于限制子view沿水平拖曳的手势.默认的实现是,不允许水平手势.如果有类继承了该类,         * 必须覆盖重写该方法,并且提供值去限制该拖曳手势.         * Restrict the motion of the dragged child view along the horizontal axis.         * The default implementation does not allow horizontal motion; the extending         * class must override this method and provide the desired clamping.         *         *         * @param child Child view being dragged-  被拖曳的子view.         * @param left Attempted motion along the X axis- 沿X轴(水平)的手势         * @param dx Proposed change in position for left- view的left变更值         * @return The new clamped position for left - 对left返回新的位置值         */        public int clampViewPositionHorizontal(View child, int left, int dx) {            return 0;        }        /**         * 该方法用于限制子view沿垂直拖曳的手势.默认的实现是,不允许垂直手势...(同上面的方法类似,就不过多解释了.)         * Restrict the motion of the dragged child view along the vertical axis.         * The default implementation does not allow vertical motion; the extending         * class must override this method and provide the desired clamping.         *         *         * @param child Child view being dragged         * @param top Attempted motion along the Y axis         * @param dy Proposed change in position for top         * @return The new clamped position for top         */        public int clampViewPositionVertical(View child, int top, int dy) {            return 0;        }    }    /**     * 定义曲线动画的插值器     * Interpolator defining the animation curve for mScroller     */    private static final Interpolator sInterpolator = new Interpolator() {        public float getInterpolation(float t) {            t -= 1.0f;            return t * t * t * t * t + 1.0f;        }    };    // 实现Runnable接口    private final Runnable mSetIdleRunnable = new Runnable() {        public void run() {            setDragState(STATE_IDLE);        }    };    /**     * 创建ViewDragHelper的工厂方法     * Factory method to create a new ViewDragHelper.     *     * @param forParent Parent view to monitor  - 所要监听的父view     * @param cb Callback to provide information and receive events - 提供信息的Callback对象     * @return a new ViewDragHelper instance     */    public static ViewDragHelper create(ViewGroup forParent, Callback cb) {        return new ViewDragHelper(forParent.getContext(), forParent, cb);    }    /**     * Factory method to create a new ViewDragHelper.     *     * @param forParent Parent view to monitor     * @param sensitivity Multiplier for how sensitive the helper should be about detecting     *                    the start of a drag. Larger values are more sensitive. 1.0f is normal.     * @param cb Callback to provide information and receive events     * @return a new ViewDragHelper instance     */    public static ViewDragHelper create(ViewGroup forParent, float sensitivity, Callback cb) {        final ViewDragHelper helper = create(forParent, cb);        helper.mTouchSlop = (int) (helper.mTouchSlop * (1 / sensitivity));        return helper;    }    /**     * 应用应该使用ViewDragHelper.create()去获取新的实例.这将允许ViewDragHelper使用内部实现去兼容不同的平台版本.     * Apps should use ViewDragHelper.create() to get a new instance.     * This will allow VDH to use internal compatibility implementations for different     * platform versions.     *     * @param context Context to initialize config-dependent params from     * @param forParent Parent view to monitor     */    private ViewDragHelper(Context context, ViewGroup forParent, Callback cb) {        if (forParent == null) {            throw new IllegalArgumentException("Parent view may not be null");        }        if (cb == null) {            throw new IllegalArgumentException("Callback may not be null");        }        mParentView = forParent;        mCallback = cb;        // ViewConfiguration是一个包含配置信息,如时间,位移等的配置类.        final ViewConfiguration vc = ViewConfiguration.get(context);        final float density = context.getResources().getDisplayMetrics().density;        mEdgeSize = (int) (EDGE_SIZE * density + 0.5f);        mTouchSlop = vc.getScaledTouchSlop();        mMaxVelocity = vc.getScaledMaximumFlingVelocity();        mMinVelocity = vc.getScaledMinimumFlingVelocity();        mScroller = ScrollerCompat.create(context, sInterpolator);    }    /**     * 设置最小速率.大于0px/s的速率能更好的被检测到.这样Callback就能恰当的运用该值去约束移动的速率.     * Set the minimum velocity that will be detected as having a magnitude greater than zero     * in pixels per second. Callback methods accepting a velocity will be clamped appropriately.     *     * @param minVel Minimum velocity to detect     */    public void setMinVelocity(float minVel) {        mMinVelocity = minVel;    }    /**     * 获取最小速率. 值得注意的是,如果最小速率小于0, 那么直接返回0,不会返回比0小的值.     * Return the currently configured minimum velocity. Any flings with a magnitude less     * than this value in pixels per second. Callback methods accepting a velocity will receive     * zero as a velocity value if the real detected velocity was below this threshold.     *     * @return the minimum velocity that will be detected     */    public float getMinVelocity() {        return mMinVelocity;    }    /**     * 获取当前helper的拖曳状态,返回结果为{@link #STATE_IDLE}, {@link #STATE_DRAGGING}      * or {@link #STATE_SETTLING}.中的其一.     *      * Retrieve the current drag state of this helper. This will return one of     * {@link #STATE_IDLE}, {@link #STATE_DRAGGING} or {@link #STATE_SETTLING}.     * @return The current drag state     */    public int getViewDragState() {        return mDragState;    }    /**     * 设置允许父view的某个边缘可追踪.CallBack对象的{@link Callback#onEdgeTouched(int, int)} and     * {@link Callback#onEdgeDragStarted(int, int)}方法只有在边缘允许被追踪时才会调用.      * (就是说,如果不设置上下左右的某个边缘可追踪,那么这2个方法是不可用的.)     *      * Enable edge tracking for the selected edges of the parent view.     * The callback's {@link Callback#onEdgeTouched(int, int)} and     * {@link Callback#onEdgeDragStarted(int, int)} methods will only be invoked     * for edges for which edge tracking has been enabled.     *     * @param edgeFlags Combination of edge flags describing the edges to watch     * @see #EDGE_LEFT     * @see #EDGE_TOP     * @see #EDGE_RIGHT     * @see #EDGE_BOTTOM     */    public void setEdgeTrackingEnabled(int edgeFlags) {        mTrackingEdges = edgeFlags;    }    /**     * 返回边缘大小的值.单位为px.这个值是该view边缘可以被监测或追踪的值的范围.     * Return the size of an edge. This is the range in pixels along the edges of this view     * that will actively detect edge touches or drags if edge tracking is enabled.     *     * @return The size of an edge in pixels     * @see #setEdgeTrackingEnabled(int)     */    public int getEdgeSize() {        return mEdgeSize;    }    /**     * 在父view内捕获指定的子view用于拖曳.同时callback对象会被通知.但{@link Callback#tryCaptureView(android.view.View, int)}     * 不会被要求获取权限来捕获该view.     *      * Capture a specific child view for dragging within the parent. The callback will be notified     * but {@link Callback#tryCaptureView(android.view.View, int)} will not be asked permission to     * capture this view.     *     * @param childView Child view to capture     * @param activePointerId ID of the pointer that is dragging the captured child view     */    public void captureChildView(View childView, int activePointerId) {        if (childView.getParent() != mParentView) {            throw new IllegalArgumentException("captureChildView: parameter must be a descendant " +                    "of the ViewDragHelper's tracked parent view (" + mParentView + ")");        }        mCapturedView = childView;        mActivePointerId = activePointerId;        mCallback.onViewCaptured(childView, activePointerId);        setDragState(STATE_DRAGGING);    }    /**     * 返回当前捕获的view.如果没有捕获到的view,则返回null.     * @return The currently captured view, or null if no view has been captured.     */    public View getCapturedView() {        return mCapturedView;    }    /**     * 当前拖曳捕获的view的点(pointer)的ID.     * @return The ID of the pointer currently dragging the captured view,     *         or {@link #INVALID_POINTER}.     */    public int getActivePointerId() {        return mActivePointerId;    }    /**     * 获取最小触发和初始化拖曳动作的值,单位px.     * @return The minimum distance in pixels that the user must travel to initiate a drag     */    public int getTouchSlop() {        return mTouchSlop;    }    /**     * 这方法等价于onTouch中MotionEvent的ACTION_CANCEL事件.     * The result of a call to this method is equivalent to     * {@link #processTouchEvent(android.view.MotionEvent)} receiving an ACTION_CANCEL event.     */    public void cancel() {        mActivePointerId = INVALID_POINTER;        clearMotionHistory();        if (mVelocityTracker != null) {            mVelocityTracker.recycle();            mVelocityTracker = null;        }    }    /**     * 中止取所有手势.并且直接结束动画.     * {@link #cancel()}, but also abort all motion in progress and snap to the end of any     * animation.     */    public void abort() {        cancel();        if (mDragState == STATE_SETTLING) {            final int oldX = mScroller.getCurrX();            final int oldY = mScroller.getCurrY();            mScroller.abortAnimation();            final int newX = mScroller.getCurrX();            final int newY = mScroller.getCurrY();            mCallback.onViewPositionChanged(mCapturedView, newX, newY, newX - oldX, newY - oldY);        }        //中止了,当然要设置拖曳状态为闲置(或者说初始态)        setDragState(STATE_IDLE);    }    /**     * (使用这个方法,可以有动画效果的移动子view到特定位置,该位置需要给出的finalLeft和 finalTop值.)     * 随着动画,子view移动到既定(给定left和top值)的位置.如果这个方法返回true,会在后面随着手势移动的     * 每一帧中回调{@link #continueSettling(boolean)}方法,直至返回false.如果这个方法返回false,     * 就不会再移动去完成手势动作的事件.      *      * Animate the view <code>child</code> to the given (left, top) position.     * If this method returns true, the caller should invoke {@link #continueSettling(boolean)}     * on each subsequent frame to continue the motion until it returns false. If this method     * returns false there is no further work to do to complete the movement.     *     * 要注意的是,即使方法{@link #getCapturedView()}在这个滑动过程中仍会一直有效,可以获取catureView的值,     * 但这个操作过程不看做是一个捕获事件(我们应当知道,捕获子view不是我们决定的,是Helper自动在父view和     * 子view之间去自动完成的过程,无论这个过程成功还是失败).     *      * <p>This operation does not count as a capture event, though {@link #getCapturedView()}     * will still report the sliding view while the slide is in progress.</p>     *     * @param child Child view to capture and animate - 要捕获和添加动画移动的view对象     * @param finalLeft Final left position of child - 最终位置的left值     * @param finalTop Final top position of child - 最终位置的top值     * @return true if animation should continue through {@link #continueSettling(boolean)} calls     */    public boolean smoothSlideViewTo(View child, int finalLeft, int finalTop) {        mCapturedView = child;        mActivePointerId = INVALID_POINTER;        boolean continueSliding = forceSettleCapturedViewAt(finalLeft, finalTop, 0, 0);        if (!continueSliding && mDragState == STATE_IDLE && mCapturedView != null) {            // If we're in an IDLE state to begin with and aren't moving anywhere, we            // end up having a non-null capturedView with an IDLE dragState            mCapturedView = null;        }        return continueSliding;    }    /**     * (通过这个方法,我们应当知道settle和slide的区别.前者是直接跳到结束位置,而后者是有过渡效果的.)     * 将捕获的view设置(settle)在给定的left,top值的位置.(表示,直接忽略过程,直接将view显示在特定位置)     * 这个过程中,该view(如果在此时已经有)适当的速度,则该速度会影响settle的过程.     * 如果这个方法返回true,方法{@link #continueSettling(boolean)}在整个settle过程中会被回调,直至返回false.     * 如果这个方法返回false,(表示此时该view已经在给定的位置)这个settle的过程就会结束,不会再工作完成事件.     *      * Settle the captured view at the given (left, top) position.     * The appropriate velocity from prior motion will be taken into account.     * If this method returns true, the caller should invoke {@link #continueSettling(boolean)}     * on each subsequent frame to continue the motion until it returns false. If this method     * returns false there is no further work to do to complete the movement.     *     * @param finalLeft Settled left edge position for the captured view     * @param finalTop Settled top edge position for the captured view     * @return true if animation should continue through {@link #continueSettling(boolean)} calls     */    public boolean settleCapturedViewAt(int finalLeft, int finalTop) {        if (!mReleaseInProgress) {            throw new IllegalStateException("Cannot settleCapturedViewAt outside of a call to " +                    "Callback#onViewReleased");        }        return forceSettleCapturedViewAt(finalLeft, finalTop,                (int) VelocityTrackerCompat.getXVelocity(mVelocityTracker, mActivePointerId),                (int) VelocityTrackerCompat.getYVelocity(mVelocityTracker, mActivePointerId));    }    /**     * 同样是将view直接设到特定位置(给定left, top值).     * (看该方法的实现,整个过程 也是靠scroller的scroll去实现的).     * Settle the captured view at the given (left, top) position.     *     * @param finalLeft Target left position for the captured view     * @param finalTop Target top position for the captured view     * @param xvel Horizontal velocity- 水平速度     * @param yvel Vertical velocity- 垂直速度     * @return true if animation should continue through {@link #continueSettling(boolean)} calls     * - settleing的过程中会一直返回true,否则返回false表示结束.     */    private boolean forceSettleCapturedViewAt(int finalLeft, int finalTop, int xvel, int yvel) {        final int startLeft = mCapturedView.getLeft();        final int startTop = mCapturedView.getTop();        final int dx = finalLeft - startLeft;        final int dy = finalTop - startTop;        if (dx == 0 && dy == 0) {            // Nothing to do. Send callbacks, be done.            mScroller.abortAnimation();            setDragState(STATE_IDLE);            return false;        }        // 仔细看computeSettleDuration()这个计算时间的方法,其实挺复杂的.使用了相当多的运算处理.因此可以不看该方法的实现;        // 除非要继承ViewDrarHelper实现子类,实现更多效果...        final int duration = computeSettleDuration(mCapturedView, dx, dy, xvel, yvel);        mScroller.startScroll(startLeft, startTop, dx, dy, duration);        setDragState(STATE_SETTLING);        return true;    }    //该方法计算settle的时间    private int computeSettleDuration(View child, int dx, int dy, int xvel, int yvel) {    //clampMag(...)方法保证水平和垂直速度值不大于最大值, 也不小于最小值.        xvel = clampMag(xvel, (int) mMinVelocity, (int) mMaxVelocity);        yvel = clampMag(yvel, (int) mMinVelocity, (int) mMaxVelocity);        final int absDx = Math.abs(dx);        final int absDy = Math.abs(dy);        final int absXVel = Math.abs(xvel);        final int absYVel = Math.abs(yvel);        final int addedVel = absXVel + absYVel;        final int addedDistance = absDx + absDy;        final float xweight = xvel != 0 ? (float) absXVel / addedVel :                (float) absDx / addedDistance;        final float yweight = yvel != 0 ? (float) absYVel / addedVel :                (float) absDy / addedDistance;        //要注意的是getViewHorizontalDragRange(...)方法默认返回0,但一般都会在创建helper时传进的mCallback中重写该方法        int xduration = computeAxisDuration(dx, xvel, mCallback.getViewHorizontalDragRange(child));        int yduration = computeAxisDuration(dy, yvel, mCallback.getViewVerticalDragRange(child));        return (int) (xduration * xweight + yduration * yweight);    }        //该方法计算settle的时间,三个输入的参数依次分别是:水平或垂直方向的移动距离,水平或垂直方向的速度大小,拖曳范围值    private int computeAxisDuration(int delta, int velocity, int motionRange) {        if (delta == 0) {            return 0;        }        final int width = mParentView.getWidth();        final int halfWidth = width / 2;        final float distanceRatio = Math.min(1f, (float) Math.abs(delta) / width);        final float distance = halfWidth + halfWidth *                distanceInfluenceForSnapDuration(distanceRatio);        int duration;        velocity = Math.abs(velocity);        if (velocity > 0) {            duration = 4 * Math.round(1000 * Math.abs(distance / velocity));        } else {            final float range = (float) Math.abs(delta) / motionRange;            duration = (int) ((range + 1) * BASE_SETTLE_DURATION);        }        return Math.min(duration, MAX_SETTLE_DURATION);    }    /**     * 该方法通过最大和最小值,算出区间值.低于最小值返回0,大于最大值则返回最大值.     * Clamp the magnitude of value for absMin and absMax.     * If the value is below the minimum, it will be clamped to zero.     * If the value is above the maximum, it will be clamped to the maximum.     *     * @param value Value to clamp     * @param absMin Absolute value of the minimum significant value to return     * @param absMax Absolute value of the maximum value to return     * @return The clamped value with the same sign as <code>value</code>     */    private int clampMag(int value, int absMin, int absMax) {        final int absValue = Math.abs(value);        if (absValue < absMin) return 0;        if (absValue > absMax) return value > 0 ? absMax : -absMax;        return value;    }    /**     * 这个方法和上面的clampMag(int value, int absMin, int absMax)几乎一样,只是换了浮点型.     * Clamp the magnitude of value for absMin and absMax.     * If the value is below the minimum, it will be clamped to zero.     * If the value is above the maximum, it will be clamped to the maximum.     *     * @param value Value to clamp     * @param absMin Absolute value of the minimum significant value to return     * @param absMax Absolute value of the maximum value to return     * @return The clamped value with the same sign as <code>value</code>     */    private float clampMag(float value, float absMin, float absMax) {        final float absValue = Math.abs(value);        if (absValue < absMin) return 0;        if (absValue > absMax) return value > 0 ? absMax : -absMax;        return value;    }    private float distanceInfluenceForSnapDuration(float f) {        f -= 0.5f; // center the values about 0.        f *= 0.3f * Math.PI / 2.0f;        return (float) Math.sin(f);    }    /**     * 该方法类似上面的forceSettleCapturedViewAt(...),可参考之.     *      * Settle the captured view based on standard free-moving fling behavior.     * The caller should invoke {@link #continueSettling(boolean)} on each subsequent frame     * to continue the motion until it returns false.     *     * @param minLeft Minimum X position for the view's left edge     * @param minTop Minimum Y position for the view's top edge     * @param maxLeft Maximum X position for the view's left edge     * @param maxTop Maximum Y position for the view's top edge     */    public void flingCapturedView(int minLeft, int minTop, int maxLeft, int maxTop) {        if (!mReleaseInProgress) {            throw new IllegalStateException("Cannot flingCapturedView outside of a call to " +                    "Callback#onViewReleased");        }        mScroller.fling(mCapturedView.getLeft(), mCapturedView.getTop(),                (int) VelocityTrackerCompat.getXVelocity(mVelocityTracker, mActivePointerId),                (int) VelocityTrackerCompat.getYVelocity(mVelocityTracker, mActivePointerId),                minLeft, maxLeft, minTop, maxTop);        setDragState(STATE_SETTLING);    }    /**     * 这个方法在上面好几地方都被提及了.     * 在整个settle的过程中,这个方法会返回true.直至返回false,表示settle的过程结束.     * (该方法是内部调用的,外部建议不适用.)     *      * Move the captured settling view by the appropriate amount for the current time.     * If <code>continueSettling</code> returns true, the caller should call it again     * on the next frame to continue.     *     * 参数deferCallbacks - 如果要推迟滑动,比如在{@link android.view.View#computeScroll()}里面回调,或者view还在layout或者draw     * 的过程中,该参数应当传true;     * @param deferCallbacks true if state callbacks should be deferred via posted message.     *                       Set this to true if you are calling this method from     *                       {@link android.view.View#computeScroll()} or similar methods     *                       invoked as part of layout or drawing.     * @return true if settle is still in progress     */    public boolean continueSettling(boolean deferCallbacks) {        if (mDragState == STATE_SETTLING) {        // 由于整个settle的过程都借助Scroller去实现,            // 因此keepGoing这个值也来自mScroller.computeScrollOffset();        // mScroller.computeScrollOffset()这方法,表示只要view处于scroll状态,都会返回true.停止scroll则返回false.            boolean keepGoing = mScroller.computeScrollOffset();            final int x = mScroller.getCurrX();            final int y = mScroller.getCurrY();            final int dx = x - mCapturedView.getLeft();            final int dy = y - mCapturedView.getTop();            if (dx != 0) {                mCapturedView.offsetLeftAndRight(dx);            }            if (dy != 0) {                mCapturedView.offsetTopAndBottom(dy);            }            if (dx != 0 || dy != 0) {            // 可见该方法在整个settle的过程中,由于位置的不断变化            // 会一直回调mCallback.onViewPositionChanged(...)的方法                mCallback.onViewPositionChanged(mCapturedView, x, y, dx, dy);            }                        //这里很明显,当view已经去到最终位置,XY的坐标均相等时,即使keepGoing依然为true,系统以为            //该view依旧处于滑动中,但很显然,应该结束了.于是方法里面强制调用Scroller.abortAnimation()去中止动画,并            //向mScroller标记完成状态.keepGoing自然就为false了.            if (keepGoing && x == mScroller.getFinalX() && y == mScroller.getFinalY()) {                // Close enough. The interpolator/scroller might think we're still moving                // but the user sure doesn't.                mScroller.abortAnimation();                keepGoing = false;            }                        //此处推迟滑动,借助Runable接口去实现            if (!keepGoing) {                if (deferCallbacks) {                    mParentView.post(mSetIdleRunnable);                } else {                //来到这里,keepGoing和deferCallbacks为false,表示整个settle过程都结束了.                //更改拖曳状态,continueSettling(...)不会再被回调.                    setDragState(STATE_IDLE);                }            }        }        return mDragState == STATE_SETTLING;    }    /**     * (该方法是当完成settle过程后释放捕获到的view对象, 内部方法,不必了解详细过程.)     * 正如所有接口事件的方法,这个方法也必须在UI主线程中使用.在释放的过程中,只会调用一次     * {@link #settleCapturedViewAt(int, int)}或者{@link #flingCapturedView(int, int, int, int)}方法.     *      * Like all callback events this must happen on the UI thread, but release     * involves some extra semantics. During a release (mReleaseInProgress)     * is the only time it is valid to call {@link #settleCapturedViewAt(int, int)}     * or {@link #flingCapturedView(int, int, int, int)}.     */    private void dispatchViewReleased(float xvel, float yvel) {        mReleaseInProgress = true;        mCallback.onViewReleased(mCapturedView, xvel, yvel);        mReleaseInProgress = false;        if (mDragState == STATE_DRAGGING) {            // onViewReleased didn't call a method that would have changed this. Go idle.            setDragState(STATE_IDLE);        }    }    //下面几个"clear"为首的方法都是清空历史记录了    private void clearMotionHistory() {        if (mInitialMotionX == null) {            return;        }        Arrays.fill(mInitialMotionX, 0);        Arrays.fill(mInitialMotionY, 0);        Arrays.fill(mLastMotionX, 0);        Arrays.fill(mLastMotionY, 0);        Arrays.fill(mInitialEdgesTouched, 0);        Arrays.fill(mEdgeDragsInProgress, 0);        Arrays.fill(mEdgeDragsLocked, 0);        mPointersDown = 0;    }    private void clearMotionHistory(int pointerId) {        if (mInitialMotionX == null) {            return;        }        mInitialMotionX[pointerId] = 0;        mInitialMotionY[pointerId] = 0;        mLastMotionX[pointerId] = 0;        mLastMotionY[pointerId] = 0;        mInitialEdgesTouched[pointerId] = 0;        mEdgeDragsInProgress[pointerId] = 0;        mEdgeDragsLocked[pointerId] = 0;        mPointersDown &= ~(1 << pointerId);    }        //这个方法很明显是内部调用的,在saveInitialMotion(...)中调用.    //因为mInitialMotionX数组里面保存有触摸X坐标的缓存信息,该方法确保mInitialMotionX一直保存最新的pointerId值    private void ensureMotionHistorySizeForId(int pointerId) {        if (mInitialMotionX == null || mInitialMotionX.length <= pointerId) {            float[] imx = new float[pointerId + 1];            float[] imy = new float[pointerId + 1];            float[] lmx = new float[pointerId + 1];            float[] lmy = new float[pointerId + 1];            int[] iit = new int[pointerId + 1];            int[] edip = new int[pointerId + 1];            int[] edl = new int[pointerId + 1];                        //这个过程,将触摸的X,Y坐标,上次触摸的X,Y坐标等信息复制过去            if (mInitialMotionX != null) {            //这里调用本地C方法去将mInitialMotionX的内存复制给imx数组,没源码...                System.arraycopy(mInitialMotionX, 0, imx, 0, mInitialMotionX.length);                System.arraycopy(mInitialMotionY, 0, imy, 0, mInitialMotionY.length);                System.arraycopy(mLastMotionX, 0, lmx, 0, mLastMotionX.length);                System.arraycopy(mLastMotionY, 0, lmy, 0, mLastMotionY.length);                System.arraycopy(mInitialEdgesTouched, 0, iit, 0, mInitialEdgesTouched.length);                System.arraycopy(mEdgeDragsInProgress, 0, edip, 0, mEdgeDragsInProgress.length);                System.arraycopy(mEdgeDragsLocked, 0, edl, 0, mEdgeDragsLocked.length);            }            mInitialMotionX = imx;            mInitialMotionY = imy;            mLastMotionX = lmx;            mLastMotionY = lmy;            mInitialEdgesTouched = iit;            mEdgeDragsInProgress = edip;            mEdgeDragsLocked = edl;        }    }    // 在这里,连同pointerId,保存X,Y轴坐标信息    // 也许看到这里,你已经猜到,pointerId这个值是递增的,由系统自动分配.    private void saveInitialMotion(float x, float y, int pointerId) {        ensureMotionHistorySizeForId(pointerId);        mInitialMotionX[pointerId] = mLastMotionX[pointerId] = x;        mInitialMotionY[pointerId] = mLastMotionY[pointerId] = y;        mInitialEdgesTouched[pointerId] = getEdgesTouched((int) x, (int) y);        // 或运算后再进行左移运算.         mPointersDown |= 1 << pointerId;    }    private void saveLastMotion(MotionEvent ev) {        final int pointerCount = MotionEventCompat.getPointerCount(ev);        for (int i = 0; i < pointerCount; i++) {            final int pointerId = MotionEventCompat.getPointerId(ev, i);            final float x = MotionEventCompat.getX(ev, i);            final float y = MotionEventCompat.getY(ev, i);            mLastMotionX[pointerId] = x;            mLastMotionY[pointerId] = y;        }    }    /**     * 检查给定id的pointer是否当前按下的pointer.     * Check if the given pointer ID represents a pointer that is currently down (to the best     * of the ViewDragHelper's knowledge).     *     * 被用于报告这个pointer信息的有以下几个方法:shouldInterceptTouchEvent()和processTouchEvent().     * 如果这其中一个方法都没有被相关的触摸事件回调,那么该方法中所汇报的信息是不准确或者过时的.     * (很明显,最新的触摸信息,必须是当前InterceptTouchEvent事件中能回调的.)     *      * <p>The state used to report this information is populated by the methods     * {@link #shouldInterceptTouchEvent(android.view.MotionEvent)} or     * {@link #processTouchEvent(android.view.MotionEvent)}. If one of these methods has not     * been called for all relevant MotionEvents to track, the information reported     * by this method may be stale or incorrect.</p>     *     * @param pointerId pointer ID to check; corresponds to IDs provided by MotionEvent     * @return true if the pointer with the given ID is still down     */    public boolean isPointerDown(int pointerId) {        return (mPointersDown & 1 << pointerId) != 0;    }    void setDragState(int state) {        mParentView.removeCallbacks(mSetIdleRunnable);        if (mDragState != state) {            mDragState = state;            mCallback.onViewDragStateChanged(state);            if (mDragState == STATE_IDLE) {                mCapturedView = null;            }        }    }    /**     * 通过传进的pointerId,试图捕获view.如果之前已成功捕获过,则不再调用mCallback.tryCaptureView()方法,而直接返回true.     * Attempt to capture the view with the given pointer ID. The callback will be involved.     * This will put us into the "dragging" state. If we've already captured this view with     * this pointer this method will immediately return true without consulting the callback.     *     * @param toCapture View to capture     * @param pointerId Pointer to capture with     * @return true if capture was successful     */    boolean tryCaptureViewForDrag(View toCapture, int pointerId) {        if (toCapture == mCapturedView && mActivePointerId == pointerId) {            // Already done!            return true;        }        if (toCapture != null && mCallback.tryCaptureView(toCapture, pointerId)) {            mActivePointerId = pointerId;            captureChildView(toCapture, pointerId);            return true;        }        return false;    }    /**     * 测试是否view v是否能滑动     * Tests scrollability within child views of v given a delta of dx.     *     * @param v View to test for horizontal scrollability     * @param checkV Whether the view v passed should itself be checked for scrollability (true),     *               or just its children (false).     * @param dx Delta scrolled in pixels along the X axis     * @param dy Delta scrolled in pixels along the Y axis     * @param x X coordinate of the active touch point     * @param y Y coordinate of the active touch point     * @return true if child views of v can be scrolled by delta of dx.     */    protected boolean canScroll(View v, boolean checkV, int dx, int dy, int x, int y) {        if (v instanceof ViewGroup) {            final ViewGroup group = (ViewGroup) v;            final int scrollX = v.getScrollX();            final int scrollY = v.getScrollY();            final int count = group.getChildCount();            // Count backwards - let topmost views consume scroll distance first.            for (int i = count - 1; i >= 0; i--) {                // TODO: Add versioned support here for transformed views.                // This will not work for transformed views in Honeycomb+                final View child = group.getChildAt(i);                if (x + scrollX >= child.getLeft() && x + scrollX < child.getRight() &&                        y + scrollY >= child.getTop() && y + scrollY < child.getBottom() &&                        canScroll(child, true, dx, dy, x + scrollX - child.getLeft(),                                y + scrollY - child.getTop())) {                    return true;                }            }        }        return checkV && (ViewCompat.canScrollHorizontally(v, -dx) ||                ViewCompat.canScrollVertically(v, -dy));    }    /**     * 检测这个作为被提供给父view的onInterceptTouchEvent的事件是否令父view拦截到当前的触摸事件流.     * Check if this event as provided to the parent view's onInterceptTouchEvent should     * cause the parent to intercept the touch event stream.     *     * @param ev MotionEvent provided to onInterceptTouchEvent - 提供给onInterceptTouchEvent()方法的触摸事件对象     * @return true if the parent view should return true from onInterceptTouchEvent     */    public boolean shouldInterceptTouchEvent(MotionEvent ev) {        final int action = MotionEventCompat.getActionMasked(ev);        final int actionIndex = MotionEventCompat.getActionIndex(ev);        if (action == MotionEvent.ACTION_DOWN) {            // Reset things for a new event stream, just in case we didn't get            // the whole previous stream.            cancel();        }        if (mVelocityTracker == null) {            mVelocityTracker = VelocityTracker.obtain();        }        mVelocityTracker.addMovement(ev);        switch (action) {            case MotionEvent.ACTION_DOWN: {                final float x = ev.getX();                final float y = ev.getY();                final int pointerId = MotionEventCompat.getPointerId(ev, 0);                saveInitialMotion(x, y, pointerId);                final View toCapture = findTopChildUnder((int) x, (int) y);                // Catch a settling view if possible.                if (toCapture == mCapturedView && mDragState == STATE_SETTLING) {                    tryCaptureViewForDrag(toCapture, pointerId);                }                final int edgesTouched = mInitialEdgesTouched[pointerId];                if ((edgesTouched & mTrackingEdges) != 0) {                    mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);                }                break;            }            case MotionEventCompat.ACTION_POINTER_DOWN: {                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);                final float x = MotionEventCompat.getX(ev, actionIndex);                final float y = MotionEventCompat.getY(ev, actionIndex);                saveInitialMotion(x, y, pointerId);                // A ViewDragHelper can only manipulate one view at a time.                if (mDragState == STATE_IDLE) {                    final int edgesTouched = mInitialEdgesTouched[pointerId];                    if ((edgesTouched & mTrackingEdges) != 0) {                        mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);                    }                } else if (mDragState == STATE_SETTLING) {                    // Catch a settling view if possible.                    final View toCapture = findTopChildUnder((int) x, (int) y);                    if (toCapture == mCapturedView) {                        tryCaptureViewForDrag(toCapture, pointerId);                    }                }                break;            }            case MotionEvent.ACTION_MOVE: {                // First to cross a touch slop over a draggable view wins. Also report edge drags.                final int pointerCount = MotionEventCompat.getPointerCount(ev);                for (int i = 0; i < pointerCount; i++) {                    final int pointerId = MotionEventCompat.getPointerId(ev, i);                    final float x = MotionEventCompat.getX(ev, i);                    final float y = MotionEventCompat.getY(ev, i);                    final float dx = x - mInitialMotionX[pointerId];                    final float dy = y - mInitialMotionY[pointerId];                    final View toCapture = findTopChildUnder((int) x, (int) y);                    final boolean pastSlop = toCapture != null && checkTouchSlop(toCapture, dx, dy);                    if (pastSlop) {                        // check the callback's                        // getView[Horizontal|Vertical]DragRange methods to know                        // if you can move at all along an axis, then see if it                        // would clamp to the same value. If you can't move at                        // all in every dimension with a nonzero range, bail.                        final int oldLeft = toCapture.getLeft();                        final int targetLeft = oldLeft + (int) dx;                        final int newLeft = mCallback.clampViewPositionHorizontal(toCapture,                                targetLeft, (int) dx);                        final int oldTop = toCapture.getTop();                        final int targetTop = oldTop + (int) dy;                        final int newTop = mCallback.clampViewPositionVertical(toCapture, targetTop,                                (int) dy);                        final int horizontalDragRange = mCallback.getViewHorizontalDragRange(                                toCapture);                        final int verticalDragRange = mCallback.getViewVerticalDragRange(toCapture);                        if ((horizontalDragRange == 0 || horizontalDragRange > 0                                && newLeft == oldLeft) && (verticalDragRange == 0                                || verticalDragRange > 0 && newTop == oldTop)) {                            break;                        }                    }                    reportNewEdgeDrags(dx, dy, pointerId);                    if (mDragState == STATE_DRAGGING) {                        // Callback might have started an edge drag                        break;                    }                    if (pastSlop && tryCaptureViewForDrag(toCapture, pointerId)) {                        break;                    }                }                saveLastMotion(ev);                break;            }            case MotionEventCompat.ACTION_POINTER_UP: {                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);                clearMotionHistory(pointerId);                break;            }            case MotionEvent.ACTION_UP:            case MotionEvent.ACTION_CANCEL: {                cancel();                break;            }        }        return mDragState == STATE_DRAGGING;    }    /**     * 加工从父view中获取的触摸事件.这个方法将分发callback回调事件.父view的触摸事件实现中应该调用该方法.     * Process a touch event received by the parent view. This method will dispatch callback events     * as needed before returning. The parent view's onTouchEvent implementation should call this.     *     * @param ev The touch event received by the parent view     */    public void processTouchEvent(MotionEvent ev) {        final int action = MotionEventCompat.getActionMasked(ev);        final int actionIndex = MotionEventCompat.getActionIndex(ev);        if (action == MotionEvent.ACTION_DOWN) {            // Reset things for a new event stream, just in case we didn't get            // the whole previous stream.            cancel();        }        if (mVelocityTracker == null) {            mVelocityTracker = VelocityTracker.obtain();        }        mVelocityTracker.addMovement(ev);        switch (action) {            case MotionEvent.ACTION_DOWN: {                final float x = ev.getX();                final float y = ev.getY();                final int pointerId = MotionEventCompat.getPointerId(ev, 0);                final View toCapture = findTopChildUnder((int) x, (int) y);                saveInitialMotion(x, y, pointerId);                // Since the parent is already directly processing this touch event,                // there is no reason to delay for a slop before dragging.                // Start immediately if possible.                tryCaptureViewForDrag(toCapture, pointerId);                final int edgesTouched = mInitialEdgesTouched[pointerId];                if ((edgesTouched & mTrackingEdges) != 0) {                    mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);                }                break;            }            case MotionEventCompat.ACTION_POINTER_DOWN: {                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);                final float x = MotionEventCompat.getX(ev, actionIndex);                final float y = MotionEventCompat.getY(ev, actionIndex);                saveInitialMotion(x, y, pointerId);                // A ViewDragHelper can only manipulate one view at a time.                if (mDragState == STATE_IDLE) {                    // If we're idle we can do anything! Treat it like a normal down event.                    final View toCapture = findTopChildUnder((int) x, (int) y);                    tryCaptureViewForDrag(toCapture, pointerId);                    final int edgesTouched = mInitialEdgesTouched[pointerId];                    if ((edgesTouched & mTrackingEdges) != 0) {                        mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);                    }                } else if (isCapturedViewUnder((int) x, (int) y)) {                    // We're still tracking a captured view. If the same view is under this                    // point, we'll swap to controlling it with this pointer instead.                    // (This will still work if we're "catching" a settling view.)                    tryCaptureViewForDrag(mCapturedView, pointerId);                }                break;            }            case MotionEvent.ACTION_MOVE: {                if (mDragState == STATE_DRAGGING) {                    final int index = MotionEventCompat.findPointerIndex(ev, mActivePointerId);                    final float x = MotionEventCompat.getX(ev, index);                    final float y = MotionEventCompat.getY(ev, index);                    final int idx = (int) (x - mLastMotionX[mActivePointerId]);                    final int idy = (int) (y - mLastMotionY[mActivePointerId]);                    dragTo(mCapturedView.getLeft() + idx, mCapturedView.getTop() + idy, idx, idy);                    saveLastMotion(ev);                } else {                    // Check to see if any pointer is now over a draggable view.                    final int pointerCount = MotionEventCompat.getPointerCount(ev);                    for (int i = 0; i < pointerCount; i++) {                        final int pointerId = MotionEventCompat.getPointerId(ev, i);                        final float x = MotionEventCompat.getX(ev, i);                        final float y = MotionEventCompat.getY(ev, i);                        final float dx = x - mInitialMotionX[pointerId];                        final float dy = y - mInitialMotionY[pointerId];                        reportNewEdgeDrags(dx, dy, pointerId);                        if (mDragState == STATE_DRAGGING) {                            // Callback might have started an edge drag.                            break;                        }                        final View toCapture = findTopChildUnder((int) x, (int) y);                        if (checkTouchSlop(toCapture, dx, dy) &&                                tryCaptureViewForDrag(toCapture, pointerId)) {                            break;                        }                    }                    saveLastMotion(ev);                }                break;            }            case MotionEventCompat.ACTION_POINTER_UP: {                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);                if (mDragState == STATE_DRAGGING && pointerId == mActivePointerId) {                    // Try to find another pointer that's still holding on to the captured view.                    int newActivePointer = INVALID_POINTER;                    final int pointerCount = MotionEventCompat.getPointerCount(ev);                    for (int i = 0; i < pointerCount; i++) {                        final int id = MotionEventCompat.getPointerId(ev, i);                        if (id == mActivePointerId) {                            // This one's going away, skip.                            continue;                        }                        final float x = MotionEventCompat.getX(ev, i);                        final float y = MotionEventCompat.getY(ev, i);                        if (findTopChildUnder((int) x, (int) y) == mCapturedView &&                                tryCaptureViewForDrag(mCapturedView, id)) {                            newActivePointer = mActivePointerId;                            break;                        }                    }                    if (newActivePointer == INVALID_POINTER) {                        // We didn't find another pointer still touching the view, release it.                        releaseViewForPointerUp();                    }                }                clearMotionHistory(pointerId);                break;            }            case MotionEvent.ACTION_UP: {                if (mDragState == STATE_DRAGGING) {                    releaseViewForPointerUp();                }                cancel();                break;            }            case MotionEvent.ACTION_CANCEL: {                if (mDragState == STATE_DRAGGING) {                    dispatchViewReleased(0, 0);                }                cancel();                break;            }        }    }        //更新边缘拖曳标记    private void reportNewEdgeDrags(float dx, float dy, int pointerId) {        int dragsStarted = 0;        if (checkNewEdgeDrag(dx, dy, pointerId, EDGE_LEFT)) {            dragsStarted |= EDGE_LEFT;        }        if (checkNewEdgeDrag(dy, dx, pointerId, EDGE_TOP)) {            dragsStarted |= EDGE_TOP;        }        if (checkNewEdgeDrag(dx, dy, pointerId, EDGE_RIGHT)) {            dragsStarted |= EDGE_RIGHT;        }        if (checkNewEdgeDrag(dy, dx, pointerId, EDGE_BOTTOM)) {            dragsStarted |= EDGE_BOTTOM;        }        if (dragsStarted != 0) {            mEdgeDragsInProgress[pointerId] |= dragsStarted;            mCallback.onEdgeDragStarted(dragsStarted, pointerId);        }    }        //内部调用方法,计算是否新的拖曳标记    private boolean checkNewEdgeDrag(float delta, float odelta, int pointerId, int edge) {        final float absDelta = Math.abs(delta);        final float absODelta = Math.abs(odelta);        if ((mInitialEdgesTouched[pointerId] & edge) != edge  || (mTrackingEdges & edge) == 0 ||                (mEdgeDragsLocked[pointerId] & edge) == edge ||                (mEdgeDragsInProgress[pointerId] & edge) == edge ||                (absDelta <= mTouchSlop && absODelta <= mTouchSlop)) {            return false;        }        if (absDelta < absODelta * 0.5f && mCallback.onEdgeLock(edge)) {            mEdgeDragsLocked[pointerId] |= edge;            return false;        }        return (mEdgeDragsInProgress[pointerId] & edge) == 0 && absDelta > mTouchSlop;    }    /**     * 检测在子view滑动时是否合理的出现溢出(这个现象是手指在滑动时部分滑动到屏幕外,但子view还能正常被拖曳移动).     * 当溢出现象严重,子view就不能沿着手指滑动的轨迹在水平或垂直轴上被拖曳移动.这时,该轴上的手势将不计入检测溢出的考虑范围.     *      * Check if we've crossed a reasonable touch slop for the given child view.     * If the child cannot be dragged along the horizontal or vertical axis, motion     * along that axis will not count toward the slop check.     *     * @param child Child to check - 要检测的子view     * @param dx Motion since initial position along X axis - 从初始开始沿着X轴的坐标     * @param dy Motion since initial position along Y axis- - 从初始开始沿着Y的坐标     * @return true if the touch slop has been crossed     */    private boolean checkTouchSlop(View child, float dx, float dy) {        if (child == null) {            return false;        }        final boolean checkHorizontal = mCallback.getViewHorizontalDragRange(child) > 0;        final boolean checkVertical = mCallback.getViewVerticalDragRange(child) > 0;        if (checkHorizontal && checkVertical) {            return dx * dx + dy * dy > mTouchSlop * mTouchSlop;        } else if (checkHorizontal) {            return Math.abs(dx) > mTouchSlop;        } else if (checkVertical) {            return Math.abs(dy) > mTouchSlop;        }        return false;    }    /**     * 检测当前手势是否的点是否超出了溢出的临界值.     *      * Check if any pointer tracked in the current gesture has crossed     * the required slop threshold.     *     * 这依赖于由方法shouldInterceptTouchEvent(...)或者processTouchEvent(...)所填充的内部状态.     * 你只有在所有有效触摸的数据都被提供到这其中的一个方法时,才应该去使用这些方法返回的结果.     *     * <p>This depends on internal state populated by     * {@link #shouldInterceptTouchEvent(android.view.MotionEvent)} or     * {@link #processTouchEvent(android.view.MotionEvent)}. You should only rely on     * the results of this method after all currently available touch data     * has been provided to one of these two methods.</p>     *     * 参数directions - 指引值. 值为{@link #DIRECTION_HORIZONTAL},     *                   {@link #DIRECTION_VERTICAL}, {@link #DIRECTION_ALL}中的其中一个.     * @param directions Combination of direction flags, see {@link #DIRECTION_HORIZONTAL},     *                   {@link #DIRECTION_VERTICAL}, {@link #DIRECTION_ALL}     *                        * 如果返回true,表示超过了溢出的临界值.否则返回false.     * @return true if the slop threshold has been crossed, false otherwise     */    public boolean checkTouchSlop(int directions) {        final int count = mInitialMotionX.length;        //mInitialMotionX装着触摸事件的所有坐标(所有pointer),因此循环去检验,只要有一个点超出临界值,就返回true.        for (int i = 0; i < count; i++) {            if (checkTouchSlop(directions, i)) {                return true;            }        }        return false;    }    /**     * 检测越出所需的溢出临界值的手势中,某个被追踪的具体的pointer.     *      * Check if the specified pointer tracked in the current gesture has crossed     * the required slop threshold.     *     * 这依赖于由方法shouldInterceptTouchEvent(...)或者processTouchEvent(...)所填充的内部状态.     * 你只有在所有有效触摸的数据都被提供到这其中的一个方法时,才应该去使用这些方法返回的结果.     *      * <p>This depends on internal state populated by     * {@link #shouldInterceptTouchEvent(android.view.MotionEvent)} or     * {@link #processTouchEvent(android.view.MotionEvent)}. You should only rely on     * the results of this method after all currently available touch data     * has been provided to one of these two methods.</p>     *     * 下面参数directions的解释跟上一个方法checkTouchSlop()是一样的.     * @param directions Combination of direction flags, see {@link #DIRECTION_HORIZONTAL},     *                   {@link #DIRECTION_VERTICAL}, {@link #DIRECTION_ALL}     * @param pointerId ID of the pointer to slop check as specified by MotionEvent     * @return true if the slop threshold has been crossed, false otherwise     */    public boolean checkTouchSlop(int directions, int pointerId) {        if (!isPointerDown(pointerId)) {            return false;        }        final boolean checkHorizontal = (directions & DIRECTION_HORIZONTAL) == DIRECTION_HORIZONTAL;        final boolean checkVertical = (directions & DIRECTION_VERTICAL) == DIRECTION_VERTICAL;        final float dx = mLastMotionX[pointerId] - mInitialMotionX[pointerId];        final float dy = mLastMotionY[pointerId] - mInitialMotionY[pointerId];        if (checkHorizontal && checkVertical) {        //如果是同时水平和垂直的运动,就检测X和Y轴坐标的平方的和.(直角三角形,两条直角边的平方的和等于第三边的平方.)            return dx * dx + dy * dy > mTouchSlop * mTouchSlop;        } else if (checkHorizontal) {        //mTouchSlop是临界值, 如果dx的绝对值大于这个值,表示已超出临界值,返回true,否则返回false.            return Math.abs(dx) > mTouchSlop;        } else if (checkVertical) {        //同前面一个解释.            return Math.abs(dy) > mTouchSlop;        }        return false;    }    /**     * 检测在当前活动的手势中一开始触摸时的屏幕边缘.如果没有活动的手势,该方法会返回false.     *      * Check if any of the edges specified were initially touched in the currently active gesture.     * If there is no currently active gesture this method will return false.     *     * 参数edges - 所检测的边缘     *     * @param edges Edges to check for an initial edge touch. See {@link #EDGE_LEFT},     *              {@link #EDGE_TOP}, {@link #EDGE_RIGHT}, {@link #EDGE_BOTTOM} and     *              {@link #EDGE_ALL}     * @return true if any of the edges specified were initially touched in the current gesture     */    public boolean isEdgeTouched(int edges) {        final int count = mInitialEdgesTouched.length;        for (int i = 0; i < count; i++) {            if (isEdgeTouched(edges, i)) {                return true;            }        }        return false;    }    /**     * 用于检测边缘中某个有具体ID值的poniter(点).     * (如果到这里还没有搞懂pointer和触摸或者和drag之间到底有什么关系,不妨这么     * 想象,一个触摸屏幕的手势是由一定数量的点去组成的,而每一个这样的点,就是一个pointer.手指一触碰屏幕,肯定会产生至少一个     * pointer.)     * 如果当前没有活动的手势,或者给定的pointerId并没有对应的pointer,那么该方法会返回false.否则返回true.     *      * Check if any of the edges specified were initially touched by the pointer with     * the specified ID. If there is no currently active gesture or if there is no pointer with     * the given ID currently down this method will return false.     *     * 参数edges - 该值是EDGE_LEFT, EDGE_TOP, EDGE_RIGHT, EDGE_RIGHT和EDGE_RIGHT中的其中一个.     * 很显然,屏幕只有左上右下这四个边缘.     * @param edges Edges to check for an initial edge touch. See {@link #EDGE_LEFT},     *              {@link #EDGE_TOP}, {@link #EDGE_RIGHT}, {@link #EDGE_RIGHT} and     *              {@link #EDGE_ALL}     * @return true if any of the edges specified were initially touched in the current gesture     */    public boolean isEdgeTouched(int edges, int pointerId) {        return isPointerDown(pointerId) && (mInitialEdgesTouched[pointerId] & edges) != 0;    }    //跟dispatchViewReleased(...)方法是类似的,当pointer消失时回收view.     private void releaseViewForPointerUp() {    //computeCurrentVelocity()方法是根据当前收集到的pointer去计算速度        mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);        final float xvel = clampMag(                VelocityTrackerCompat.getXVelocity(mVelocityTracker, mActivePointerId),                mMinVelocity, mMaxVelocity);        final float yvel = clampMag(                VelocityTrackerCompat.getYVelocity(mVelocityTracker, mActivePointerId),                mMinVelocity, mMaxVelocity);        dispatchViewReleased(xvel, yvel);    }    //拖曳的方法    private void dragTo(int left, int top, int dx, int dy) {        int clampedX = left;        int clampedY = top;        final int oldLeft = mCapturedView.getLeft();        final int oldTop = mCapturedView.getTop();        if (dx != 0) {            clampedX = mCallback.clampViewPositionHorizontal(mCapturedView, left, dx);            //设置水平偏移值            mCapturedView.offsetLeftAndRight(clampedX - oldLeft);        }        if (dy != 0) {            clampedY = mCallback.clampViewPositionVertical(mCapturedView, top, dy);            //设置垂直偏移值            mCapturedView.offsetTopAndBottom(clampedY - oldTop);        }        if (dx != 0 || dy != 0) {            final int clampedDx = clampedX - oldLeft;            final int clampedDy = clampedY - oldTop;            //既然拖曳肯定发送位置变更,就回调该接口方法            mCallback.onViewPositionChanged(mCapturedView, clampedX, clampedY,                    clampedDx, clampedDy);        }    }    /**     * 获取当前捕获的view是否在父view的坐标系统的某个点之下.如果不存在捕获的view,该方法返回false.     * (实则,这个方法可获知,触摸的点,捕获的view以及该view所在的父view是否同时处于一个坐标系统.如果是,     * 这才是一个有效的触摸拖曳事件.)     *      * Determine if the currently captured view is under the given point in the     * parent view's coordinate system. If there is no captured view this method     * will return false.     *     * @param x X position to test in the parent's coordinate system - 父view坐标系统的X轴坐标     * @param y Y position to test in the parent's coordinate system - 父view坐标系统的Y轴坐标     * @return true if the captured view is under the given point, false otherwise     */    public boolean isCapturedViewUnder(int x, int y) {        return isViewUnder(mCapturedView, x, y);    }    /**     * (该方法在上面被调用,也可以外部调用.)     * 某个view是否在父view给定的点(所处的坐标系统)之下.意思跟上一个方法isCapturedViewUnder(...)是一样的.     *      * Determine if the supplied view is under the given point in the     * parent view's coordinate system.     *     * @param view Child view of the parent to hit test - 父view中所要测试的子view对象     * @param x X position to test in the parent's coordinate system - 父view中坐标系统的X轴坐标     * @param y Y position to test in the parent's coordinate system - 父view中坐标系统的Y轴坐标     * @return true if the supplied view is under the given point, false otherwise     */    public boolean isViewUnder(View view, int x, int y) {        if (view == null) {            return false;        }        //显然,当X,Y所定位的点同时在子view内时才返回true.        return x >= view.getLeft() &&                x < view.getRight() &&                y >= view.getTop() &&                y < view.getBottom();    }    /**     * 找出父view坐标系统中指定点所定位的最顶层的子view.     * (这里需要点空间想象力,除了X,Y轴还存在Z轴,Z轴同时垂直X,Y轴.)父view中最顶层的子view,自然能就是Z轴中队对应     * 的Z值最大的子view.这种排列方法参考方法{@link Callback#getOrderedChildIndex(int)}.     * (其实我们无需过多关心这个Z轴,如果在布局文件XML中,我们只需要保证添加Drag事件的子view处于父view的顶层可见即可.)     *      * Find the topmost child under the given point within the parent view's coordinate system.     * The child order is determined using {@link Callback#getOrderedChildIndex(int)}.     *     * @param x X position to test in the parent's coordinate system - 父view坐标系统的X轴坐标     * @param y Y position to test in the parent's coordinate system - 父view坐标系统的Y轴坐标     * @return The topmost child view under (x, y) or null if none found. - 如果没找到这个最顶层子view则返回null.     */    public View findTopChildUnder(int x, int y) {        final int childCount = mParentView.getChildCount();        //应该不难理解,直接从i最大的位置开始遍历,第一个就是最顶层.同时还要保证该point的X,Y值都在该子view之内.        for (int i = childCount - 1; i >= 0; i--) {            final View child = mParentView.getChildAt(mCallback.getOrderedChildIndex(i));            if (x >= child.getLeft() && x < child.getRight() &&                    y >= child.getTop() && y < child.getBottom()) {                return child;            }        }        return null;    }        //获取边缘大小    private int getEdgesTouched(int x, int y) {        int result = 0;         // 因为子view是处于屏幕内,view的坐标轴原点是在左上角.向下为X轴正方形,向右为Y轴正方向.        // 因此,左侧和顶部都需要加mEdgeSize,而底部和右侧则是减去mEdgeSize,如果X或者Y轴坐标处于加上或减去mEdgeSize        // 的范围内,则对result和诸如EDGE_LEFT这样的值进行按位或运算,然后赋值给result再返回.        // 结合上面整个类,到了这里不禁要想为何这里和前面的一些方法都进行按位运算,好简单的理由呵呵:        // 因为EDGE_LEFT,EDGE_TOP等这些值本身就是二进制取值的.        if (x < mParentView.getLeft() + mEdgeSize) result |= EDGE_LEFT;        if (y < mParentView.getTop() + mEdgeSize) result |= EDGE_TOP;        if (x > mParentView.getRight() - mEdgeSize) result |= EDGE_RIGHT;        if (y > mParentView.getBottom() - mEdgeSize) result |= EDGE_BOTTOM;        return result;    }}