Android Scroller 的理解

Scroller对象只是记录了当前view 的运动坐标和时间以及速度等物理变量。
 
public void startScroll(int startX, int startY, int dx, int dy, int duration);只是给 view 开始滚动的位置，要滚动到哪里，以及滚动过程所需要的时间进行赋值。


public void fling(int startX, int startY, int velocityX, int velocityY,int minX, int maxX, int minY, int maxY) 是根据fing的手势，开始的位置，以及手势的初始速度，经过计算，对Sroller对象里表示运动和时间的变量进行赋值。与startScroll本质上一样。


public boolean computeScrollOffset() 更新各个物理变量，查看当前view是否已经scroll到目标位置。


通常将一个Scroller 对象放到自定义的view 或者 ViewGroup 中。当调用view 调用Invalidate()方法引起 界面重绘时，view会依次 onMeasure->onLayout->onDraw，在view 调用onDraw()中 会调用view 的computeScroll中会根据Scroller对象中记录的物理变量 对view进行scroll，通过调用view.scrollTo()或者view.scrollBy()。

以下是Scroller 的source code

/* * Copyright (C) 2006 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.widget;import android.content.Context;import android.view.ViewConfiguration;import android.view.animation.AnimationUtils;import android.view.animation.Interpolator;/** * This class encapsulates scrolling.  The duration of the scroll * can be passed in the constructor and specifies the maximum time that * the scrolling animation should take.  Past this time, the scrolling is  * automatically moved to its final stage and computeScrollOffset() * will always return false to indicate that scrolling is over. */public class Scroller  {    private int mMode;    private int mStartX;    private int mStartY;    private int mFinalX;    private int mFinalY;    private int mMinX;    private int mMaxX;    private int mMinY;    private int mMaxY;    private int mCurrX;    private int mCurrY;    private long mStartTime;    private int mDuration;    private float mDurationReciprocal;    private float mDeltaX;    private float mDeltaY;    private float mViscousFluidScale;    private float mViscousFluidNormalize;    private boolean mFinished;    private Interpolator mInterpolator;    private float mCoeffX = 0.0f;    private float mCoeffY = 1.0f;    private float mVelocity;    private static final int DEFAULT_DURATION = 250;    private static final int SCROLL_MODE = 0;    private static final int FLING_MODE = 1;    private final float mDeceleration;    /**     * Create a Scroller with the default duration and interpolator.     */    public Scroller(Context context) {        this(context, null);    }    /**     * Create a Scroller with the specified interpolator. If the interpolator is     * null, the default (viscous) interpolator will be used.     */    public Scroller(Context context, Interpolator interpolator) {        mFinished = true;        mInterpolator = interpolator;        float ppi = context.getResources().getDisplayMetrics().density * 160.0f;        mDeceleration = 9.8f   // g (m/s^2)                      * 39.37f // inch/meter                      * ppi    // pixels per inch                      * ViewConfiguration.getScrollFriction();    }        /**     *      * Returns whether the scroller has finished scrolling.     *      * @return True if the scroller has finished scrolling, false otherwise.     */    public final boolean isFinished() {        return mFinished;    }        /**     * Force the finished field to a particular value.     *       * @param finished The new finished value.     */    public final void forceFinished(boolean finished) {        mFinished = finished;    }        /**     * Returns how long the scroll event will take, in milliseconds.     *      * @return The duration of the scroll in milliseconds.     */    public final int getDuration() {        return mDuration;    }        /**     * Returns the current X offset in the scroll.      *      * @return The new X offset as an absolute distance from the origin.     */    public final int getCurrX() {        return mCurrX;    }        /**     * Returns the current Y offset in the scroll.      *      * @return The new Y offset as an absolute distance from the origin.     */    public final int getCurrY() {        return mCurrY;    }        /**     * Returns the start X offset in the scroll.      *      * @return The start X offset as an absolute distance from the origin.     */    public final int getStartX() {        return mStartX;    }        /**     * Returns the start Y offset in the scroll.      *      * @return The start Y offset as an absolute distance from the origin.     */    public final int getStartY() {        return mStartY;    }        /**     * Returns where the scroll will end. Valid only for "fling" scrolls.     *      * @return The final X offset as an absolute distance from the origin.     */    public final int getFinalX() {        return mFinalX;    }        /**     * Returns where the scroll will end. Valid only for "fling" scrolls.     *      * @return The final Y offset as an absolute distance from the origin.     */    public final int getFinalY() {        return mFinalY;    }    /**     * Call this when you want to know the new location.  If it returns true,     * the animation is not yet finished.  loc will be altered to provide the     * new location.     */     public boolean computeScrollOffset() {        if (mFinished) {            return false;        }        int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);            if (timePassed < mDuration) {            switch (mMode) {            case SCROLL_MODE:                float x = (float)timePassed * mDurationReciprocal;                    if (mInterpolator == null)                    x = viscousFluid(x);                 else                    x = mInterpolator.getInterpolation(x);                    mCurrX = mStartX + Math.round(x * mDeltaX);                mCurrY = mStartY + Math.round(x * mDeltaY);                if ((mCurrX == mFinalX) && (mCurrY == mFinalY)) {                    mFinished = true;                }                break;            case FLING_MODE:                float timePassedSeconds = timePassed / 1000.0f;                float distance = (mVelocity * timePassedSeconds)                        - (mDeceleration * timePassedSeconds * timePassedSeconds / 2.0f);                                mCurrX = mStartX + Math.round(distance * mCoeffX);                // Pin to mMinX <= mCurrX <= mMaxX                mCurrX = Math.min(mCurrX, mMaxX);                mCurrX = Math.max(mCurrX, mMinX);                                mCurrY = mStartY + Math.round(distance * mCoeffY);                // Pin to mMinY <= mCurrY <= mMaxY                mCurrY = Math.min(mCurrY, mMaxY);                mCurrY = Math.max(mCurrY, mMinY);                if (mCurrX == mFinalX && mCurrY == mFinalY) {                    mFinished = true;                }                                break;            }        }        else {            mCurrX = mFinalX;            mCurrY = mFinalY;            mFinished = true;        }        return true;    }        /**     * Start scrolling by providing a starting point and the distance to travel.     * The scroll will use the default value of 250 milliseconds for the     * duration.     *      * @param startX Starting horizontal scroll offset in pixels. Positive     *        numbers will scroll the content to the left.     * @param startY Starting vertical scroll offset in pixels. Positive numbers     *        will scroll the content up.     * @param dx Horizontal distance to travel. Positive numbers will scroll the     *        content to the left.     * @param dy Vertical distance to travel. Positive numbers will scroll the     *        content up.     */    public void startScroll(int startX, int startY, int dx, int dy) {        startScroll(startX, startY, dx, dy, DEFAULT_DURATION);    }    /**     * Start scrolling by providing a starting point and the distance to travel.     *      * @param startX Starting horizontal scroll offset in pixels. Positive     *        numbers will scroll the content to the left.     * @param startY Starting vertical scroll offset in pixels. Positive numbers     *        will scroll the content up.     * @param dx Horizontal distance to travel. Positive numbers will scroll the     *        content to the left.     * @param dy Vertical distance to travel. Positive numbers will scroll the     *        content up.     * @param duration Duration of the scroll in milliseconds.     */    public void startScroll(int startX, int startY, int dx, int dy, int duration) {        mMode = SCROLL_MODE;        mFinished = false;        mDuration = duration;        mStartTime = AnimationUtils.currentAnimationTimeMillis();        mStartX = startX;        mStartY = startY;        mFinalX = startX + dx;        mFinalY = startY + dy;        mDeltaX = dx;        mDeltaY = dy;        mDurationReciprocal = 1.0f / (float) mDuration;        // This controls the viscous fluid effect (how much of it)        mViscousFluidScale = 8.0f;        // must be set to 1.0 (used in viscousFluid())        mViscousFluidNormalize = 1.0f;        mViscousFluidNormalize = 1.0f / viscousFluid(1.0f);    }    /**     * Start scrolling based on a fling gesture. The distance travelled will     * depend on the initial velocity of the fling.     *      * @param startX Starting point of the scroll (X)     * @param startY Starting point of the scroll (Y)     * @param velocityX Initial velocity of the fling (X) measured in pixels per     *        second.     * @param velocityY Initial velocity of the fling (Y) measured in pixels per     *        second     * @param minX Minimum X value. The scroller will not scroll past this     *        point.     * @param maxX Maximum X value. The scroller will not scroll past this     *        point.     * @param minY Minimum Y value. The scroller will not scroll past this     *        point.     * @param maxY Maximum Y value. The scroller will not scroll past this     *        point.     */    public void fling(int startX, int startY, int velocityX, int velocityY,            int minX, int maxX, int minY, int maxY) {        mMode = FLING_MODE;        mFinished = false;        float velocity = (float)Math.hypot(velocityX, velocityY);             mVelocity = velocity;        mDuration = (int) (1000 * velocity / mDeceleration); // Duration is in                                                            // milliseconds        mStartTime = AnimationUtils.currentAnimationTimeMillis();        mStartX = startX;        mStartY = startY;        mCoeffX = velocity == 0 ? 1.0f : velocityX / velocity;         mCoeffY = velocity == 0 ? 1.0f : velocityY / velocity;        int totalDistance = (int) ((velocity * velocity) / (2 * mDeceleration));                mMinX = minX;        mMaxX = maxX;        mMinY = minY;        mMaxY = maxY;                        mFinalX = startX + Math.round(totalDistance * mCoeffX);        // Pin to mMinX <= mFinalX <= mMaxX        mFinalX = Math.min(mFinalX, mMaxX);        mFinalX = Math.max(mFinalX, mMinX);                mFinalY = startY + Math.round(totalDistance * mCoeffY);        // Pin to mMinY <= mFinalY <= mMaxY        mFinalY = Math.min(mFinalY, mMaxY);        mFinalY = Math.max(mFinalY, mMinY);    }                private float viscousFluid(float x)    {        x *= mViscousFluidScale;        if (x < 1.0f) {            x -= (1.0f - (float)Math.exp(-x));        } else {            float start = 0.36787944117f;   // 1/e == exp(-1)            x = 1.0f - (float)Math.exp(1.0f - x);            x = start + x * (1.0f - start);        }        x *= mViscousFluidNormalize;        return x;    }        /**     *      */    public void abortAnimation() {        mCurrX = mFinalX;        mCurrY = mFinalY;        mFinished = true;    }        /**     * Extend the scroll animation. This allows a running animation to      * scroll further and longer, when used with setFinalX() or setFinalY().     *     * @param extend Additional time to scroll in milliseconds.     */    public void extendDuration(int extend) {        int passed = timePassed();        mDuration = passed + extend;        mDurationReciprocal = 1.0f / (float)mDuration;        mFinished = false;    }        public int timePassed() {        return (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);    }        public void setFinalX(int newX) {        mFinalX = newX;        mDeltaX = mFinalX - mStartX;        mFinished = false;    }   public void setFinalY(int newY) {        mFinalY = newY;        mDeltaY = mFinalY - mStartY;        mFinished = false;    }}