android 群英传笔记----Android scroll

现在流行的app中都存在大量的滑动操作，但是一直没有去研究过，实在是不应该，正好这本书有讲解，就参考本书学习一下

• 滑动产生的原因
• 如何实现、处理滑动效果

1 . 滑动产生的原因

滑动一个View,本质上来说就是移动一个View。改变其当前所处位置，它的原理与动画效果非常相似，都是通过不断改变View的坐标来实现这一效果。所以，要实现View的滑动，就必须监听用户的触摸事件，从而实现View跟随用户触摸的滑动而滑动。

1.1 坐标系

如下图所示：

1.2 常见的获取坐标方法

2 .实现滑动的7中方法

要实现的效果：让自定义View随着手指在屏幕上的滑动而滑动

• layout()方法

public class DragViewLayout extends View {    private int lastX;    private int lastY;    public DragViewLayout(Context context) {        super(context);    }    public DragViewLayout(Context context, AttributeSet attrs) {        super(context, attrs);    }    public DragViewLayout(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int action = event.getAction();        int x = (int) event.getX();        int y = (int) event.getY();        switch (action){            case MotionEvent.ACTION_DOWN:                lastX = x;                lastY = y;                break;            case MotionEvent.ACTION_MOVE:                int offsetX = x - lastX;                int offsetY = y-lastY;                layout(getLeft()+offsetX,getTop()+offsetY,getRight()+offsetX,getBottom()+offsetY);                break;            case MotionEvent.ACTION_UP:                break;            default:                break;        }        return true;    }}

• offsetLeftAndrRight()与offsetTopAndBottom()

public class DragViewOffset extends View {    private int lastX;    private int lastY;    public DragViewOffset(Context context) {        super(context);    }    public DragViewOffset(Context context, AttributeSet attrs) {        super(context, attrs);    }    public DragViewOffset(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int action = event.getAction();        int x = (int) event.getX();        int y = (int) event.getY();        switch (action){            case MotionEvent.ACTION_DOWN:                lastX = x;                lastY = y;                break;            case MotionEvent.ACTION_MOVE:                int offsetX = x - lastX;                int offsetY = y-lastY;                offsetLeftAndRight(offsetX);                offsetTopAndBottom(offsetY);                break;            case MotionEvent.ACTION_UP:                break;            default:                break;        }        return true;    }}

• LayoutParams

public class DragViewMargin extends View {    private int lastX;    private int lastY;    private ViewGroup.MarginLayoutParams params;    public DragViewMargin(Context context) {        super(context);    }    public DragViewMargin(Context context, AttributeSet attrs) {        super(context, attrs);    }    public DragViewMargin(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int action = event.getAction();        int x = (int) event.getX();        int y = (int) event.getY();        switch (action){            case MotionEvent.ACTION_DOWN:                lastX = x;                lastY = y;                break;            case MotionEvent.ACTION_MOVE:                int offsetX = x - lastX;                int offsetY = y-lastY;                params = (ViewGroup.MarginLayoutParams) getLayoutParams();                params.leftMargin = getLeft()+offsetX;                params.topMargin = getTop()+offsetY;                setLayoutParams(params);                break;            case MotionEvent.ACTION_UP:                break;            default:                break;        }        return true;    }}

• scrollTo 与scrollby,注意参考系的不同

/*** 如果是View使用Scrollby,移动的是view的内容，TextView，content是他的文本，ImageView，content是他的drawable* 如果是ViewGroup,移动的是所有子View*/public class DragViewScrollBy extends View {    private int lastX;    private int lastY;    private ViewGroup.MarginLayoutParams params;    public DragViewScrollBy(Context context) {        super(context);    }    public DragViewScrollBy(Context context, AttributeSet attrs) {        super(context, attrs);    }    public DragViewScrollBy(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int action = event.getAction();        int x = (int) event.getX();        int y = (int) event.getY();        switch (action) {            case MotionEvent.ACTION_DOWN:                lastX = x;                lastY = y;                break;            case MotionEvent.ACTION_MOVE:                int offsetX = x - lastX;                int offsetY = y - lastY;                //此处，是用-offset                ((ViewGroup) getParent()).scrollBy(-offsetX, -offsetY);                break;            case MotionEvent.ACTION_UP:                break;            default:                break;        }        return true;    }}

• Scroller，实现平滑移动效果

/** * Created by acer on 2015/11/5. * 松开手指，回到原位置 *  */public class DragViewScroller extends View {    private int lastX;    private int lastY;    private Scroller mscroller;    public DragViewScroller(Context context) {        super(context);        init(context);    }    public DragViewScroller(Context context, AttributeSet attrs) {        super(context, attrs);        init(context);    }    public DragViewScroller(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);        init(context);    }    private void init(Context context) {        mscroller = new Scroller(context);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int action = event.getAction();        int x = (int) event.getX();        int y = (int) event.getY();        switch (action) {            case MotionEvent.ACTION_DOWN:                lastX = x;                lastY = y;                break;            case MotionEvent.ACTION_MOVE:                int offsetX = x - lastX;                int offsetY = y - lastY;                ((View) getParent()).scrollBy(-offsetX, -offsetY);                break;            case MotionEvent.ACTION_UP:            //手指离开，开始滑动回原位置                View viewGroup = (View) getParent();                mscroller.startScroll(viewGroup.getScrollX(), viewGroup.getScrollY(), -viewGroup.getScrollX(), -viewGroup.getScrollY());                invalidate();                break;            default:                break;        }        return true;    }    /**    *computeScroll()方法不会自动调用，    *只能通过invalidate()---draw()--computeScroll()来间接调用    *    */    @Override    public void computeScroll() {        if (mscroller.computeScrollOffset()) {            ((View) getParent()).scrollTo(mscroller.getCurrX(), mscroller.getCurrY());            //通过重绘，不断调用computeScrol            invalidate();        }    }}

• ViewGragHelper 强大的滑动控制类，实现各种不同的滑动，拖放需求

public class LearnViewDragHelper extends FrameLayout {    private ViewDragHelper mHelper;    private View mMenuView;    private View mMainView;    private int mWidth;    public LearnViewDragHelper(Context context) {        super(context);        init();    }    public LearnViewDragHelper(Context context, AttributeSet attrs) {        super(context, attrs);        init();    }    public LearnViewDragHelper(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);        init();    }    @TargetApi(Build.VERSION_CODES.LOLLIPOP)    public LearnViewDragHelper(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {        super(context, attrs, defStyleAttr, defStyleRes);        init();    }    /**     * 加载布局文件完成后调用     */    @Override    public void onFinishInflate(){        super.onFinishInflate();        mMenuView = getChildAt(0);        mMainView = getChildAt(1);    }    @Override    public void onSizeChanged(int w,int h,int oldw,int oldh){        super.onSizeChanged(w, h, oldw, oldh);        mWidth = mMenuView.getMeasuredWidth();    }    @Override    public boolean onInterceptTouchEvent(MotionEvent ev) {        return mHelper.shouldInterceptTouchEvent(ev);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        //将触摸事件传递给ViewDragHelper,此操作必不可少        mHelper.processTouchEvent(event);        return true;    }    private void init() {        mHelper = ViewDragHelper.create(this, callback);    }    /**     * 核心回調     **/    private ViewDragHelper.Callback callback = new ViewDragHelper.Callback() {        /**         * 开始检测触摸事件         * @param child         * @param pointerId         * @return         */        @Override        public boolean tryCaptureView(View child, int pointerId) {            //如果当前触摸的child是mMainView时开始检测            return mMainView==child;        }        /**         * 用户触摸到View后回调         * @param capturedChild         * @param activePointerId         */        @Override        public void onViewCaptured(View capturedChild, int activePointerId) {            super.onViewCaptured(capturedChild, activePointerId);        }        /**         * view拖拽状态改变时回调         * @param state         */        @Override        public void onViewDragStateChanged(int state) {            super.onViewDragStateChanged(state);        }        /**         * view position改变时回调，常用于更改scale进行缩放等效果         * @param changedView         * @param left         * @param top         * @param dx         * @param dy         */        @Override        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {            super.onViewPositionChanged(changedView, left, top, dx, dy);        }        /**         *         * 通常情况下，只需要返回top 和 left即可，但是当需要更加精确的计算padding等属性的时候，就需要对left进行一些处理，并返回合适大小的值         * */        /**         * 水平方向滑动         * @param child         * @param left 在水平方向child移动的距离         * @param dx 较前一次的增量         * @return 默认返回0         */        @Override        public int clampViewPositionHorizontal(View child, int left, int dx) {            return left;        }        /**         * 竖直方向滑动         * @param child         * @param top 在竖直方向child移动的距离         * @param dy 较前一次移动的距离         * @return 默认返回0         */        @Override        public int clampViewPositionVertical(View child, int top, int dy) {            return super.clampViewPositionVertical(child,top,dy);        }        /**         * 拖动结束后调用         * @param releasedChild         * @param xvel         * @param yvel         */        @Override        public void onViewReleased(View releasedChild, float xvel, float yvel) {            super.onViewReleased(releasedChild, xvel, yvel);            //手指抬起后，缓慢移动到指定位置            if(mMainView.getLeft()<500){                //关闭菜单，相当于Scroller.startScroll                mHelper.smoothSlideViewTo(mMainView,0,0);                ViewCompat.postInvalidateOnAnimation(LearnViewDragHelper.this);            }else{                //打开菜单                mHelper.smoothSlideViewTo(mMainView,300,0);/***Cause an invalidate to happen on the next animation time *step, typically the next display frame**/                ViewCompat.postInvalidateOnAnimation(LearnViewDragHelper.this);            }        }    };    /**     * 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.     *     * @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     */    @Override    public void computeScroll() {        if (mHelper.continueSettling(true)) {            ViewCompat.postInvalidateOnAnimation(this);        }    }}

滑动效果图如下：

DragViewHelper效果图如下：