上下面板切换效果（类似大黑板效果）

上篇blog介绍了ViewDragHelper的简单的使用，这篇blog再用ViewDragHelper来实现一个效果，先上图

再上代码：

public class BlackboardView extends ViewGroup {    private int mWidth;//控件的宽    private int mHeight;//控件的高    private View topView, bottonView, capturedView;    private ViewDragHelper mHelper;    private ViewDragHelper.Callback callback = new ViewDragHelper.Callback() {        @Override        public boolean tryCaptureView(View child, int pointerId) {            return (topView == child) || (bottonView == child);        }        @Override        public int clampViewPositionVertical(View child, int top, int dy) {            if (top < 0)                return 0;            if (top > mHeight / 2)                return mHeight / 2;            return top;        }        @Override        public int clampViewPositionHorizontal(View child, int left, int dx) {            return 0;        }        @Override        public void onViewReleased(View releasedChild, float xvel, float yvel) {            super.onViewReleased(releasedChild, xvel, yvel);            if (releasedChild.getTop() < mHeight / 4)                mHelper.smoothSlideViewTo(releasedChild, 0, 0);            else                mHelper.smoothSlideViewTo(releasedChild, 0, mHeight / 2);            ViewCompat.postInvalidateOnAnimation(BlackboardView.this);        }        @Override        public void onViewCaptured(View capturedChild, int activePointerId) {            super.onViewCaptured(capturedChild, activePointerId);            capturedView = capturedChild;        }        @Override        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {            super.onViewPositionChanged(changedView, left, top, dx, dy);            if (changedView == capturedView) {                if (changedView == topView)                    bottonView.offsetTopAndBottom(-dy);                else if (changedView == bottonView)                    topView.offsetTopAndBottom(-dy);            }        }    };    public BlackboardView(Context context, AttributeSet attrs) {        super(context, attrs);        init();    }    public BlackboardView(Context context) {        super(context);        init();    }    public BlackboardView(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);        init();    }    private void init() {        mHelper = ViewDragHelper.create(this, callback);    }    @Override    protected void onSizeChanged(int w, int h, int oldw, int oldh) {        super.onSizeChanged(w, h, oldw, oldh);        mWidth = getMeasuredWidth();        mHeight = getMeasuredHeight();    }    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        super.onMeasure(widthMeasureSpec, heightMeasureSpec);        for (int i = 0; i < getChildCount(); i++) {            measureChild(getChildAt(i), widthMeasureSpec, heightMeasureSpec);        }    }    @Override    protected void onLayout(boolean changed, int l, int t, int r, int b) {        topView = getChildAt(0);        bottonView = getChildAt(1);        topView.layout(0, 0, mWidth, mHeight / 2);        bottonView.layout(0, mHeight / 2, mWidth, mHeight);    }    @Override    public boolean onInterceptTouchEvent(MotionEvent ev) {        return mHelper.shouldInterceptTouchEvent(ev);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        mHelper.processTouchEvent(event);        return true;    }    @Override    public void computeScroll() {        super.computeScroll();        if (mHelper.continueSettling(true)) {            ViewCompat.postInvalidateOnAnimation(this);        }    }

代码不难，比起上一篇blog，只是callback多2个回调函数，以及控制了两个子view的滑动。
public void onViewCaptured(View capturedChild, int activePointerId)
当滑动某一个子view时回调，down-move-up过程中只回调一次
public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy)
当子view位置变化时调用，不一定是拖动的那个。