android仿ios弹性页

public class Drag extends LinearLayout {
    private ViewDragHelper mDraw;
    private Point mPoint = new Point();

    public Drag(Context context, AttributeSet attrs) {
        super(context, attrs);
        mDraw = ViewDragHelper.create(this, 1.0f, new ViewDragHelper.Callback() {
            @Override
            public boolean tryCaptureView(View child, int pointerId) {
                return true;
            }

            @Override
            public int clampViewPositionHorizontal(View child, int left, int dx) {
                int boundaryLeft = getPaddingLeft();
                int boundaryRight = getWidth() - child.getWidth() - getPaddingRight();
                left = Math.min(Math.max(left, boundaryLeft), boundaryRight);
                return left;
            }

            @Override
            public int clampViewPositionVertical(View child, int top, int dy) {
                Log.e("zxc", "top-------->" + top);
                //垂直下拉弹性
                if (top == 0) {
                    top = 0;
                }
                if (top > 300) {
                    top = 300;
                }
              //垂直上拉弹性
                if (top < 0){
                    if (top < -300){
                        top = -300;
                    }
                }
                return top;
            }

            //手指释放的时候回调
            @Override
            public void onViewReleased(View releasedChild, float xvel, float yvel) {
                mDraw.settleCapturedViewAt(0,0);
                invalidate();
            }
        });
    }

    @Override
    public void computeScroll()
    {
        if(mDraw.continueSettling(true))
        {
            invalidate();
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        return mDraw.shouldInterceptTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mDraw.processTouchEvent(event);
        return true;
    }
}