Android View 的事件体系

width = right - left;
height = bottom - top;

x = left = translationX;
y = right + translationY;

ACTION_DOWN // 收支接触
ACTION_MOVE // 手指在屏幕上移动
ACTION_UP //手指从屏幕松开
ACTION_CANCEL //取消
...

//1.在 onTouchEvent 方法中追踪当前事件的速度
VelocityTracker  tracker = VelocityTracker.obtain();
tracker.addMovement(event);
//2.获取当前速度
tracker.computeCurrentVelocity(1000);//计算速度
int xVelocity = (int) tracker.getXVelocity();
int yVelocity = (int) tracker.getYVelocity();
//3.在不需要的时候重置回收内存
tracker.recycle();

//设置监听
mGestureDetector = new GestureDetector(this);
//避免长按后无法拖动，自己测试时发现不设置，长按后也可以拖动
mGestureDetector.setIsLongpressEnabled(false);

boolean consume = mGestureDetector.onTouchEvent(event);
return consume;

/**
   * Set the scrolled position of your view. This will cause a call to
   * {@link #onScrollChanged(int, int, int, int)} and the view will be
   * invalidated.
   * @param x the x position to scroll to
   * @param y the y position to scroll to
   */
  public void scrollTo(int x, int y) {
      if (mScrollX != x || mScrollY != y) {
          int oldX = mScrollX;
          int oldY = mScrollY;
          mScrollX = x;
          mScrollY = y;
          invalidateParentCaches();
          onScrollChanged(mScrollX, mScrollY, oldX, oldY);
          if (!awakenScrollBars()) {
              postInvalidateOnAnimation();
          }
      }
  }
  
    /**
   * Move the scrolled position of your view. This will cause a call to
   * {@link #onScrollChanged(int, int, int, int)} and the view will be
   * invalidated.
   * @param x the amount of pixels to scroll by horizontally
   * @param y the amount of pixels to scroll by vertically
   */
  public void scrollBy(int x, int y) {
      scrollTo(mScrollX + x, mScrollY + y);
  }

 ViewGroup.MarginLayoutParams params = (ViewGroup.MarginLayoutParams)
                mButton.getLayoutParams();
params.width += 100;
params.leftMargin += 100;
mButton.setLayoutParams(params);  // 或者  mButton.requestLayout();

Scroller mScroller = new Scroller(context);

/**
 * 滑动到指定位置
 *
 * @param destX  X 滑动距离
 * @param destY  Y 滑动距离
 */
private void smoothScrollTo(int destX, int destY) {
    //滑动起点X
    int scrollX = getScrollX();
    //滑动起点Y
    int scrollY = getScrollY();
    //1000 ms内慢慢滑向 （destX，destY）
    mScroller.startScroll(scrollX, scrollY, destX, destY, 1000);
    //重绘
    invalidate();
}
 /**
 * 使View 不断重绘
 */
@Override
public void computeScroll() {
    /**
     *  computeScrollOffset 方法通过时间流逝百分比计算 scrollX和scrollY 
     *  返回true 表示滑动未结束
     */
    if (mScroller.computeScrollOffset()) {
        //滑动到当前位置，通过小幅度滑动实现弹性滑动
        scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
        //再次重绘
        postInvalidate();
    }
}

//事件分发
public boolean dispatchTouchEvent(MotionEvent event) {
    boolean consume = false;
    //是否被拦截
    if (onInterceptTouchEvent(event))
    {
        //被拦截，处理事件
        consume = onTouchEvent(event);
    } else {
        //未被拦截，向下分发
        consume = childView.dispatchTouchEvent(event);
    }
    return consume;
}