官方介绍

/**
 * Helper for tracking the velocity of touch events, for implementing flinging and other such gestures. 
 * Use obtain to retrieve a new instance of the class when you are going to begin tracking. 
 * Put the motion events you receive into it with addMovement(MotionEvent). 
 * When you want to determine the velocity call computeCurrentVelocity(int) and then call getXVelocity(int) and getYVelocity(int) to retrieve the velocity for each pointer id.
 */

android.view.VelocityTracker主要用跟踪触摸屏事件（flinging事件和其他gestures手势事件）的速率。用addMovement(MotionEvent)函数将Motion event加入到VelocityTracker类实例中.你可以使用getXVelocity() 或getXVelocity()获得横向和竖向的速率到速率时，但是使用它们之前请先调用computeCurrentVelocity(int)来初始化速率的单位 。

使用步骤

1.声明

  private VelocityTracker mVelocityTracker;

2.初始化

  private void init() {
      mVelocityTracker = VelocityTracker.obtain();
  }

3.在onTouchEvent的开头添加滑动事件

  public boolean onTouchEvent(MotionEvent event) {
      mVelocityTracker.addMovement(event);
      ...
  }

当你需要跟踪触摸屏事件的速度的时候,使用obtain()方法来获得VelocityTracker类的一个实例对象

在onTouchEvent回调函数中，使用addMovement(MotionEvent)函数将当前的移动事件传递给VelocityTracker对象

使用computeCurrentVelocity (int units)函数来计算当前的速度，使用getXVelocity ()、 getYVelocity ()函数来获得当前的速度

private VelocityTracker mVelocityTracker;//生命变量  
//在onTouchEvent(MotionEvent ev)中  
if (mVelocityTracker == null) {  
    mVelocityTracker = VelocityTracker.obtain();//获得VelocityTracker类实例  
}  
mVelocityTracker.addMovement(ev);//将事件加入到VelocityTracker类实例中  
//判断当ev事件是MotionEvent.ACTION_UP时：计算速率  
final VelocityTracker velocityTracker = mVelocityTracker;  
// 1000 provides pixels per second  
velocityTracker.computeCurrentVelocity(1, (float)0.01);//设置maxVelocity值为0.1时，速率大于0.01时，显示的速率都是0.01,速率小于0.01时，显示正常  
Log.i("test","velocityTraker"+velocityTracker.getXVelocity());  
velocityTracker.computeCurrentVelocity(1000); //设置units的值为1000，意思为一秒时间内运动了多少个像素  
Log.i("test","velocityTraker"+velocityTracker.getXVelocity());

4.在onDetachedFromWindow中回收

  @Override
  protected void onDetachedFromWindow() {
      mVelocityTracker.recycle();
      super.onDetachedFromWindow();
  }