VelocityTracker是用来计算触摸事件速度的帮助类，主要用法如下：

• 获取对象：

if(mTracker == null){
     mTracker = VelocityTracker.obtain();
}else{
    mTracker.clear();
}

• 跟踪事件

mTracker.addMovement(event);

• 计算速度

mTracker.computeCurrentVelocity(1000);
float xVel = mTracker.getXVelocity();
float yVel = mTracker.getYVelocity();

• 释放资源： mTracker.recycle(); mTracker = null; ——————————————————————– VelocityTraker有两个方法，需要注意：
• recycle：把一个VelocityTracker对象恢复到可被重复使用的状态，执行完该函数后不应再使用VelocityTracker对象 public void recycle() { if (mStrategy == null) { clear(); sPool.release(this); } }

1. release：把VelocityTracker复位到初始状态 public void clear() { nativeClear(mPtr); }

完整实例代码：

 private VelocityTracker mTracker;
 @Override
    public boolean onTouchEvent(MotionEvent event){
        if(mTracker == null){
            mTracker = VelocityTracker.obtain();
        }else{
            mTracker.clear();
        }
        mTracker.addMovement(event);
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:

                break;
            case MotionEvent.ACTION_MOVE:
                mTracker.computeCurrentVelocity(1000);
                float xVel = mTracker.getXVelocity();
                float yVel = mTracker.getYVelocity();
                Log.d("test","x = "+xVel+"; y = "+yVel);
                break;
            case MotionEvent.ACTION_CANCEL:
            case MotionEvent.ACTION_UP:
                mTracker.recycle();
                mTracker = null;
                break;
        }
        return  super.onTouchEvent(event);
    }