VelocityTracker顾名思义即速度跟踪，在android中主要应用于touch even。VelocityTracker通过跟踪一连串事件实时计算出当前的速度，这样的用法在android系统空间中随处可见，比如Gestures中的Fling, Scrolling等。
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 。
| Public Methods | |
|---|---|
| void | addMovement( MotionEventevent) Add a user's movement to the tracker. |
| void | clear() Reset the velocity tracker back to its initial state. |
| void | computeCurrentVelocity(int units, float maxVelocity) Compute the current velocity based on the points that have been collected. intunitis表示速率的基本时间单位。unitis值为1的表示是，一毫秒时间单位内运动了多少个像素， unitis值为1000表示一秒（1000毫秒）时间单位内运动了多少个像素 floatVelocity表示速率的最大值 |
| void | computeCurrentVelocity(int units) Equivalent to invoking computeCurrentVelocity(int, float)with a maximum velocity of Float.MAX_VALUE. 一般使用此函数即可 |
| abstract T | getNextPoolable() |
| float | getXVelocity() Retrieve the last computed X velocity. |
| float | getXVelocity(int id) Retrieve the last computed X velocity. |
| float | getYVelocity(int id) Retrieve the last computed Y velocity. |
| float | getYVelocity() Retrieve the last computed Y velocity. |
| abstract boolean | isPooled() |
| static VelocityTracker | obtain() Retrieve a new VelocityTracker object to watch the velocity of a motion. |
| void | recycle() Return a VelocityTracker object back to be re-used by others. |
| abstract void | setNextPoolable(T element) |
| abstract void | setPooled(boolean isPooled) |
示例代码：
@Override
public boolean onTouchEvent(MotionEvent ev) {
if (null == mVelocityTracker) {
mVelocityTracker = VelocityTracker.obtain();
}
mVelocityTracker.addMovement(ev);
switch (ev.getAction()) {
case MotionEvent.ACTION_UP:
// 隐藏在左边的宽度
int scrollX = getScrollX();
Loger.e(ObjEarth.TAG, "V=" + mVelocityTracker.getXVelocity());
if (Math.abs(mVelocityTracker.getXVelocity()) > 4000f) {
if (mVelocityTracker.getXVelocity() < 0f) {
//正向逻辑代码
} else {
//反向逻辑代码
}
}
return true;
case MotionEvent.ACTION_MOVE:
mVelocityTracker.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
}
return super.onTouchEvent(ev);
}
