VelocityTracker简介
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
。
主要函数
| Public Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| void |
addMovement(MotionEvent event) 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.
int unitis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. |
||||||||||
| 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：
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());
大体的使用是这样的：
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
