android.view.VelocityTracker主要用跟踪触摸屏事件

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 float Velocity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

example:自定义listview  对listview监听上下滑动的速率

public class RankListView extends ListView {


/**
* @param context
*/
public RankListView(Context context) {
super(context);
// TODO Auto-generated constructor stub
}
/**
* @param context
* @param attrs
*/
public RankListView(Context context, AttributeSet attrs) {
super(context, attrs);
// TODO Auto-generated constructor stub
}
/**
* @param context
* @param attrs
* @param defStyle
*/
public RankListView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
// TODO Auto-generated constructor stub
}


private VelocityTracker mVelocityTracker;
boolean visibleType;
private Callback callback;


public void setCallback(Callback callback) {
this.callback = callback;
}


public void setVisibleType(boolean visibleType) {
this.visibleType = visibleType;
}


@Override
public boolean dispatchTouchEvent(MotionEvent event) {
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
mVelocityTracker.addMovement(event);
if (event.getAction() == MotionEvent.ACTION_MOVE) {

//功能实现部分
} else if (event.getAction() == MotionEvent.ACTION_UP) {
final VelocityTracker velocityTracker = mVelocityTracker;

//设置每秒移动的速度是1000像素
velocityTracker.computeCurrentVelocity(1000);

//获取y轴方向的移动量
int velocityY = (int) velocityTracker.getYVelocity();

//具体功能实现
if (velocityY > 600) {
if (!visibleType) {
if (callback != null) {
callback.moveDown();
}
return true;
}
} else if (velocityY < -600) {
if (visibleType) {
if (callback != null) {
callback.moveUp();
}
return true;
}
}
mVelocityTracker.recycle();
mVelocityTracker = null;
}
return super.dispatchTouchEvent(event);
}
//不用的时候回收资源
public void reset() {
if (mVelocityTracker != null) {
mVelocityTracker.recycle();
mVelocityTracker = null;
}
visibleType = false;
}

//定义回调接口
public static interface Callback {
void moveDown();
void moveUp();
}
}