android 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

demo

package com.bxwu.demo.component.activity;

import android.app.Activity;

import android.graphics.Color;

import android.os.Bundle;

import android.view.MotionEvent;

import android.view.VelocityTracker;

import android.view.ViewConfiguration;

import android.view.ViewGroup.LayoutParams;

import android.widget.TextView;

public class VelocityTrackerTest extends Activity {

private TextView mInfo;

private VelocityTracker mVelocityTracker;

private int mMaxVelocity;

private int mPointerId;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

mInfo = new TextView(this);

mInfo.setLines(4);

mInfo.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT));

mInfo.setTextColor(Color.WHITE);

setContentView(mInfo);

mMaxVelocity = ViewConfiguration.get(this).getMaximumFlingVelocity();

}

@Override

public boolean onTouchEvent(MotionEvent event) {

final int action = event.getAction();

acquireVelocityTracker(event);

final VelocityTracker verTracker = mVelocityTracker;

switch (action) {

case MotionEvent.ACTION_DOWN:

//求第一个触点的id， 此时可能有多个触点，但至少一个

mPointerId = event.getPointerId(0);

break;

case MotionEvent.ACTION_MOVE:

//求伪瞬时速度

verTracker.computeCurrentVelocity(1000, mMaxVelocity);

final float velocityX = verTracker.getXVelocity(mPointerId);

final float velocityY = verTracker.getYVelocity(mPointerId);

recodeInfo(velocityX, velocityY);

break;

case MotionEvent.ACTION_UP:

releaseVelocityTracker();

break;

case MotionEvent.ACTION_CANCEL:

releaseVelocityTracker();

break;

default:

break;

}

return super.onTouchEvent(event);

}

/**

*

* @param event 向VelocityTracker添加MotionEvent

*

* @see android.view.VelocityTracker#obtain()

* @see android.view.VelocityTracker#addMovement(MotionEvent)

*/

private void acquireVelocityTracker(final MotionEvent event) {

if(null == mVelocityTracker) {

mVelocityTracker = VelocityTracker.obtain();

}

mVelocityTracker.addMovement(event);

}

/**

     * 释放VelocityTracker

     *

     * @see android.view.VelocityTracker#clear()

     * @see android.view.VelocityTracker#recycle()

     */

private void releaseVelocityTracker() {

if(null != mVelocityTracker) {

mVelocityTracker.clear();

mVelocityTracker.recycle();

mVelocityTracker = null;

}

}

private static final String sFormatStr = "velocityX=%f\nvelocityY=%f";

/**

* 记录当前速度

*

* @param velocityX x轴速度

* @param velocityY y轴速度

*/

private void recodeInfo(final float velocityX, final float velocityY) {

final String info = String.format(sFormatStr, velocityX, velocityY);

mInfo.setText(info);

}

}