VelocityTracker的简单使用

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。

    /** 
     * Helper for tracking the velocity of touch events, for implementing 
     * flinging and other such gestures.  Use {@link #obtain} to retrieve a 
     * new instance of the class when you are going to begin tracking, put 
     * the motion events you receive into it with {@link #addMovement(MotionEvent)}, 
     * and when you want to determine the velocity call 
     * {@link #computeCurrentVelocity(int)} and then {@link #getXVelocity()} 
     * and {@link #getXVelocity()}. 
     */  

简单翻译下：帮助你追踪一个touch事件（flinging事件和其他手势事件）的速率。当你要跟踪一个touch事件的时候，使用obtain()方法得到这个类的实例，然后 用addMovement(MotionEvent)函数将你接受到的motion event加入到VelocityTracker类实例中。当你使用到速率时，使用computeCurrentVelocity(int)初始化速率的单位，并获得当前的事件的速率，然后使用getXVelocity() 或getXVelocity()获得横向和竖向的速率。

Velocity Tracker主要包括的函数如下：

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)

从上面的介绍中，我们可以很使用VelocityTracker类去追踪一个移动事件的速率。VelocityTracker的详细使用不走如下：

1.首先获得VelocityTracker的实例

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
         } 

   2.把事件event,将事件加入到VelocityTracker类实例中

        mVelocityTracker.addMovement(event);

3. 判断当事件MotionEvent.ACTION_MOVE的时候，调用下面的方法  ，并获取到x，y方向的速度。

                final float velocityX = verTracker.getXVelocity(mPoniterId);
                final float velocityY = verTracker.getYVelocity(mPoniterId);
                String info = String.format(sFormatStr, velocityX, velocityY);
                tv_info.setText(info);

4. 判断当事件MotionEvent.ACTION_UP或者MotionEvent.ACTION_CANCEL的时候释放Velocity Tracker

        if (null != mVelocityTracker) {
            mVelocityTracker.clear();
            mVelocityTracker.recycle();
            mVelocityTracker = null;
        }

到此Velocity Tracker的基本使用就介绍完毕了，最后附上Demo的源码，进攻参考。

package cn.chinaiptn.velocitytracker;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.ViewConfiguration;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    private VelocityTracker mVelocityTracker;
    private int mMaxVelocity;
    private int mPoniterId;
    private TextView tv_info;

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

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv_info = (TextView) findViewById(R.id.tv_info);
        mMaxVelocity = ViewConfiguration.get(MainActivity.this).getMaximumFlingVelocity();

    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        final int action = event.getAction();
        initVelocityTracker(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
                mPoniterId = event.getPointerId(0);
                break;

            case MotionEvent.ACTION_MOVE:
                //求伪瞬时速度
                verTracker.computeCurrentVelocity(1000, mMaxVelocity);
                final float velocityX = verTracker.getXVelocity(mPoniterId);
                final float velocityY = verTracker.getYVelocity(mPoniterId);
                String info = String.format(sFormatStr, velocityX, velocityY);
                tv_info.setText(info);
                break;

            case MotionEvent.ACTION_UP:
                releaseVelocityTracker();
                break;

            case MotionEvent.ACTION_CANCEL:
                releaseVelocityTracker();
                break;

            default:
                break;
        }
        return super.onTouchEvent(event);
    }

    /**
     * 向mVelocityTracker添加MotionEvent
     *
     * @param event
     */
    private void initVelocityTracker(final MotionEvent event) {
        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(event);
    }

    /**
     * 释放VelocityTracker
     */
    private void releaseVelocityTracker() {
        if (null != mVelocityTracker) {
            mVelocityTracker.clear();
            mVelocityTracker.recycle();
            mVelocityTracker = null;
        }
    }

}

本文主要参考了 http://blog.csdn.net/lonelyroamer/article/details/7560598

下篇博文将从源码的角度去熟悉Velocity Tracker的工作原理。