Android VelocityTracker简介

jopen 11年前
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()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
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