安卓VelocityTracker使用小例子

安卓VelocityTracker使用小例子

定义:

VelocityTracker是位于android.view.VelocityTracker的一个工具类，用于监听滑动的速度，顾名思义Velocity(速度)Tracker(追踪者)

常用api

    /**     * Retrieve a new VelocityTracker object to watch the velocity of a     * motion.  Be sure to call {@link #recycle} when done.  You should     * generally only maintain an active object while tracking a movement,     * so that the VelocityTracker can be re-used elsewhere.     *     * @return Returns a new VelocityTracker.     */    static public VelocityTracker obtain() {        VelocityTracker instance = sPool.acquire();        return (instance != null) ? instance : new VelocityTracker(null);    }

获取一个实例 VelocityTracker v= VelocityTracker.obtain();

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已经获得初始化的VelocityTracker实例，那么就要和事件中的android.view.MotionEvent做关联，调用如下函数

    /**     * Add a user's movement to the tracker.  You should call this for the     * initial {@link MotionEvent#ACTION_DOWN}, the following     * {@link MotionEvent#ACTION_MOVE} events that you receive, and the     * final {@link MotionEvent#ACTION_UP}.  You can, however, call this     * for whichever events you desire.     *      * @param event The MotionEvent you received and would like to track.     */    public void addMovement(MotionEvent event) {        if (event == null) {            throw new IllegalArgumentException("event must not be null");        }        nativeAddMovement(mPtr, event);    }

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关联完毕后，就应该获取了，这个时候就可以先设置使用
public void computeCurrentVelocity(int units) ;
int 参数指的就是监测时间

    /**     * Equivalent to invoking {@link #computeCurrentVelocity(int, float)} with a maximum     * velocity of Float.MAX_VALUE.     *      * @see #computeCurrentVelocity(int, float)      */    public void computeCurrentVelocity(int units) {        nativeComputeCurrentVelocity(mPtr, units, Float.MAX_VALUE);    }

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所有的准备工作都可以了，然后就能调用
getXVelocity(),getYVelocity()来获取速度了
函数原型,其他同理

    /**     * Retrieve the last computed X velocity.  You must first call     * {@link #computeCurrentVelocity(int)} before calling this function.     *      * @return The previously computed X velocity.     */    public float getXVelocity() {        return nativeGetXVelocity(mPtr, ACTIVE_POINTER_ID);    }

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最后一步就是记得回收，调用recycle()

     * Return a VelocityTracker object back to be re-used by others.  You must     * not touch the object after calling this function.     */    public void recycle() {        if (mStrategy == null) {            clear();            sPool.release(this);        }    }

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简单示例:

VelocityTracker mVelocityTracker = VelocityTracker.obtain();mVelocityTracker.addMovement(/*要监听的MotionEvent*/);mVelocityTracker.computeCurrentVelocity(1000);//设置时间mVelocityTracker.getXVelocity());//获取x轴1000毫秒单位速度mVelocityTracker.recycle();//使用完毕回收资源，比如一个onTouch事件中开始是obtain获取实例//,那么，在onTouch事件结束的时候就需要回收(recycle())mVelocityTracker = null;

注意事项:
有时候如果没有通过obtain()获取就使用的话，会导致错误
必须先调用computeCurrentVelocity(毫秒时间)，才能用get系列方法来