android VelocityTracker的简单用法

VelocityTracker这个类主要是一个滑动跟踪器，常在touch event里面使用，比较简单，在这里，我们只需要熟练它的几个方法就行了。
直接来看源码方法，源码不多：

 /** * 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() {
        return sPool.acquire();
    }

我们看注释知道，这是当我需要实例化VelocityTracker对象的时候。调用这个方法，并且使用完成后要通过recycle（）方法回收。我们应该保证只有一个VelocityTracker实例，所以VelocityTracker是可以重用的。

    /** * 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) {
            sPool.release(this);
        }
    }

使用完后回收方法。

 /** * Reset the velocity tracker back to its initial state. */
    public void clear() {
        nativeClear(mPtr);
    }

重置velocity tracker初始化

 /** * 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);
    }

初始化的时候调用传递touch事件给VelocityTracker实例。

    /**
     * Compute the current velocity based on the points that have been
     * collected.  Only call this when you actually want to retrieve velocity
     * information, as it is relatively expensive.  You can then retrieve
     * the velocity with {@link #getXVelocity()} and
     * {@link #getYVelocity()}.
     * 
     * @param units The units you would like the velocity in.  A value of 1
     * provides pixels per millisecond, 1000 provides pixels per second, etc.
     * @param maxVelocity The maximum velocity that can be computed by this method.
     * This value must be declared in the same unit as the units parameter. This value
     * must be positive.
     */
    public void computeCurrentVelocity(int units, float maxVelocity) {
        nativeComputeCurrentVelocity(mPtr, units, maxVelocity);
    }

计算当前速度, 其中units是单位表示， 1代表px/毫秒, 1000代表px/秒,

public float getXVelocity();
public float getYVelocity();
public float getXVelocity(int id);
public float getYVelocity(int id);

//经过一次computeCurrentVelocity后你就可以用一下上面方法获取此次计算的值
//id是touch event触摸点的ID, 来为多点触控标识，有这个标识在计算时可以忽略
//其他触点干扰，当然干扰肯定是有的

下面我们通过一个demo来使用下这写方法：

public class VelocityTrackerTest extends Activity{
    private VelocityTracker mVelocityTracker;
     private int mMaxVelocity;
    private TextView mTextInfo;
     private int mPointerId; 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        mTextInfo = new TextView(this);
        LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT);
        mTextInfo.setLayoutParams(params);
        mTextInfo.setTextColor(Color.BLACK);
        mTextInfo.setBackgroundColor(Color.WHITE);
        setContentView(mTextInfo);
        mMaxVelocity = ViewConfiguration.get(this).getMaximumFlingVelocity();
    }
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        // TODO Auto-generated method stub
        acquireVelocityTracker(event);
         final VelocityTracker verTracker = mVelocityTracker; 
        switch(event.getAction()){
        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; 
        }
        return super.onTouchEvent(event);
    }
    private void acquireVelocityTracker(final MotionEvent event){
         if(null == mVelocityTracker) { 
             mVelocityTracker = VelocityTracker.obtain(); 
         }
         mVelocityTracker.addMovement(event);  
    }
     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);  
            mTextInfo.setText(info);  
        }  
        /** * 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;  
            }  
        }  
}

这样我们就可以求出move过程中的伪瞬时速度，这样在做很多控件的时候都是可以用到的，比如系统Launcher的分页，ScrollView滑动等，可根据此时的速度来计算ACTION_UP后的减速运动等。实现一些非常棒的效果。