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/**
 * 速率追踪类的使用demo：
 * 1、实例化，在需要的地方通过obtain方法获取
 * 2、在ontouch时间把event设置给追踪类，并设置computeCurrentVelocity，一般是给1000，这里也可以设置
 * 3、在down动作下捕捉触点id，然后在move动作就可以计算实时的速率了
 * 4、可以在cancel或者up进行追踪类的释放(有误)：在这里释放再次再次滑动就会报我的追踪类空了，所以我们在activity的destroy的时候去释放
 */
public class MainActivity extends Activity {
    private VelocityTracker mVelocityTracker;
    private int id;
    private TextView mInfo;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mInfo = new TextView(this);
        mInfo.setLines(4);
        mInfo.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.FILL_PARENT, ViewGroup.LayoutParams.FILL_PARENT));
        mInfo.setTextColor(Color.BLACK);
        setContentView(mInfo);
        mVelocityTracker = VelocityTracker.obtain();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mVelocityTracker.addMovement(event);
        int action = event.getAction();
        switch (action) {
            case MotionEvent.ACTION_DOWN:
                //获取触点的ID，因为可能有多个。0代表第一个
                id = event.getPointerId(0);
                break;
            case MotionEvent.ACTION_MOVE:
                mVelocityTracker.computeCurrentVelocity(1000);
                float vX = mVelocityTracker.getXVelocity(id);
                float vY = mVelocityTracker.getYVelocity(id);
                mInfo.setText("顺时速度x   " + vX + "顺时速度y   " + vY);
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                //releaseVelocityTracker();
                break;
        }

        return super.onTouchEvent(event);
    }

    //释放VelocityTracker

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