void printSamples(MotionEvent ev) {
   //获取MotionEvent中捆绑的坐标点
   final int historySize = ev.getHistorySize();
   //获取手指数目
   final int pointerCount = ev.getPointerCount();
   for (int h = 0; h < historySize; h++) {
     System.out.printf("At time %d:", ev.getHistoricalEventTime(h));
     for (int p = 0; p < pointerCount; p++) {
       System.out.printf(" pointer %d: (%f,%f)",
         ev.getPointerId(p), ev.getHistoricalX(p, h), ev.getHistoricalY(p, h));
     }
   }
 }

public class MainActivity extends Activity {
...
// This example shows an Activity, but you would use the same approach if
// you were subclassing a View.
@Override
public boolean onTouchEvent(MotionEvent event){

  int action = MotionEventCompat.getActionMasked(event);

  switch(action) {
    case (MotionEvent.ACTION_DOWN) :
      Log.d(DEBUG_TAG,"Action was DOWN");
      return true;
    case (MotionEvent.ACTION_MOVE) :
      Log.d(DEBUG_TAG,"Action was MOVE");
      return true;
    case (MotionEvent.ACTION_UP) :
      Log.d(DEBUG_TAG,"Action was UP");
      return true;
    case (MotionEvent.ACTION_CANCEL) :
      Log.d(DEBUG_TAG,"Action was CANCEL");
      return true;
    case (MotionEvent.ACTION_OUTSIDE) :
      Log.d(DEBUG_TAG,"Movement occurred outside bounds " +
          "of current screen element");
      return true;
    default :
    //当前View不处理事件，交由上层处理。
      return super.onTouchEvent(event);
  }
}

private GestureDetectorCompat mDetector;
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mDetector = new GestureDetectorCompat(this,this);
    mDetector.setOnDoubleTapListener(this);
  }
  @Override
  public boolean onTouchEvent(MotionEvent event){
    this.mDetector.onTouchEvent(event);
    // Be sure to call the superclass implementation
    return super.onTouchEvent(event);
  }

    switch(action) {
      case MotionEvent.ACTION_DOWN:
        if(mVelocityTracker == null) {         
          mVelocityTracker = VelocityTracker.obtain();
        }
        else {
          mVelocityTracker.clear();
        }
        mVelocityTracker.addMovement(event);
        break;
      case MotionEvent.ACTION_MOVE:
        mVelocityTracker.addMovement(event);
        mVelocityTracker.computeCurrentVelocity(1000);
        Log.d("", "X velocity: " +
            VelocityTrackerCompat.getXVelocity(mVelocityTracker,
            pointerId));
        Log.d("", "Y velocity: " +
            VelocityTrackerCompat.getYVelocity(mVelocityTracker,
            pointerId));
        break;
      case MotionEvent.ACTION_UP:
      case MotionEvent.ACTION_CANCEL:
        mVelocityTracker.recycle();
        break;