【摘】捕获触摸事件
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重写onTouchEvent()回调函数
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public class MainActivity extends Activity {@Overridepublic 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 : return super.onTouchEvent(event); }}?
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然后，我们可以对这些事件做些自己的处理，以判断某个手势是否出现了。这种是针对自定义手势，我们所需要进行的处理。然而，如果我们的app仅仅需要一些常见的手势，如双击，长按，快速滑动（fling）等，那么我们可以使用GestureDetector类来完成。
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public class MainActivity extends Activity implements GestureDetector.OnGestureListener, GestureDetector.OnDoubleTapListener{ private static final String DEBUG_TAG = "Gestures"; private GestureDetectorCompat mDetector; // Called when the activity is first created. @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // Instantiate the gesture detector with the // application context and an implementation of // GestureDetector.OnGestureListener mDetector = new GestureDetectorCompat(this,this); // Set the gesture detector as the double tap // listener. mDetector.setOnDoubleTapListener(this); } @Override public boolean onTouchEvent(MotionEvent event){ this.mDetector.onTouchEvent(event); // Be sure to call the superclass implementation return super.onTouchEvent(event); } @Override public boolean onDown(MotionEvent event) { Log.d(DEBUG_TAG,"onDown: " + event.toString()); return true; } @Override public boolean onFling(MotionEvent event1, MotionEvent event2, float velocityX, float velocityY) { Log.d(DEBUG_TAG, "onFling: " + event1.toString()+event2.toString()); return true; } @Override public void onLongPress(MotionEvent event) { Log.d(DEBUG_TAG, "onLongPress: " + event.toString()); } @Override public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) { Log.d(DEBUG_TAG, "onScroll: " + e1.toString()+e2.toString()); return true; } @Override public void onShowPress(MotionEvent event) { Log.d(DEBUG_TAG, "onShowPress: " + event.toString()); } @Override public boolean onSingleTapUp(MotionEvent event) { Log.d(DEBUG_TAG, "onSingleTapUp: " + event.toString()); return true; } @Override public boolean onDoubleTap(MotionEvent event) { Log.d(DEBUG_TAG, "onDoubleTap: " + event.toString()); return true; } @Override public boolean onDoubleTapEvent(MotionEvent event) { Log.d(DEBUG_TAG, "onDoubleTapEvent: " + event.toString()); return true; } @Override public boolean onSingleTapConfirmed(MotionEvent event) { Log.d(DEBUG_TAG, "onSingleTapConfirmed: " + event.toString()); return true; }}?
如果我们只想处理几种手势，那么可以选择继承?GestureDetector.SimpleOnGestureListener?类，而不是实现GestureDetector.OnGestureListener?接口。
GestureDetector.SimpleOnGestureListener 类实现了所有的?on<TouchEvent>?型函数，其中，这些函数都返回false。因此，我们可以仅仅重写我们需要的函数。比如，下面的代码段中，创建了一个继承自GestureDetector.SimpleOnGestureListener?的类，并重写了 onFling() 和 onDown() 函数。
无论我们是否使用GestureDetector.OnGestureListener类，最好都实现 onDown() 函数并且返回?true。这是因为所有的手势都是由 onDown() 消息开始的。如果让 onDown() 函数返回?false，就像GestureDetector.SimpleOnGestureListener类中默认实现的那样，系统会假定我们想忽略剩余的手势，GestureDetector.OnGestureListener中的其他函数也就永远不会被调用。这可能会导致我们的app出现意想不到的问题。仅仅当我们真的想忽略全部手势时，我们才应该让 onDown() 函数返回?false。
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public class MainActivity extends Activity { private GestureDetectorCompat mDetector; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mDetector = new GestureDetectorCompat(this, new MyGestureListener()); } @Override public boolean onTouchEvent(MotionEvent event){ this.mDetector.onTouchEvent(event); return super.onTouchEvent(event); } class MyGestureListener extends GestureDetector.SimpleOnGestureListener { private static final String DEBUG_TAG = "Gestures"; @Override public boolean onDown(MotionEvent event) { Log.d(DEBUG_TAG,"onDown: " + event.toString()); return true; } @Override public boolean onFling(MotionEvent event1, MotionEvent event2, float velocityX, float velocityY) { Log.d(DEBUG_TAG, "onFling: " + event1.toString()+event2.toString()); return true; } }}?
根据应用的需求，有多种追踪手势移动的方式可以选择。比如：
- 追踪手指的起始和终止位置（比如，把屏幕上的对象从A点移动到B点）
- 根据x、y轴坐标，追踪手指移动的方向。
- 追踪历史状态。我们可以通过调用MotionEvent的getHistorySize()方法，来获得一个手势的历史尺寸。我们可以通过移动事件的
getHistorical<Value>系列函数，来获得事件之前的位置、尺寸、时间以及按压力(pressures)。当我们需要绘制用户手指痕迹时，历史状态非常有用，比如触摸绘图。查看MotionEvent来了解更多细节。 - 追踪手指在触摸屏上滑过的速度。Android提供了VelocityTracker类以及Support Library中的VelocityTrackerCompat类。VelocityTracker类可以帮助我们追踪触摸事件中的速度因素。
public class MainActivity extends Activity { private static final String DEBUG_TAG = "Velocity"; ... private VelocityTracker mVelocityTracker = null; @Override public boolean onTouchEvent(MotionEvent event) { int index = event.getActionIndex(); int action = event.getActionMasked(); int pointerId = event.getPointerId(index); switch(action) { case MotionEvent.ACTION_DOWN: if(mVelocityTracker == null) { // Retrieve a new VelocityTracker object to watch the velocity of a motion. mVelocityTracker = VelocityTracker.obtain(); } else { // Reset the velocity tracker back to its initial state. mVelocityTracker.clear(); } // Add a user's movement to the tracker. mVelocityTracker.addMovement(event); break; case MotionEvent.ACTION_MOVE: mVelocityTracker.addMovement(event); // When you want to determine the velocity, call // computeCurrentVelocity(). Then call getXVelocity() // and getYVelocity() to retrieve the velocity for each pointer ID. mVelocityTracker.computeCurrentVelocity(1000); // Log velocity of pixels per second // Best practice to use VelocityTrackerCompat where possible. 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: // Return a VelocityTracker object back to be re-used by others. mVelocityTracker.recycle(); break; } return true; }}?
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追踪多点
当多个手指同时触摸屏幕时，系统会产生如下的触摸事件：
- ACTION_DOWN?- 针对触摸屏幕的第一个点。此事件是手势的开端。第一触摸点的数据在MotionEvent中的索引总是0。
- ACTION_POINTER_DOWN?- 针对第一点后，出现在屏幕上额外的点。这个点的数据在MotionEvent中的索引，可以通过getActionIndex()获得。
- ACTION_MOVE?- 在按下手势期间发生变化。
- ACTION_POINTER_UP?- 当非主要点（non-primary pointer）离开屏幕时，发送此事件。
- ACTION_UP?- 当最后一点离开屏幕时发送此事件。
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拖拽一个对象
对于触摸手势来说，一个很常见的操作是在屏幕上拖拽一个对象。接下来的代码段让用户可以拖拽屏幕上的图片。需要注意以下几点：
- 拖拽操作时，即使有额外的手指放置到屏幕上了，app也必须保持对最初的点（手指）的追踪。比如，想象在拖拽图片时，用户放置了第二根手指在屏幕上，并且抬起了第一根手指。如果我们的app只是单独地追踪每个点，它会把第二个点当做默认的点，并且把图片移到该点的位置。
- 为了防止这种情况发生，我们的app需要区分初始点以及随后任意的触摸点。要做到这一点，它需要追踪处理多触摸手势章节中提到过的?ACTION_POINTER_DOWN?和?ACTION_POINTER_UP?事件。每当第二根手指按下或拿起时，ACTION_POINTER_DOWN?和?ACTION_POINTER_UP?事件就会传递给
onTouchEvent()回调函数。 - 当ACTION_POINTER_UP事件发生时，示例程序会移除对该点的索引值的引用，确保操作中的点的ID(the active pointer ID)不会引用已经不在触摸屏上的触摸点。这种情况下，app会选择另一个触摸点来作为操作中(active)的点，并保存它当前的x、y值。由于在ACTION_MOVE事件时，这个保存的位置会被用来计算屏幕上的对象将要移动的距离，所以app会始终根据正确的触摸点来计算移动的距离。
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// The ‘active pointer’ is the one currently moving our object.private int mActivePointerId = INVALID_POINTER_ID;@Overridepublic boolean onTouchEvent(MotionEvent ev) { // Let the ScaleGestureDetector inspect all events. mScaleDetector.onTouchEvent(ev); final int action = MotionEventCompat.getActionMasked(ev); switch (action) { case MotionEvent.ACTION_DOWN: { final int pointerIndex = MotionEventCompat.getActionIndex(ev); final float x = MotionEventCompat.getX(ev, pointerIndex); final float y = MotionEventCompat.getY(ev, pointerIndex); // Remember where we started (for dragging) mLastTouchX = x; mLastTouchY = y; // Save the ID of this pointer (for dragging) mActivePointerId = MotionEventCompat.getPointerId(ev, 0); break; } case MotionEvent.ACTION_MOVE: { // Find the index of the active pointer and fetch its position final int pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId); final float x = MotionEventCompat.getX(ev, pointerIndex); final float y = MotionEventCompat.getY(ev, pointerIndex); // Calculate the distance moved final float dx = x - mLastTouchX; final float dy = y - mLastTouchY; mPosX += dx; mPosY += dy; invalidate(); // Remember this touch position for the next move event mLastTouchX = x; mLastTouchY = y; break; } case MotionEvent.ACTION_UP: { mActivePointerId = INVALID_POINTER_ID; break; } case MotionEvent.ACTION_CANCEL: { mActivePointerId = INVALID_POINTER_ID; break; } case MotionEvent.ACTION_POINTER_UP: { final int pointerIndex = MotionEventCompat.getActionIndex(ev); final int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex); if (pointerId == mActivePointerId) { // This was our active pointer going up. Choose a new // active pointer and adjust accordingly. final int newPointerIndex = pointerIndex == 0 ? 1 : 0; mLastTouchX = MotionEventCompat.getX(ev, newPointerIndex); mLastTouchY = MotionEventCompat.getY(ev, newPointerIndex); mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex); } break; } } return true;}?