[移动开发]android事件分发基础知识点

1.单个view捕获触摸事件
除了通过覆写onTouchEvent()方法外，还可以通过setOnTouchListener()方法设置OnTouchListener监听器来监听相关的事件。
注意：如果设置的onTouchListener中 ACTION_DOWN 返回了false，那么后续就接收不到 ACTION_MOVE 和 ACTION_UP事件。

2.使用GestureDetectorCompat检测所有支持的手势
代码示例

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 event1, MotionEvent event2, float distanceX,
            float distanceY) {
        Log.d(DEBUG_TAG, "onScroll: " + event1.toString() + event2.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;
    }
}

3.检测所需要的事件
如果只是想检测部分关心的事件而不是所有的事件，则可以扩展GestureDetector.SimpleOnGestureListener类。该类implements了GestureDetector.OnGestureListener接口。
不管用GestureDetector.SimpleOnGestureListener还是GestureDetector.OnGestureListener来检测手势，onDown()方法最好返回true。因为一旦onDown()返回false，那么
系统会认为要忽略接下来的事件，那么其他方法就不会被调用。

4.因为基于手指触摸并不是一种非常精确的交互形式，所以检测触摸事件更多基于动作(movement)，而不是简单的接触(contact)。所以为了区分移动手势(例如swipe)和非移动手势(例如single tap)。Android提出了touch slop的概念。

5.通过getHistorical方法可以获取触摸事件的位置、大小、时间、压力等。在绘制手指触摸路径时该方法非常有用（大概可以绘制那种移动尾部逐渐消失的效果）

6.记录触摸事件移动速度
代码示例

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.
                // Now, on android 5, if the .recycle() is called twice, it will throw the "IllegalStateException" claiming that the the memory has 
                // already been added to the pool.
                // trycatch to fix android 5 only bug
                mVelocityTracker.recycle();
                break;
        }
        return true;
    }
}

7、滚动手势scroller
Scrollers track scroll offsets for you over time, but they don’t automatically apply those positions to your view. It’s your responsibility to get and apply new coordinates at a rate that will make the scrolling animation look smooth.
滚动相关的术语：
当x与y轴上均有滚动时称之为panning。
Dragging,简单的dragging操作通常通过覆写 GestureDetector.OnGestureListener接口中的onScroll()方法实现。
Flinging,即用户拖拽并松开手指后滚动的状态，fling通常通过覆写GestureDetector.OnGestureListener接口中的onFling()方法实现。