android中的手势探测

我们常说的手势包括：单击、长按、双击、滑动、缩放, android提供了几个手势探测类：GestureDetector（点击类探测），ScaleGestureDetector（缩放探测），

如何用这几种探测类来捕捉用户的手势呢？

1. 首先构建一个GestureDetector对象，创建GestureDetector对象时会传入一个回调对象（实现GestureDetector.OnGestureListener）
2. view有一个虚拟函数onTouchEvent(MotionEvent)， 只要重载这个函数，并将MotionEvent对象丢给GestureDetector对象就可以了
3. GestureDetector类中会自己判断MotionEvent的事件类型，并回调
4. 问题来了，怎么回调业务的接口?
5. 回到1中，传入的回调对象其实是我们需要实现的，并继承GestureDetector中的接口来实现手势的回调
6. 如果是缩放，将GestureDetector换成ScaleGestureDetector对象即可

来看一下5中，继承GestureDetector/ScaleGestureDetecto的接口指的哪些？

GestureDetector.OnGestureListener：单击相关事件判断

Public Methods
abstract boolean	onDown( MotionEvent e) Notified when a tap occurs with the down  MotionEvent that triggered it.
abstract boolean	onFling( MotionEvent e1,  MotionEvent e2, float velocityX, float velocityY) Notified of a fling event when it occurs with the initial on down  MotionEvent and the matching up  MotionEvent.
abstract void	onLongPress( MotionEvent e) Notified when a long press occurs with the initial on down  MotionEvent that trigged it.
abstract boolean	onScroll( MotionEvent e1,  MotionEvent e2, float distanceX, float distanceY) Notified when a scroll occurs with the initial on down  MotionEvent and the current move  MotionEvent.
abstract void	onShowPress( MotionEvent e) The user has performed a down  MotionEvent and not performed a move or up yet.
abstract boolean	onSingleTapUp( MotionEvent e) Notified when a tap occurs with the up  MotionEvent that triggered it.

GestureDetector.OnDoubleTapListener:双击事件判断

Public Methods
abstract boolean	onDoubleTap( MotionEvent e) Notified when a double-tap occurs.
abstract boolean	onDoubleTapEvent( MotionEvent e) Notified when an event within a double-tap gesture occurs, including the down, move, and up events.
abstract boolean	onSingleTapConfirmed( MotionEvent e) Notified when a single-tap occurs.

ScaleGestureDetector.OnScaleGestureListener：缩放事件判断

Public Methods
abstract boolean	onScale( ScaleGestureDetector detector) Responds to scaling events for a gesture in progress.
abstract boolean	onScaleBegin( ScaleGestureDetector detector) Responds to the beginning of a scaling gesture.
abstract void	onScaleEnd( ScaleGestureDetector detector) Responds to the end of a scale gesture.

注意这些都是接口， 如果实现这些接口则所有函数都要实现，比较麻烦。幸运的是android系统已经为你考虑好了，你只需要继承这几个类，重载你需要实现的函数就可以了，如下：

GestureDetector.SimpleOnGestureListener（实现了GestureDetector.OnGestureListener&GestureDetector.OnDoubleTapListener两个接口）

ScaleGestureDetector.SimpleOnScaleGestureListener（实现了ScaleGestureDetector.OnScaleGestureListener）

分析一下android源码中SimpleOnGestureListener和SimpleOnScaleGestureListener的实现就可以知道，它们其实只是实现的空函数，只是为了方便你继承的。

回过头来，6中的具体做法可以描述为：继承GestureDetector.SimpleOnGestureListener、ScaleGestureDetector.SimpleOnScaleGestureListener中的你关注的函数就可以了。

继续分析android的源码： 最重要的关于手势的分析是函数：GestureDetector::onTouchEvent/ScaleGestureDetector::onTouchEvent, 它具体了分析手势然后回调相关的接口，理论上你完全可以自己写一套类似的机制来实现手势的分析。

示例代码：

import android.content.Context;
import android.util.Log;
import android.view.GestureDetector;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;


public class MyView extends View {

GestureDetector mGesture = null; 


public MyView(Context context)
{
super(context);

mGesture = new GestureDetector(this.getContext(), new MyGestureDetector());


                Log.d("MyGestureDetector", "MyView()");  
}


protected void onDraw(Canvas canvas)
{
android.util.Log.d("MyGestureDetector", "onDraw");
}

@Override
public boolean onTouchEvent(MotionEvent event) {
        Log.d("MyGestureDetector", "onTouch");  


   // TODO Auto-generated method stub  
   return mGesture.onTouchEvent(event);  
}

class MyGestureDetector implements GestureDetector.OnGestureListener, GestureDetector.OnDoubleTapListener
{


@Override
public boolean onDown(MotionEvent e) {
// TODO Auto-generated method stub
return false;
}


@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,
float velocityY) {
// TODO Auto-generated method stub
return false;
}


@Override
public void onLongPress(MotionEvent e) {
// TODO Auto-generated method stub

}


@Override
public boolean onScroll(MotionEvent e1, MotionEvent e2,
float distanceX, float distanceY) {
// TODO Auto-generated method stub
return false;
}


@Override
public void onShowPress(MotionEvent e) {
// TODO Auto-generated method stub

}


@Override
public boolean onSingleTapUp(MotionEvent e) {
// TODO Auto-generated method stub
return false;
}


@Override
public boolean onDoubleTap(MotionEvent e) {
// TODO Auto-generated method stub
return false;
}


@Override
public boolean onDoubleTapEvent(MotionEvent e) {
// TODO Auto-generated method stub
return false;
}


@Override
public boolean onSingleTapConfirmed(MotionEvent e) {
// TODO Auto-generated method stub
return false;
}

}

/*或者直接这种继承也为常见
class MyGestureDetector extends GestureDetector.SimpleOnGestureListener
{
        @Override  
        public void onLongPress(MotionEvent e)  
        {  
            Log.d("MyGestureDetector", "onLongPress");  
            super.onLongPress(e);  
        }  
}
*/

}