一、一些常量
常见的动作常量：
public static final int ACTION_DOWN = 0;单点触摸动作
public static final int ACTION_UP = 1;单点触摸离开动作
public static final int ACTION_MOVE = 2;触摸点移动动作
public static final int ACTION_CANCEL = 3;触摸动作取消
public static final int ACTION_OUTSIDE = 4;触摸动作超出边界
public static final int ACTION_POINTER_DOWN = 5;多点触摸动作
public static final int ACTION_POINTER_UP = 6;多点离开动作
以下是一些非touch事件
public static final int ACTION_HOVER_MOVE = 7;
public static final int ACTION_SCROLL = 8;
public static final int ACTION_HOVER_ENTER = 9;
public static final int ACTION_HOVER_EXIT = 10;
掩码常量
ACTION_MASK = 0X000000ff
动作掩码
ACTION_POINTER_INDEX_MASK = 0X0000ff00
触摸点索引掩码
ACTION_POINTER_INDEX_SHIFT = 8 获取触摸点索引需要移动的位数

二、相关方法
getAction()方法返回的是int类型，用到的只有低16位，其中：低八位是动作的类型，高8位是触摸点索引值的表示（单点为0，双点为1）
获得动作类型： int action = event.getAction() & ACTION_MASK 或者使用 getActionMasked()
获得触摸点索引类型： int pointerIndex = (event.getAction() & ACTION_POINTER_INDEX_MASK ) >> ACTION_POINTER_INDEX_SHIFT
或者使用 getActionIndex()
为什么要有索引信息？
有了索引信息，我们可以在onTOuchEvent事件中判断传进来的MotionEvent对象对应的是单点信息还是多点信息。
下面的代码段能使用户在屏幕上拖动一个对象。它记录了初始点的位置，计算点移动的距离，并将对象移动到新的位置。它正确的处理了这种情况：当第一个手指把控件拖到一个位置，然后按下第二个手指，且第二个手指与同一个控件上。当用户抬起第一个手指时，控件不会跑到第二个手指的位置同时第二个手指可以继续拖动控件。

private int mActivePointerId = INVALID_POINTER_ID;  

@Override  
public 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);  

        // Only move if the ScaleGestureDetector isn't processing a gesture.  
        if (!mScaleDetector.isInProgress()) {  
            // 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;  
}