Android触控基础:MotionEvent
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之前的文章层从Framework层介绍了Android Touch事件即(MotionEvent)的传递机制。本文将详细介绍MotionEvent的一些成员和方法。了解了MotionEvent对开发一些特效如拖动控件或多点缩放控件有很大的作用。同时,掌握MotionEvent类也是学好android触控技术的基础。
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。
- // The ‘active pointer’ is the one currently moving our object.
- 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;
- }
MotionEvent还包含了移动操作中其它历史移动数据以方便处理触控的移动操作.
android sdk对于这个类的描述中就有这么一句:
For efficiency, motion events with ACTION_MOVE may batch together multiple movement samples within a single object.
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