【笔记】android捕获触摸事件

【摘】捕获触摸事件

重写onTouchEvent()回调函数

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);        }}

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。

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 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。

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;    }}

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。

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。

// 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;}

追踪多点

拖拽一个对象

更多相关文章

随机推荐