[Android分享] 使用触摸手势（五）

来源：互联网发布：表格文字识别软件编辑：程序博客网时间：2024/05/20 17:40

原文链接：http://developer.android.com/training/gestures/scale.html

拖动和缩放

拖动一个对象

触屏的常见操作是在屏幕上拖动一个对象。

1.在一个拖动操作中，应用程序需要保持对最初始点的跟踪，即使有其他的手指放置在屏幕上。例如，想象一下，当用户在屏幕上拖动图像，用户将第二个手指放在了屏幕上，并将第一个手指抬起。如果你的应用程序仅仅追踪单个点，那么它将会把第二个点作为目标位置并将图像移向那个位置。
2.为了阻止这种情况的发生，你的应用程序需要区分初始的点和后续的点。当后续的点按下或抬起时，ACTION_POINTER_DOWN和ACTION_POINTER_UP将会被传递到onTouchEvent（）回调函数。
3.在ACTION_POINTER_UP的情况下，示例提取该触摸点的index，并确定该点ID有没有表示一个不再触摸屏幕的点。如果是这样，应用程序会选取一个不同的点进行激，并保存它的x和y坐标。由于这个保存的位置被用到ACTION_MOVE情况下来计算移动对象的距离，所以应用程序将使用正确的点计算移动的距离和位置。

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。它正确的处理了附加点的可能性。

你应该始终使用getActionMasked方法来获取一个MotionEvent的action。
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代码片段，双击复制
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// The ‘active pointer’ is the one currently moving our object.
privateint mActivePointerId = INVALID_POINTER_ID;

@Override
publicboolean onTouchEvent(MotionEvent ev) {
   // Let the ScaleGestureDetector inspect all events.
   mScaleDetector.onTouchEvent(ev);

   finalint action = MotionEventCompat.getActionMasked(ev);

   switch(action) {
   caseMotionEvent.ACTION_DOWN: {
        finalint pointerIndex = MotionEventCompat.getActionIndex(ev);
        finalfloat x = MotionEventCompat.getX(ev, pointerIndex);
        finalfloat 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;
   }

   caseMotionEvent.ACTION_MOVE: {
        // Find the index of the active pointer and fetch its position
        finalint pointerIndex =
              MotionEventCompat.findPointerIndex(ev, mActivePointerId);

        finalfloat x = MotionEventCompat.getX(ev, pointerIndex);
        finalfloat y = MotionEventCompat.getY(ev, pointerIndex);

        // Only move if the ScaleGestureDetector isn't processing a gesture.
        if(!mScaleDetector.isInProgress()) {
           // Calculate the distance moved
           finalfloat dx = x - mLastTouchX;
           finalfloat dy = y - mLastTouchY;

           mPosX += dx;
           mPosY += dy;

           invalidate();
        }
        // Remember this touch position for the next move event
        mLastTouchX = x;
        mLastTouchY = y;

        break;
   }

   caseMotionEvent.ACTION_UP: {
        mActivePointerId = INVALID_POINTER_ID;
        break;
   }

   caseMotionEvent.ACTION_CANCEL: {
        mActivePointerId = INVALID_POINTER_ID;
        break;
   }

   caseMotionEvent.ACTION_POINTER_UP: {

        finalint pointerIndex = MotionEventCompat.getActionIndex(ev);
        finalint pointerId = MotionEventCompat.getPointerId(ev, pointerIndex);

        if(pointerId == mActivePointerId) {
           // This was our active pointer going up. Choose a new
           // active pointer and adjust accordingly.
           finalint newPointerIndex = pointerIndex == 0? 1: 0;
           mLastTouchX = MotionEventCompat.getX(ev, newPointerIndex);
           mLastTouchY = MotionEventCompat.getY(ev, newPointerIndex);
           mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex);
        }
        break;
   }
   }
   returntrue;
}

使用触摸执行缩放

GestureDetector能够帮助你监听常见的手势。对于缩放，Android提供了ScaleGestureDetector. GestureDetector，ScaleGestureDetector能被用来检测更多的手势。

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代码片段，双击复制
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privateScaleGestureDetector mScaleDetector;
privatefloat mScaleFactor = 1.f;

publicMyCustomView(Context mContext){
   ...
   // View code goes here
   ...
   mScaleDetector = newScaleGestureDetector(context, newScaleListener());
}

@Override
publicboolean onTouchEvent(MotionEvent ev) {
   // Let the ScaleGestureDetector inspect all events.
   mScaleDetector.onTouchEvent(ev);
   returntrue;
}

@Override
publicvoid onDraw(Canvas canvas) {
   super.onDraw(canvas);

   canvas.save();
   canvas.scale(mScaleFactor, mScaleFactor);
   ...
   // onDraw() code goes here
   ...
   canvas.restore();
}

privateclass ScaleListener
        extendsScaleGestureDetector.SimpleOnScaleGestureListener {
   @Override
   publicboolean onScale(ScaleGestureDetector detector) {
        mScaleFactor *= detector.getScaleFactor();

        // Don't let the object get too small or too large.
        mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));

        invalidate();
        returntrue;
   }
}