Android官方开发文档Training系列课程中文版：手势处理之拖拽或缩放
原文地址：https://developer.android.com/training/gestures/scale.html 这节课主要学习如何使用触摸手势来拖动、放大屏幕上的对象。 拖动对象 如果你的重点在Android 3.0以上的版本，那么你可以使用内置的拖拽事件监听器View.OnDragListener。 触摸手势最常见的操作就是使用它来拖动屏幕上的对象。下面的代码允许用户拖动屏幕上的图像。要注意以下几点： 在拖动操作中，APP会一直保持手指拖动的轨迹，就算是另一只手指触到屏幕也是。举个例子，想象一根手指在拖动着一张图像，这时用户将第二根手指放置到屏幕上，如果APP只是追踪单根手指的轨迹，那么它会将第二根手指作为默认位置，并会将图像移动到这个位置。 为了防止这样的事件发生，APP需要区分第一根手指与其它手指。为此，需要追踪 ACTION_POINTER_DOWN 及 ACTION_POINTER_UP 。ACTION_POINTER_DOWN 及 ACTION_POINTER_UP在第二根手指落下或抬起的时候由onTouchEvent()方法传回。 在ACTION_POINTER_UP的情况下，示例提取了这个事件的索引，并确保当前活动的指针不是那个已经不在屏幕上的指针。如果是那个指针的话，那么APP会选择一个不同的指针使其活动并保存它的X及Y的位置。一旦这个值被保存下来，那么APP将会使用正确指针的数据一直计算剩余移动的距离。 下面的代码允许用户在屏幕上拖动对象。它记录了当前活动指针的初始位置，计算了它所位移的距离，并将对象移动到新的位置上。 这里要注意，代码段使用了getActionMasked()方法。你应该一直使用这个方法来接收MotionEvent对象的活动。与getAction()方法不同，getActionMasked()工作于多点触控模式下。它会返回被执行的掩饰活动，不包括指针的索引比特。 // 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); // 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; } 平移 上面的部分展示了如何在屏幕上拖动对象。另一个通用的场景就是平移了，平移的意思是：用户的拖动动作引起的x及y轴方向上的滚动。上面的代码直接将MotionEvent拦截实现拖动。这部分的代码将会采用另一种更具有优势的方法，以便支持通用手势。它重写了GestureDetector.SimpleOnGestureListener的onScroll()方法。 只有用户在使用手指移动内容时，onScroll()才会被调用。onScroll()只有在手指按下的时候才会调用，一旦手指离开屏幕，那么平移手势也随之终止。 下面是onScroll()的使用摘要： // The current viewport. This rectangle represents the currently visible // chart domain and range. private RectF mCurrentViewport = new RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX); // The current destination rectangle (in pixel coordinates) into which the // chart data should be drawn. private Rect mContentRect; private final GestureDetector.SimpleOnGestureListener mGestureListener = new GestureDetector.SimpleOnGestureListener() { ... @Override public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) { // Scrolling uses math based on the viewport (as opposed to math using pixels). // Pixel offset is the offset in screen pixels, while viewport offset is the // offset within the current viewport. float viewportOffsetX = distanceX * mCurrentViewport.width() / mContentRect.width(); float viewportOffsetY = -distanceY * mCurrentViewport.height() / mContentRect.height(); ... // Updates the viewport, refreshes the display. setViewportBottomLeft( mCurrentViewport.left + viewportOffsetX, mCurrentViewport.bottom + viewportOffsetY); ... return true; } 下面是setViewportBottomLeft()方法的实现，它主要实现了移动内容的逻辑： /** * Sets the current viewport (defined by mCurrentViewport) to the given * X and Y positions. Note that the Y value represents the topmost pixel position, * and thus the bottom of the mCurrentViewport rectangle. */ private void setViewportBottomLeft(float x, float y) { /* * Constrains within the scroll range. The scroll range is simply the viewport * extremes (AXIS_X_MAX, etc.) minus the viewport size. For example, if the * extremes were 0 and 10, and the viewport size was 2, the scroll range would * be 0 to 8. */ float curWidth = mCurrentViewport.width(); float curHeight = mCurrentViewport.height(); x = Math.max(AXIS_X_MIN, Math.min(x, AXIS_X_MAX - curWidth)); y = Math.max(AXIS_Y_MIN + curHeight, Math.min(y, AXIS_Y_MAX)); mCurrentViewport.set(x, y - curHeight, x + curWidth, y); // Invalidates the View to update the display. ViewCompat.postInvalidateOnAnimation(this); } 缩放 在Detecting Common Gestures中，我们讨论到GestureDetector可以帮助我们来检测比如滑动、滚动、长按等手势。而对于缩放，Android提供了ScaleGestureDetector. GestureDetector 以及 ScaleGestureDetector。 为了可以反馈检测到的手势事件，手势探测器使用了监听器对象ScaleGestureDetector.OnScaleGestureListener。如果你只关心部分手势的话，Android还提供了ScaleGestureDetector.SimpleOnScaleGestureListener，你可以通过重写它的方法来使用。 缩放基础示例 下面的代码是缩放所需要的基础： private ScaleGestureDetector mScaleDetector; private float mScaleFactor = 1.f; public MyCustomView(Context mContext){ ... // View code goes here ... mScaleDetector = new ScaleGestureDetector(context, new ScaleListener()); } @Override public boolean onTouchEvent(MotionEvent ev) { // Let the ScaleGestureDetector inspect all events. mScaleDetector.onTouchEvent(ev); return true; } @Override public void onDraw(Canvas canvas) { super.onDraw(canvas); canvas.save(); canvas.scale(mScaleFactor, mScaleFactor); ... // onDraw() code goes here ... canvas.restore(); } private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener { @Override public boolean 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(); return true; } } 稍微复杂点的示例 下面是一个稍微复杂一点的示例，它摘自与这节课所提供的示例InteractiveChart(PS:示例工程请参见原网页)。InteractiveChart同时支持平移、缩放，它使用了ScaleGestureDetector的“平移”(getCurrentSpanX/Y)及“焦点” (getFocusX/Y)特性： @Override private RectF mCurrentViewport = new RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX); private Rect mContentRect; private ScaleGestureDetector mScaleGestureDetector; ... public boolean onTouchEvent(MotionEvent event) { boolean retVal = mScaleGestureDetector.onTouchEvent(event); retVal = mGestureDetector.onTouchEvent(event) || retVal; return retVal || super.onTouchEvent(event); } /** * The scale listener, used for handling multi-finger scale gestures. */ private final ScaleGestureDetector.OnScaleGestureListener mScaleGestureListener = new ScaleGestureDetector.SimpleOnScaleGestureListener() { /** * This is the active focal point in terms of the viewport. Could be a local * variable but kept here to minimize per-frame allocations. */ private PointF viewportFocus = new PointF(); private float lastSpanX; private float lastSpanY; // Detects that new pointers are going down. @Override public boolean onScaleBegin(ScaleGestureDetector scaleGestureDetector) { lastSpanX = ScaleGestureDetectorCompat. getCurrentSpanX(scaleGestureDetector); lastSpanY = ScaleGestureDetectorCompat. getCurrentSpanY(scaleGestureDetector); return true; } @Override public boolean onScale(ScaleGestureDetector scaleGestureDetector) { float spanX = ScaleGestureDetectorCompat. getCurrentSpanX(scaleGestureDetector); float spanY = ScaleGestureDetectorCompat. getCurrentSpanY(scaleGestureDetector); float newWidth = lastSpanX / spanX * mCurrentViewport.width(); float newHeight = lastSpanY / spanY * mCurrentViewport.height(); float focusX = scaleGestureDetector.getFocusX(); float focusY = scaleGestureDetector.getFocusY(); // Makes sure that the chart point is within the chart region. // See the sample for the implementation of hitTest(). hitTest(scaleGestureDetector.getFocusX(), scaleGestureDetector.getFocusY(), viewportFocus); mCurrentViewport.set( viewportFocus.x - newWidth * (focusX - mContentRect.left) / mContentRect.width(), viewportFocus.y - newHeight * (mContentRect.bottom - focusY) / mContentRect.height(), 0, 0); mCurrentViewport.right = mCurrentViewport.left + newWidth; mCurrentViewport.bottom = mCurrentViewport.top + newHeight; ... // Invalidates the View to update the display. ViewCompat.postInvalidateOnAnimation(InteractiveLineGraphView.this); lastSpanX = spanX; lastSpanY = spanY; return true; } };
