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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。
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。
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：
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，代码段使用了方法。你应该一直使用这个方法来接收对象的活动。与方法不同，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;@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;} 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。它重写了的方法。
只有用户在使用手指移动内容时，才会被调用。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() {...@Overridepublic 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);} 在中，我们讨论到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 以及 。
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。如果你只关心部分手势的话，Android还提供了，你可以通过重写它的方法来使用。
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());}@Overridepublic boolean onTouchEvent(MotionEvent ev) { // Let the ScaleGestureDetector inspect all events. mScaleDetector.onTouchEvent(ev); return true;}@Overridepublic 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; }} 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，它使用了的“平移”()及“焦点” ()特性：
@Overrideprivate 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; }}; 转载地址：http://santl.baihongyu.com/