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 positionfinal int pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId);  final float x = MotionEventCompat.getX(ev, pointerIndex);final float y = MotionEventCompat.getY(ev, pointerIndex);// Calculate the distance movedfinal float dx = x - mLastTouchX;final float dy = y - mLastTouchY;mPosX += dx;mPosY += dy;invalidate();// Remember this touch position for the next move eventmLastTouchX = 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 ScaleListenerextends ScaleGestureDetector.SimpleOnScaleGestureListener {@Overridepublic 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.@Overridepublic boolean onScaleBegin(ScaleGestureDetector scaleGestureDetector) {lastSpanX = ScaleGestureDetectorCompat.getCurrentSpanX(scaleGestureDetector);lastSpanY = ScaleGestureDetectorCompat.getCurrentSpanY(scaleGestureDetector);return true;}@Overridepublic 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;}
};