1.Use Touch to Perform Scaling

  As discussed in Detecting Common GesturesGestureDetector helps you detect common gestures used by Android such as scrollingflinging, and long press. For scaling, Android provides ScaleGestureDetector .   GestureDetector and ScaleGestureDetector can be used together when you want a view to recognize additional gestures.

  ScaleGestureDetector 是用来识别缩放手势的。它可以和GestureDetector同时使用,来识别额外的手势.

  To report detected gesture events, gesture detectors use listener objects passed to their constructors.ScaleGestureDetector uses ScaleGestureDetector.OnScaleGestureListener. Android provides ScaleGestureDetector.SimpleOnScaleGestureListener as a helper class that you can extend if you don’t care about all of the reported events.

  ScaleGestureDetector.SimpleOnScaleGestureListener 是一个封装好的缩放手势类.用来接收探测到的手势,构造手势探测器 ScaleGestureDetector 的时候要用到它.

2.Basic scaling example

  Here is a snippet that illustrates the basic ingredients involved in scaling.

 1 private ScaleGestureDetector mScaleDetector;
2 private float mScaleFactor = 1.f;
3
4 public MyCustomView(Context mContext){
5 ...
6 // View code goes here
7 ...
8 mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
9 }
10
11 @Override
12 public boolean onTouchEvent(MotionEvent ev) {
13 // Let the ScaleGestureDetector inspect all events.
14 mScaleDetector.onTouchEvent(ev);
15 return true;
16 }
17
18 @Override
19 public void onDraw(Canvas canvas) {
20 super.onDraw(canvas);
21
22 canvas.save();
23 canvas.scale(mScaleFactor, mScaleFactor);
24 ...
25 // onDraw() code goes here
26 ...
27 canvas.restore();
28 }
29
30 private class ScaleListener
31 extends ScaleGestureDetector.SimpleOnScaleGestureListener {
32 @Override
33 public boolean onScale(ScaleGestureDetector detector) {
34 mScaleFactor *= detector.getScaleFactor();
35
36 // Don't let the object get too small or too large.
37 mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));
38
39 invalidate();
40 return true;
41 }
42 }

3.More complex scaling example

  Here is a more complex example from the InteractiveChart sample provided with this class. TheInteractiveChart sample supports both scrolling (panning) and scaling with multiple fingers, using the ScaleGestureDetector "span" (getCurrentSpanX/Y) and "focus" (getFocusX/Y) features:

  下面是一个复杂的手势识别示例的片段,使用了两个识别器.完整示例下载地址 InteractiveChart
 @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(),
,
);
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;
}
};

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