Android 手势识别—缩放

      上一篇讲解了手势识别中的点击和双击事件的识别，用到的是GestureDetector类和GestureDetectorCompat类，用于监听用户触摸屏幕中的简单动作。

缩放

基本用法如下，可以通过缩放手势缩放一个View

public class ScaleGestureActivity extends BaseActivity {

    private View mView;
    private ScaleGestureDetector mScaleGestureDetector;
    private static final String TAG = "ScaleGestureActivity";
    private float mScaleFactor = 1.0f;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_scale_gesture);
        mView = findViewById(R.id.scale_view);
        mScaleGestureDetector = new ScaleGestureDetector(this,mScaleGestureListener);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mScaleGestureDetector.onTouchEvent(event);
        return super.onTouchEvent(event);
    }

    private ScaleGestureDetector.OnScaleGestureListener mScaleGestureListener = new ScaleGestureDetector.OnScaleGestureListener() {
        @Override
        public boolean onScale(ScaleGestureDetector scaleGestureDetector) {
            mScaleFactor  *= scaleGestureDetector.getScaleFactor();
            mView.setScaleX(mScaleFactor);
            mView.setScaleY(mScaleFactor);
            Log.i(TAG, "onScale: "+ scaleGestureDetector.getScaleFactor()+"----"+mScaleFactor);
            return true;
        }

        @Override
        public boolean onScaleBegin(ScaleGestureDetector scaleGestureDetector) {
            return true;
        }

        @Override
        public void onScaleEnd(ScaleGestureDetector scaleGestureDetector) {

        }
    };
}

ScaleGestureDetector 官方对该类的解释如下：

Detects scaling transformation gestures using the supplied MotionEvents. The ScaleGestureDetector.OnScaleGestureListener callback will notify users when a particular gesture event has occurred. This class should only be used with MotionEvents reported via touch. To use this class:

• Create an instance of the ScaleGestureDetector for your View
• In the onTouchEvent(MotionEvent) method ensure you call onTouchEvent(MotionEvent). The methods defined in your callback will be executed when the events occur.

该类的构造方法如下：

public ScaleGestureDetector(Context context, ScaleGestureDetector.OnScaleGestureListener listener)

由构造方法可知需要为一个缩放类设置一个缩放的监听接口，具体实现见代码中监听类接口的实现，下面具体介绍几个函数的含义：

1. public boolean onScale(ScaleGestureDetector detector);

         缩放时会被调用，返回值代表本次缩放事件是否已被处理。如果已被处理，那么detector就会重置缩放事件；如果未被处理，detector会继续进行计算，修改getScaleFactor()的返回值，直到被处理为止。因此，它常用在判断只有缩放值达到一定数值时才进行缩放。下面的代码表示只有当缩放倍数达到2倍时才对其进行缩放，否则该事件不处理。

public boolean onScale(ScaleGestureDetector detector) {
		System.out.println(detector.getScaleFactor());
		if(detector.getScaleFactor()< 2){
			return false;
		}
		mMatrix.preScale(detector.getScaleFactor(), detector.getScaleFactor());
		setImageMatrix(mMatrix);
		return true;
	}

    2.public boolean onScaleBegin(ScaleGestureDetector detector)

       缩放开始时会被调用。返回值表示该detector是否处理后继的缩放事件。返回false时，不会执行后续的onScale()。

    3. public void onScaleEnd(ScaleGestureDetector scaleGestureDetector)

      缩放结束时的回调

http://blog.csdn.net/qiwenmingshiwo/article/details/70850406

https://github.com/Almeros/android-gesture-detectors

https://github.com/Almeros/android-gesture-detectors-example/blob/master/app/src/main/java/com/almeros/android/multitouch/example/TouchActivity.java