上一篇講解了手勢識別中的點擊和雙擊事件的識別，用到的是GestureDetector類和GestureDetectorCompat類，用於監聽用戶觸摸屏幕中的簡單動作。

縮放

基本用法如下，可以通過縮放手勢縮放一個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 官方對該類的解釋如下：

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.

該類的構造方法如下：

public ScaleGestureDetector(Context context, ScaleGestureDetector.OnScaleGestureListener listener)

由構造方法可知需要為一個縮放類設置一個縮放的監聽接口，具體實現見代碼中監聽類接口的實現，下面具體介紹幾個函數的含義：

1. public boolean onScale(ScaleGestureDetector detector);

縮放時會被調用，返回值代表本次縮放事件是否已被處理。如果已被處理，那麽detector就會重置縮放事件；如果未被處理，detector會繼續進行計算，修改getScaleFactor()的返回值，直到被處理為止。因此，它常用在判斷只有縮放值達到一定數值時才進行縮放。下面的代碼表示只有當縮放倍數達到2倍時才對其進行縮放，否則該事件不處理。

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)

縮放開始時會被調用。返回值表示該detector是否處理後繼的縮放事件。返回false時，不會執行後續的onScale()。

3. public void onScaleEnd(ScaleGestureDetector scaleGestureDetector)

縮放結束時的回調

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

Android 手勢識別—縮放