如何通过双击来放大和缩小TouchImageView？

private ScaleGestureDetector mScaleDetector;

private Matrix mMatrix;

private float mScaleFactor = 1.f;

public TouchImageView(Context context) {

    this(context, null);

}

public TouchImageView(Context context, AttributeSet attrs) {

    this(context, attrs, 0);

}

public TouchImageView(Context context, AttributeSet attrs, int defStyle) {

    super(context, attrs, defStyle);

    mMatrix = new Matrix();

    mScaleDetector = new Scale