将缩放图像的初始位置也设置为触摸中心

Question

我正在尝试在android中实现缩放功能。一切正常，但问题是，第一次点击图像时，它将图像带到了左上角。

我希望图像可以随时居中。

我将图像视图比例类型用作矩阵和OnTouchListener。

public class ZoomInZoomOut extends Activity implements 
View.OnTouchListener, ViewTreeObserver.OnGlobalLayoutListener {
    private static final String TAG = "ZoomInZoomOut";

    ImageView imgZoom;

    Matrix matrix = new Matrix();
    Matrix savedMatrix = new Matrix();

    static final int NONE = 0;
    static final int DRAG = 1;
    static final int ZOOM = 2;
    int mode = NONE;

    PointF start = new PointF();
    PointF mid = new PointF();
    float oldDist = 1f;


 @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.zoom_in_zoom_out_image);

    imgZoom = (ImageView) findViewById(R.id.imgZoom);
    imgZoom.setOnTouchListener(this);

    imgZoom.getViewTreeObserver().addOnGlobalLayoutListener(this);

}


    @Override
    public void onGlobalLayout() {
    if (Build.VERSION.SDK_INT >= 16) {

 imgZoom.getViewTreeObserver().removeOnGlobalLayoutListener(this);
    } else {      
 imgZoom.getViewTreeObserver().removeGlobalOnLayoutListener(this);
    }

    centerImage();
  }


   private void centerImage() {

    Drawable drawable = imgZoom.getDrawable();
    RectF imageRectF = new RectF(0, 0, drawable.getIntrinsicWidth(), drawable.getIntrinsicHeight());
    RectF viewRectF = new RectF(0, 0, imgZoom.getWidth(), imgZoom.getHeight());
    Matrix myMatrix = new Matrix();
    myMatrix.setRectToRect(imageRectF, viewRectF, Matrix.ScaleToFit.CENTER);

    imgZoom.setImageMatrix(myMatrix);

    }


     @Override
     public boolean onTouch(View v, MotionEvent event) {
      float scale;
      dumpEvent(event);

      switch (event.getAction() & MotionEvent.ACTION_MASK) {
        case MotionEvent.ACTION_DOWN:   // first finger down only
            savedMatrix.set(matrix);
            start.set(event.getX(), event.getY());
            Log.d(TAG, "mode=DRAG"); // write to LogCat
            mode = DRAG;

            centerImage();
            break;


        case MotionEvent.ACTION_UP: 
        case MotionEvent.ACTION_POINTER_UP: 
            mode = NONE;
            Log.d(TAG, "mode=NONE");
            break;


        case MotionEvent.ACTION_POINTER_DOWN: 
            oldDist = spacing(event);
            Log.d(TAG, "oldDist=" + oldDist);
            if (oldDist > 5f) {
                savedMatrix.set(matrix);
                midPoint(mid, event);
                mode = ZOOM;
                Log.d(TAG, "mode=ZOOM");
            }
            break;


        case MotionEvent.ACTION_MOVE:
            if (mode == DRAG) {
                matrix.set(savedMatrix);
                matrix.postTranslate(event.getX() - start.x, event.getY() - start.y); 
            } else if (mode == ZOOM) {
                // pinch zooming
                float newDist = spacing(event);
                Log.d(TAG, "newDist=" + newDist);
                if (newDist > 5f) {
                    matrix.set(savedMatrix);
                    scale = newDist / oldDist;
                    matrix.postScale(scale, scale, mid.x, mid.y);
                }
            }
            centerImage();
            break;
      }


      imgZoom.setImageMatrix(matrix); 


    return true; 
}


   private float spacing(MotionEvent event) {
    float x = event.getX(0) - event.getX(1);
    float y = event.getY(0) - event.getY(1);
    return (float) Math.sqrt(x * x + y * y);
  }

   private void midPoint(PointF point, MotionEvent event) {
    float x = event.getX(0) + event.getX(1);
    float y = event.getY(0) + event.getY(1);
    point.set(x / 2, y / 2);
   }

   private void dumpEvent(MotionEvent event) {
    String names[] = {"DOWN", "UP", "MOVE", "CANCEL", "OUTSIDE", "POINTER_DOWN", "POINTER_UP", "7?", "8?", "9?"};
    StringBuilder sb = new StringBuilder();
    int action = event.getAction();
    int actionCode = action & MotionEvent.ACTION_MASK;
    sb.append("event ACTION_").append(names[actionCode]);

    if (actionCode == MotionEvent.ACTION_POINTER_DOWN || actionCode == MotionEvent.ACTION_POINTER_UP) {
        sb.append("(pid ").append(action >> MotionEvent.ACTION_POINTER_ID_SHIFT);
        sb.append(")");
    }

    sb.append("[");
    for (int i = 0; i < event.getPointerCount(); i++) {
        sb.append("#").append(i);
        sb.append("(pid ").append(event.getPointerId(i));
        sb.append(")=").append((int) event.getX(i));
        sb.append(",").append((int) event.getY(i));
        if (i + 1 < event.getPointerCount())
            sb.append(";");
    }

    sb.append("]");
    Log.d(TAG, "Touch Events ---------" + sb.toString());
}
}

我的期望是实现类似缩放效果的whatsapp。图片应始终居中。

谢谢。