code: public void setZoom(float scale, float focusX, float focusY, ScaleType scaleType) {
          //
          // setZoom can be called before the image is on the screen, but at this point,
          // image and view sizes have not yet been calculated in onMeasure. Thus, we should
          // delay calling setZoom until the view has been measured.
          //
          if (!onDrawReady) {
              delayedZoomVariables = new ZoomVariables(scale, focusX, focusY, scaleType);
              return;
          }
  
          if (scaleType != mScaleType) {
              setScaleType(scaleType);
          }
          resetZoom();
          scaleImage(scale, viewWidth / 2, viewHeight / 2, true);
          matrix.getValues(m);
          m[Matrix.MTRANS_X] = -((focusX * getImageWidth()) - (viewWidth * 0.5f));
          m[Matrix.MTRANS_Y] = -((focusY * getImageHeight()) - (viewHeight * 0.5f));
          matrix.setValues(m);
          fixTrans();
          setImageMatrix(matrix);
      }
docs: Set zoom to the specified scale. Image will be centered around the point
  (focusX, focusY). These floats range from 0 to 1 and denote the focus point
  as a fraction from the left and top of the view. For example, the top left
  corner of the image would be (0, 0). And the bottom right corner would be (1, 1).
  
  @param scale
  @param focusX
  @param focusY
  @param scaleType