Implementing pinch zoom and drag using Android's build in gesture listener and scale listener

Question

I am trying to implement pinch zoom and drag using Android\'s gesture listener and scale listener. The problem is that when I perform pinch zoom, the image (which I am tryin

Answer 1

I have implemented this behaviour, and I used a matrix to handle all the zooming and scrolling (and rotation, in my case). It makes for neat code and works like clockwork.

Store a Matrix as a class member:

Matrix drawMatrix;

Edit: Store old focus point, used to get the focus shift during scaling.

float lastFocusX;
float lastFocusY;

Edit: Set lastFocus variables in onScaleBegin

@Override
public boolean onScaleBegin(ScaleGestureDetector detector) {
    lastFocusX = detector.getFocusX();
    lastFocusY = detector.getFocusY();
    return true;
}

Replace your onScale:

@Override
public boolean onScale(ScaleGestureDetector detector) {
    Matrix transformationMatrix = new Matrix();
    float focusX = detector.getFocusX();
    float focusY = detector.getFocusY();

    //Zoom focus is where the fingers are centered, 
    transformationMatrix.postTranslate(-focusX, -focusY);

    transformationMatrix.postScale(detector.getScaleFactor(), detector.getScaleFactor());

/* Adding focus shift to allow for scrolling with two pointers down. Remove it to skip this functionality. This could be done in fewer lines, but for clarity I do it this way here */
    //Edited after comment by chochim
    float focusShiftX = focusX - lastFocusX;
    float focusShiftY = focusY - lastFocusY;
    transformationMatrix.postTranslate(focusX + focusShiftX, focusY + focusShiftY);
    drawMatrix.postConcat(transformationMatrix);
    lastFocusX = focusX;
    lastFocusY = focusY;
    invalidate();
    return true;
}

Similarly in onScroll:

@Override
public boolean onScroll(MotionEvent downEvent, MotionEvent currentEvent,
            float distanceX, float distanceY) {
    drawMatrix.postTranslate(-distanceX, -distanceY);
    invalidate();
    return true;
}

in onDraw; Draw with your drawMatrix:

canvas.drawBitmap(image, drawMatrix, paint);

Happy coding!