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

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

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

Replace your onScale:

@Overridepublic 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:

@Overridepublic 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!