Android: Is this not how drawing a canvas works?

Question

Am I misunderstanding how translating and scaling a canvas should work? I created a custom view that I can drag and zoom, but it's inside the bounds I set. I thought translating the canvas would redraw the bounds? I want to be able to drag and zoom the image on the whole screen, not in a viewport! I assumed translating the canvas would adjust the viewport.

Am I thinking about this the wrong way?

@Override
public void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.save();
    canvas.translate(mPosX, mPosY);
    canvas.scale(mScaleFactor, mScaleFactor);
    mIcon.draw(canvas);
    canvas.restore();
}

This is how I created the view. So basically it is stuck in this 250 x 250 box. I need the view to actually drag around.. I get the feeling I'm doing something stupid but I can't produce the behavior I'm looking for.

    MultiTouchImageView view = new MultiTouchImageView(this);
    view.setLayoutParams(new RelativeLayout.LayoutParams(250, 250));

Answer 1

You can set margins to move view in layout. I've created test project to show the idea, it looks like that:

public class DragView extends View {

    private float mLastTouchX;
    private float mLastTouchY;

    private float mDeltaX;
    private float mDeltaY;

    public DragView(Context context) {
        super(context);
        init();
    }

    public DragView(final Context context, final AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    private void init() {
        setOnTouchListener(new OnTouchListener() {

            @Override
            public boolean onTouch(View v, MotionEvent event) {
                final int action = event.getAction();

                mLastTouchX = event.getRawX();
                mLastTouchY = event.getRawY();

                switch (action) {
                case MotionEvent.ACTION_DOWN: {
                    RelativeLayout.LayoutParams lParams = (RelativeLayout.LayoutParams) getLayoutParams();
                    mDeltaX = mLastTouchX - lParams.leftMargin;
                    mDeltaY = mLastTouchY - lParams.topMargin;

                    break;
                }
                case MotionEvent.ACTION_MOVE: {
                    mLastTouchX = event.getRawX();
                    mLastTouchY = event.getRawY();

                    final RelativeLayout.LayoutParams params = (LayoutParams) getLayoutParams();
                    params.leftMargin = (int) (mLastTouchX - mDeltaX);
                    params.topMargin = (int) (mLastTouchY - mDeltaY);
                    setLayoutParams(params);

                    break;
                }
                }

                return true;
            }
        });
    }

}