Android: How to move a BitmapDrawable?

Question

I'm trying to move a BitmapDrawable in a custom view. It works fine with a ShapeDrawable as follows:

public class MyView extends View {
    private Drawable image;

    public MyView() {
        image = new ShapeDrawable(new RectShape());
        image.setBounds(0, 0, 100, 100);
        ((ShapeDrawable) image).getPaint().setColor(Color.BLACK);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        image.draw(canvas);
    }

    public void move(int x, int y) {
        Rect bounds = image.getBounds();
        bounds.left += x;
        bounds.right += x;
        bounds.top += y;
        bounds.bottom += y;
        invalidate();
    }
}

However, if I use a BitmapDrawable, the drawable's bounds change, the onDraw method is called, but the image stays where it is on the screen.

The following constructor will reproduce the problem by creating a BitmapDrawable instead:

public MyView() {
    image = getResources().getDrawable(R.drawable.image);
    image.setBounds(0, 0, 100, 100);
}

How can I move a BitmapDrawable?

Answer 1

The documentation for Drawable.getBounds() says the following:

Note: for efficiency, the returned object may be the same object stored in the drawable (though this is not guaranteed), so if a persistent copy of the bounds is needed, call copyBounds(rect) instead. You should also not change the object returned by this method as it may be the same object stored in the drawable.

This is not cristal clear but it looks like we must not change value returned by getBounds(), it fires some nasty side effects.

By using copyBounds() and setBounds() it works like a charm.

public void move(int x, int y) {
    Rect bounds = image.copyBounds();
    bounds.left += x;
    bounds.right += x;
    bounds.top += y;
    bounds.bottom += y;
    image.setBounds(bounds);
    invalidate();
}

Another way of moving a Drawable could be to move the Canvas on wich you are drawing:

@Override
protected void onDraw(Canvas canvas) {
    canvas.translate(x, y);
    image.draw(canvas);
}