Question

I’m extending LinearLayout:

public class MyLinearLayout extends LinearLayout {
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        Paint paint = new Paint();
        paint.setColor(0xFFFF0000);
        canvas.drawLine(0, 0, getWidth(), getHeight(), paint);
    }
}

If the layout has child elements, shouldn’t the red line above be drawn on top of them? Considering a layout like:

<MyLinearLayout>
    <ImageView />
</MyLinearLayout>

I’d expect to get the red line drawn above the child ImageView. But it gets drawn below it. I was assuming all child drawing would have been completed after the super.onDraw() line is finished.

Is there a way to get to the canvas and draw something on it after all child drawing has been completed?

Thanks

Answer 1

Layouts don’t draw unless you call setWillNotDraw(false);. They do that for efficiency reasons.

EDIT:

onDraw() really is meant to just allow you to modify the Canvas before the drawing operation happens. It doesn’t actually draw. What you want to do is override draw() like so:

@Override
protected void draw(Canvas canvas) {
    super.draw(canvas);

    Paint paint = new Paint();
    paint.setColor(0xFFFF0000);
    canvas.drawLine(0, 0, getWidth(), getHeight(), paint);
}

Calling the super will draw all the children in the view. Then it will draw all the lines. I do still believe you need setWillNotDraw(false) to be called though.