android - Flutter mask a circle into a container

Question

I want to add a circle effect over a container, but I want the circle to not extend the dimensions of the container, but instead get clipped by it. This is what I'm trying to achieve:

As you can see the white circle naturally would extend the red container but instead, I'm trying to make it stay into the borders. How can I do it?

Answer 1

The simplest way to do this is to using an overlap and cliprect.

class OverlapSquare extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      height: 200,
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(10),
        color: Colors.red,
      ),
      child: ClipRect(
        clipBehavior: Clip.hardEdge,
        child: OverflowBox(
          maxHeight: 250,
          maxWidth: 250,
          child: Center(
            child: Container(
              decoration: BoxDecoration(
                color: Colors.white,
                shape: BoxShape.circle,
              ),
            ),
          ),
        ),
      ),
    );
  }
}

The OverFlowBox allows the circle to draw outside the bounds of its parent, and then the cliprect cuts it back to the edge.

Just an FYI - on my device I'm getting a tiny red line at the top & bottom of the white circle. I'm fairly sure that's a recently-introduced bug in flutter as I'm also seeing something similar if I put a white border around the container. I've raised an issue on github for that.