Creating custom UI elements in Flutter using RenderBox

In Flutter, a RenderBox is a graphical element that has been painted. It has a origin and a paintable area. It is, in essence, what gets drawn on the screen.

A RenderBox is, therefore, a fundamental building block for creating custom user interface (UI) elements in Flutter.

To create a custom RenderBox, you will need to subclass the RenderBox class and override a few of its methods:

paint: This method is called when the render box needs to paint itself. You should use the provided PaintingContext and Offset to paint.

hitTest: This method is called when the user interacts with the app. It should return true if the provided Offset is within the paintable area of the render box.

performLayout: This method is called when the dimensions of the render box change. You should use this method to set the dimensions of the paintable area.

Here is an example of a simple custom RenderBox that draws a red square:

class RedSquareRenderBox extends RenderBox {
  @override
  void paint(PaintingContext context, Offset offset) {
    final Paint paint = Paint();
    paint.color = Colors.red;
    context.canvas.drawRect(offset & size, paint);
  }


  @override
  bool hitTest(HitTestResult result, {Offset position}) {
    return true;
  }


  @override
  void performLayout() {
    size = Size(100, 100);
  }
}

To use this RenderBox, you will need to wrap it in a RenderObjectWidget, which can then be used like any other widget in your app.

class RedSquare extends RenderObjectWidget {
  @override
  RenderObject createRenderObject(BuildContext context) {
    return RedSquareRenderBox();
  }
}

You can then use the RedSquare widget in your app like this:

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: RedSquare(),
    );
  }
}

I hope this helps! Let me know if you have any questions.

Author: Anil Rao K