Flutter: Understanding how does a Stateless/Statefull widget layout its child?

Question

TL;DR

How can I know the layout rules of a widget (what size will it request from its parent and what constraints will it pass to its children) if there is no documentation about it?

The problem details

I have this very basic app

void main() {
  runApp(
    Container(
        color: Colors.red,
        width: 20,
        height: 20,
      ),
  );
}

I was expecting the Container to have width and height of 20 but I got a Container that filled up the whole screen.

Reading this article on flutter.dev about understanding constraints, in its last part called "Learning the layout rules for specific widgets ", they mention how to do this by finding the createRenderObject method and then finding the performLayout method.

However this createRenderObject method is only available for subclasses of RenderObjectWidget. For example, navigating through code of Transform widget, I find createRenderObject that returns a RenderTransform, that extends RenderProxyBox, which finally implements performLayout as :

  @override
  void performLayout() {
    if (child != null) {
      child!.layout(constraints, parentUsesSize: true);
      size = child!.size;
    } else {
      size = computeSizeForNoChild(constraints);
    }
  }

I can conclude that Transform widget will finally take the size of its child due to this line size = child!.size;.

But in case of Container above, is directly extends StatelessWidget. I couldn't find by navigating through its code the methods performLayout and createRenderObject, I could only find createElement, but I am looking for the RenderObject in the render tree associated with the Container and not the element.

The Question

So the question is how to find this render object associated with a stateless widget/stateful widget in order to know the layout rules that this widget will give to its children and will follow them itself in this case?

Solution

You have a point. I'd say my article is imprecise in that regard.

A widget doesn't need to create a RenderObject. It can, instead, use a composition of other widgets that create RenderObjects themselves.

If a widget is a composition of other widgets, then instead of looking at the performLayout you can simply look at that widget's build method to see what it's doing. In the case of a Container, this is its build method:

Widget build(BuildContext context) {
    Widget? current = child;

    if (child == null && (constraints == null || !constraints!.isTight)) {
      current = LimitedBox(
        maxWidth: 0.0,
        maxHeight: 0.0,
        child: ConstrainedBox(constraints: const BoxConstraints.expand()),
      );
    }

    if (alignment != null)
      current = Align(alignment: alignment!, child: current);

    final EdgeInsetsGeometry? effectivePadding = _paddingIncludingDecoration;
    if (effectivePadding != null)
      current = Padding(padding: effectivePadding, child: current);

    if (color != null)
      current = ColoredBox(color: color!, child: current);

    if (clipBehavior != Clip.none) {
      assert(decoration != null);
      current = ClipPath(
        clipper: _DecorationClipper(
          textDirection: Directionality.maybeOf(context),
          decoration: decoration!,
        ),
        clipBehavior: clipBehavior,
        child: current,
      );
    }

    if (decoration != null)
      current = DecoratedBox(decoration: decoration!, child: current);

    if (foregroundDecoration != null) {
      current = DecoratedBox(
        decoration: foregroundDecoration!,
        position: DecorationPosition.foreground,
        child: current,
      );
    }

    if (constraints != null)
      current = ConstrainedBox(constraints: constraints!, child: current);

    if (margin != null)
      current = Padding(padding: margin!, child: current);

    if (transform != null)
      current = Transform(transform: transform!, alignment: transformAlignment, child: current);

    return current!;
  }

As you can see, it is defined in terms of other widgets. And these widgets may also be defined in terms of other widgets and so on. But at some point you will reach the widgets that create the RenderObjects.

---

Regarding the reason why the Container is not 20x20, it's because, as the article explains, sizes are set by parents. So the Containers size is set by the Container's parent, which in this case is the screen. And the screen always forces its child to occupy all the available space, in this case ignoring the Container's desire to be 20x20. The fix here is giving the Container another parent. One which allows the Container to choose its own size. For example, both Center and Align will let that happen, and that's why you can fix the problem by doing:

void main() {
  runApp(
    Center( 
      child: Container(
        color: Colors.red,
        width: 20,
        height: 20,
      ),),);
}

As to why the screen forces its child to occupy all the available space: That's just the way Flutter creators decided it should be. If you dig into Flutter's code you will find it there. But it's probably best that you just remember this fact.

Hope it helps!