performLayout

Position children with performLayout.

performLayout

performLayout is the method that computes the size and position of the render object's children. It uses the incoming constraints from the parent to perform layout, and distributes them to children via child.layout(constraints).

@override
void performLayout() {
  size = constraints.biggest;
  if (child != null) {
    child!.layout(constraints);
  }
}

parentUsesSize

When child.layout(constraints, parentUsesSize: true) is called, the parent can read the child's size. If parentUsesSize: false, the parent does not depend on the child's size, which provides an extra optimization.

Exercise Define a RenderBox with a child

Define MyContainer as a RenderBox with the RenderObjectWithChildMixin<RenderBox> mixin. In performLayout, set size = constraints.biggest and call child?.layout(constraints, parentUsesSize: true). In main, create a MyContainer instance and print its runtimeType.

Show example solution

import 'package:flutter/rendering.dart';

class MyContainer extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
  @override
  void performLayout() {
    size = constraints.biggest;
    child?.layout(constraints, parentUsesSize: true);
  }
}

void main() {
  final container = MyContainer();
  print(container.runtimeType);
}

Test yourself

What does child.layout do?
- Draws the child
- Deletes the child
- Sends constraints to the child and makes it lay out
- Sets the child color
Show answer

Sends constraints to the child and makes it lay out — child.layout(constraints) triggers the child's performLayout, passing the parent's bounds to the child; it does not deal with drawing or color.

child.layout call

child!.layout(____, parentUsesSize: true);

Show answer

child!.layout(constraints, parentUsesSize: true);

performLayout matches

performLayout

Computes size and position
layout

Distributes constraints to child
child

Sub render object
parentUsesSize

Parent reads child size

Next lesson: paint method

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