performResize()

performResize() sets the render object's size using only the incoming BoxConstraints, before children are laid out. It is only called when sizedByParent returns true.

Separating sizing from child layout has a performance benefit: if the constraints haven't changed since the last frame, the framework can skip performResize() and go straight to performLayout(). For render objects whose size depends only on constraints — not on child content — this avoids redundant work.

When sizedByParent is true:

• performResize() must set size. It may only use constraints — children are not accessible here.
• performLayout() must not set size. It runs afterward to position and lay out children.

Default — Not Called#

sizedByParent defaults to false, so performResize() is never called. Size is set inside performLayout() instead. This is the right choice whenever the render object's size depends on its children.

Fill Available Space#

The most common use — the render object always expands to fill whatever space the parent offers.

@override
bool get sizedByParent => true;

@override
void performResize() {
  size = constraints.biggest;
}

@override
void performLayout() {
  // Size is already set. Lay out and position children here.
  child?.layout(constraints, parentUsesSize: false);
}

Shrink to Minimum#

A render object that always takes the smallest allowed size — useful for a decorative element that should never push other content aside.

@override
bool get sizedByParent => true;

@override
void performResize() {
  size = constraints.smallest;
}

Fixed Aspect Ratio#

Derive height from the available width to maintain a specific aspect ratio, then clamp to the constraints.

// Width-to-height ratio (e.g. 16/9 for widescreen).
static const double _aspectRatio = 16 / 9;

@override
bool get sizedByParent => true;

@override
void performResize() {
  final double width = constraints.maxWidth;
  final double height = (width / _aspectRatio).clamp(
    constraints.minHeight,
    constraints.maxHeight,
  );
  size = Size(width, height);
}

Fixed Natural Size Clamped to Constraints#

A render object with a preferred size that yields to whatever constraints the parent imposes.

static const Size _naturalSize = Size(200, 48);

@override
bool get sizedByParent => true;

@override
void performResize() {
  size = constraints.constrain(_naturalSize);
}