layout method Null safety

  1. @override
void layout(
  1. Constraints constraints,
  2. {bool parentUsesSize = false}

Compute the layout for this render object.

This method is the main entry point for parents to ask their children to update their layout information. The parent passes a constraints object, which informs the child as to which layouts are permissible. The child is required to obey the given constraints.

If the parent reads information computed during the child's layout, the parent must pass true for parentUsesSize. In that case, the parent will be marked as needing layout whenever the child is marked as needing layout because the parent's layout information depends on the child's layout information. If the parent uses the default value (false) for parentUsesSize, the child can change its layout information (subject to the given constraints) without informing the parent.

Subclasses should not override layout directly. Instead, they should override performResize and/or performLayout. The layout method delegates the actual work to performResize and performLayout.

The parent's performLayout method should call the layout of all its children unconditionally. It is the layout method's responsibility (as implemented here) to return early if the child does not need to do any work to update its layout information.


void layout(Constraints constraints, {bool parentUsesSize = false}) {
  if (hasSize && constraints != this.constraints &&
      _cachedBaselines != null && _cachedBaselines!.isNotEmpty) {
    // The cached baselines data may need update if the constraints change.
  super.layout(constraints, parentUsesSize: parentUsesSize);