The generic type of ParentDataWidget changed to ParentData

The generic type of ParentDataWidget has changed from RenderObjectWidget to ParentData.

Prior to this change, a ParentDataWidget was bound to a specific RenderObjectWidget type as ancestor. For example, a Positioned widget could only be used within a Stack widget. With this change, a ParentDataWidget can be used with any RenderObjectWidget type as ancestor as long as the RenderObject of said RenderObjectWidget sets up the correct ParentData type. In this new world, the Positioned widget can be reused with a hypothetical new SuperStack widget.

The generic type argument of ParentDataWidget has changed from RenderObjectWidget to ParentData, and a new debug property, debugTypicalAncestorWidgetClass, is added to ParentDataWidget. The latter is used for error messages to give users a better idea of the context a given ParentDataWidget is supposed to be used in.

You must migrate your code as described in this section if you're subclassing or implementing ParentDataWidget. If you do, the analyzer shows the following warnings when you upgrade to the Flutter version that includes this change:

  error • Missing concrete implementation of 'getter ParentDataWidget.debugTypicalAncestorWidgetClass' • lib/main.dart:114:7 • non_abstract_class_inherits_abstract_member
  error • 'FrogJar' doesn't extend 'ParentData' • lib/main.dart:114:41 • type_argument_not_matching_bounds

Code before migration:

dartclass FrogSize extends ParentDataWidget<FrogJar> {
  FrogSize({
    Key key,
    required this.size,
    required Widget child,
  }) : assert(child != null),
        assert(size != null),
        super(key: key, child: child);

  final Size size;

  @override
  void applyParentData(RenderObject renderObject) {
    final FrogJarParentData parentData = renderObject.parentData;
    if (parentData.size != size) {
      parentData.size = size;
      final RenderFrogJar targetParent = renderObject.parent;
      targetParent.markNeedsLayout();
    }
  }
}

class FrogJarParentData extends ParentData {
  Size size;
}

class FrogJar extends RenderObjectWidget {
  // ...
}

Code after migration:

dartclass FrogSize extends ParentDataWidget<FrogJarParentData> { // FrogJar changed to FrogJarParentData
  FrogSize({
    Key key,
    required this.size,
    required Widget child,
  }) : assert(child != null),
        assert(size != null),
        super(key: key, child: child);

  final Size size;

  @override
  void applyParentData(RenderObject renderObject) {
    final FrogJarParentData parentData = renderObject.parentData;
    if (parentData.size != size) {
      parentData.size = size;
      final RenderFrogJar targetParent = renderObject.parent;
      targetParent.markNeedsLayout();
    }
  }

  @override
  Type get debugTypicalAncestorWidgetClass => FrogJar; // Newly added
}

The generic type of the ParentDataWidget superclass changes from FrogJar (a RenderObjectWidget) to FrogJarParentData (the ParentData type that FrogSize.applyParentData wants to operate on). Additionally, the new debugTypicalAncestorWidgetClass is implemented for this ParentDataWidget subclass. It returns the type of a typical ancestor RenderObjectWidget for this ParentDataWidget. Most of the time, you just want to return the old generic type here (FrogJar in this example).