# The generic type of ParentDataWidget changed to ParentData

> The ParentDataWidget is now bound to the ParentData type.




:::important
These breaking change docs are accurate, as of the release
under which they are published. Over time, the
workarounds described here might become inaccurate.
We don't, in general, keep these breaking change docs up
to date as of each release.

The [breaking change index file](/release/breaking-changes)
lists the docs created for each release.
:::


## Summary

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

## Context

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.

## Description of change

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.

## Migration guide

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:

```plaintext
  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:

```dart
class 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:

```dart
class 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).

## Timeline

Landed in version: 1.16.3<br>
In stable release: 1.17

## References

API documentation:

* [`ParentDataWidget`][]

Relevant PR:

* [Make ParentDataWidget usable with different ancestor RenderObjectWidget types][]


[Make ParentDataWidget usable with different ancestor RenderObjectWidget types]: https://github.com/flutter/flutter/pull/48541
[`ParentDataWidget`]: https://api.flutter.dev/flutter/widgets/ParentDataWidget-class.html