Looking up a deactivated widget’s ancestor is unsafe.

When looking up a deactivated widget’s ancestor, it can be unsafe because the widget may not be mounted in the widget tree anymore. This means that any attempt to access its ancestor may lead to unexpected errors or behavior. It is important to consider the widget’s lifecycle and current state before attempting to access its ancestor.

Here is an example to better understand the scenario. Let’s consider a simple Flutter app with two widgets: ParentWidget and ChildWidget. ParentWidget is the ancestor of ChildWidget.

class ParentWidget extends StatefulWidget {
  @override
  _ParentWidgetState createState() => _ParentWidgetState();
}

class _ParentWidgetState extends State {
  @override
  void deactivate() {
    // Deactivate ParentWidget
    super.deactivate();
  }

  @override
  Widget build(BuildContext context) {
    return ChildWidget();
  }
}

class ChildWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // Attempting to access the ancestor (ParentWidget) without checking its state
    final parentWidget = context.findAncestorWidgetOfExactType();

    return Container(
      child: Text(
        parentWidget != null ? 'Ancestor found' : 'Ancestor not found',
      ),
    );
  }
}
  

In this example, the ParentWidget is deactivated when the deactivate() method is called. The ChildWidget tries to access its ancestor (ParentWidget) using the findAncestorWidgetOfExactType() method without checking the state of the widget.

Now, if the ChildWidget is still built after the ParentWidget is deactivated, the attempt to access the ancestor will fail. This can lead to unexpected behavior or errors since the ancestor is no longer available.

To avoid such unsafe situations, it is recommended to check the state of the deactivated widget before accessing its ancestor. You can use a condition like parentWidget?.mounted == true to ensure the ancestor is still mounted in the widget tree.

class ChildWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final parentWidget = context.findAncestorWidgetOfExactType();

    if (parentWidget != null && parentWidget.mounted) {
      return Container(
        child: Text('Ancestor found'),
      );
    } else {
      return Container(
        child: Text('Ancestor not found'),
      );
    }
  }
}
  

By checking if the parentWidget is not null and is still mounted, we ensure the safe retrieval of the ancestor widget. This way, if the ancestor is deactivated, we can handle the situation accordingly and avoid any errors or unexpected behavior.