Flutter change state from outside

The solution is to pass a new Key to WidgetB every time we need it to be rebuilt : WidgetA will see that WidgetB has changed and will rebuild it when setState is called. In other words, whenever a stateful widget’s Key property changes, calling setState on its parent will force a rebuild of the stateful widget.12 Tem 2021

Why is the build method on State, and not StatefulWidget? Putting a Widget build(BuildContext context) method on State rather than putting a Widget build(BuildContext context, State state) method on StatefulWidget gives developers more flexibility when subclassing StatefulWidget .

The tldr version is that State objects are long lived, but StatefulWidget s (and all Widget subclasses) are thrown away and rebuilt whenever configuration changes .