- 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.
- For example,
AnimatedWidget is a subclass of StatefulWidget that introduces an abstract Widget build(BuildContext context) method for its subclasses to implement. If StatefulWidget already had a build method that took a State argument, AnimatedWidget would be forced to provide its State object to subclasses even though its State object is an internal implementation detail of AnimatedWidget.
- Conceptually,
StatelessWidget could also be implemented as a subclass of StatefulWidget in a similar manner. If the build method were on StatefulWidget rather than State, that would not be possible anymore.
- Putting the build function on
State rather than StatefulWidget also helps avoid a category of bugs related to closures implicitly capturing this. If you defined a closure in a build function on a StatefulWidget, that closure would implicitly capture this, which is the current widget instance, and would have the (immutable) fields of that instance in scope.
VietMX Staff asked 4 years ago
