FAQ request: why is the build() method on State, and not StatefulWidget ?

[sethladd]

This comes up often when users are learning our framework. At our recent event, multiple Flutter team members didn't have a canned response to this FAQ.

Suggestion: Add this to our FAQ so for the next event, or next time this pops up in a chat or a conversation with a new user, we can point to our answer.

Can someone write up an answer to "Why is the build() method on State, and not StatefulWidget?" and we'll add it to the FAQ.

Thanks!

[abarth]

If we put the build function on StatefulWidget rather than State, the build function would receive the state object as a parameter, similar to the following:

class MyStatefulWidget extends Widget {
  ...
  Widget build(BuildContext context, covariant State state);
}

If you defined a closure in that function, the closure would implicitly capture this, which is the current widget instance, and would have the (immutable) fields of that instance in scope:

class MyButton extends MyStatefulWidget {
  ...
  final Color color;

  @override
  Widget build(BuildContext context, MyButtonState state) {
    ... () { print("color: $color"); } ...
  }
}

For example, suppose the parent builds MyButton with color being blue, the $color in the print function refers to blue, as expected. Now, suppose the parent rebuilds MyButton with green. The closure created by the first build still implicitly refers to the original widget and the $color still prints blue even through the widget has been updated to green.

In contrast, with the build function on the state object, closures created during build implicitly capture the state object instead of the widget:

class MyButtonState extends State<MyButton> {
  ...
  @override
  Widget build(BuildContext context) {
    ... () { print("color: ${config.color}"); } ...
  }
}

Now when the parent rebuilds MyButton with green, the closure created by the first build still refers to state object, which is preserved across rebuilds, but the framework has updated that state object's config property to refer to the new MyButton instance and ${config.color} prints green, as expected.

[abarth]

Let me know if that answer makes sense.

[sethladd]

Thanks! I'm going to challenge us to try to come up with the two-sentence explanation (maybe not the full reason, but enough to convince yourself that the difference is well-meaning, well thought out). I can start to understand that part of the reason is the way closures work, and understanding the lifecycle of our widgets (stateful and stateless).

Back to your example, I don't see why the problem of creating a closure that could implicitly keep a reference to an old value is unique to StatefulWidget. In your example, we don't refer to MyButtonState in the closure. So wouldn't we also have the problem with StatelessWidgets ?

class MyButton extends StatelessWidget {
  ...
  final Color color;

  @override
  Widget build(BuildContext context) {
    ... () { print("color: $color"); } ...   // <== this captures this.color
  }
}

(There's something here about Dart I probably don't understand, forgive me if I'm missing the obvious thing. :)

[abarth]

With a stateful widget, it's common to make closures whose lifecycle are tied to the state's lifecycle, which lasts through multiple widgets. With a stateless widget, it's common to make closures whose lifecycle are tied to the widget's lifecycle, which doesn't cause a problem.

[abarth]

Why is the build() method on State, and not StatefulWidget?

Putting a Widget build(BuildContext context) method on State rather 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.

[Hixie]

We should include all the above explanations in a ## Design discussion section on State.build.