Stateful Widget Lifecycle

When a Flutter builds a StatefulWidget, it creates a State object. This object is where all the mutable state for that widget is held.

The concept of state is defined by two things:

  1. The data used by the widget might change.
  2. The data can't be read synchronously when the widget is built. (All state must be established by the time the build method is called).

Why Are StatefulWidget and State Separate Classes?

In one word: performance.

The tldr version is that State objects are long lived, but StatefulWidgets (and all Widget subclasses) are thrown away and rebuilt whenever configuration changes. It's very inexpensive for Flutter to rebuild a mutable widget.

Because State isn't blown away on every rebuild, you can maintain it and keep from having to make expensive computations to get the state property everytime something is rebuilt.

Also, this is what allows Flutter animations to exist. Because State isn't thrown away, it can constantly be rebuilding it's Widget in response to data changes.

1. createState()

When the Framework is instructed to build a StatefulWidget, it immediately calls createState(). This method must exist. A StatefulWidget rarely needs to be more complicated than this.

class MyHomePage extends StatefulWidget {
  
  _MyHomePageState createState() => new _MyHomePageState();
}

2. mounted is true

When createState creates your state class, a buildContext is assigned to that state. BuildContext is, overly simplified, the place in the widget tree in which this widget is placed. Here's a longer explanation.

All widgets have a bool this.mounted property. It is turned true when the buildContext is assigned. It is an error to call setState when a widget is unmounted.

tip: This property is useful when a method on your state calls setState() but it isn't clear when or how often that method will be called. Perhaps its being called in response to a stream updating. You can use if (mounted) {... to make sure the State exists before calling setState().

3. initState()

This is the first method called when the widget is created (after the class constructor, of course.)

initState is called once and only once. It must called super.initState().

This method is the best time to:

  1. Initialize data that relies on the specific BuildContext for the created instance of the widget.
  2. Initialize properties that rely on this widgets 'parent' in the tree.
  3. Subscribe to Streams, ChangeNotifiers, or any other object that could change the data on this widget.

initState() {
  super.initState();
  // Add listeners to this class
  cartItemStream.listen((data) {
    _updateWidget(data);
  });
}

4. didChangeDependencies()

This method is called immediately after initState on the first time the widget is built.

It will also be called whenever an object that this widget depends on data from is called. For example, if it relies on an InheritedWidget, which updates.

build is always called after didChangeDependencies is called, so this is rarely needed. However, this method is the first change you have to call BuildContext.inheritFromWidgetOfExactType. This essentially would make this State 'listen' to changes on a Widget it's inheriting data from.

The docs also suggest that it could be useful if you need to do network calls (or any other expensive action) when an InheritedWidget updates.

5. build()

This method is called often. It is required, and it must return a Widget.

6. didUpdateWidget(Widget oldWidget)

If the parent widget changes and has to rebuild this widget (because it needs to give it different data), but it's being rebuilt with the same runtimeType, then this method is called.

This is because Flutter is re-using the state, which is long lived. In this case, you may want to initialize some data again, as you would in initState.

If your state's build method relies on a Stream or other object that can change, unsubscribe from the old object and re-subscribe to the new instance in didUpdateWidget.

tip: This method is basically the replacement for 'initState' if you expect your Widget associated with this state to be rebuilt!

The framework always called build after this, so any call to setState is redundant.


void didUpdateWidget(Widget oldWidget) {
  if (oldWidget.importantProperty != widget.importantProperty) {
    _init();
  }
}

7. setState()

This method is called often from the framework itself and from the developer. Its used to notify the framework that data has changed, and the widget at this build context should be rebuilt.

setState takes a call back which cannot be async. Otherwise, it can be used as often as you'd like because repainting is cheap.

void updateProfile(String name) {
 setState(() => this.name = name);
}

8. deactivate()

Deactivate is called when State is removed from the tree, but it might be reinserted before the current frame change is finished. This method exists basically because State objects can be moved from one point in a tree to another.

This is rarely used.

9. dispose()

Dispose is called when the State object is removed, which is permanent.

This method is where you should unsubscribe and cancel all animations, streams, etc.

10. mounted is false

The state object can never remount, and an error is thrown is setState is called.