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Document additional cases (#137957)

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Fixes https://github.com/flutter/flutter/issues/137924
Fixes https://github.com/flutter/flutter/issues/136885

The framework defaults to keeping the widget tree alive and ready to render regardless of application lifecycle events such as showing or hiding a view controller.

Add-to-app developers may be surprised by this, and tend to file bugs asking why `dispose` isn't called when the view is dismissed (or why memory usage is higher than expected after dismissing Flutter UI).

Adds documentation to explain the limitation and what to do instead.

/cc @zanderso @chinmaygarde @jonahwilliams @jason-simmons since we were discussing this in triage.
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Dan Field 2023-11-08 11:10:22 -08:00 committed by GitHub
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2 changed files with 30 additions and 6 deletions
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15
packages/flutter/lib/src/widgets/binding.dart
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@ -1184,6 +1184,21 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
/// To listen for platform shutdown messages (and other lifecycle changes),
/// consider the [AppLifecycleListener] API.
///
/// ## Dismissing Flutter UI via platform native methods
///
/// {@template flutter.widgets.runApp.dismissal}
/// An application may have both Flutter and non-Flutter UI in it. If the
/// application calls non-Flutter methods to remove Flutter based UI such as
/// platform native API to manipulate the platform native navigation stack,
/// the framework does not know if the developer intends to eagerly free
/// resources or not. The widget tree remains mounted and ready to render
/// as soon as it is displayed again.
///
/// To release resources more eagerly, establish a [platform channel](https://flutter.dev/platform-channels/)
/// and use it to call [runApp] with a widget such as [SizedBox.shrink] when
/// the framework should dispose of the active widget tree.
/// {@endtemplate}
///
/// See also:
///
/// * [WidgetsBinding.attachRootWidget], which creates the root widget for the

21
packages/flutter/lib/src/widgets/framework.dart
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@ -1303,13 +1303,18 @@ abstract class State<T extends StatefulWidget> with Diagnosticable {
/// Implementations of this method should end with a call to the inherited
/// method, as in `super.dispose()`.
///
/// ## Application shutdown
/// ## Caveats
///
/// This method is _not_ invoked when the application shuts down, because
/// there is no way to predict when that will happen. For example, a user's
/// battery could catch fire, or the user could drop the device into a
/// swimming pool, or the operating system could unilaterally terminate the
/// application process due to memory pressure.
/// This method is _not_ invoked at times where a developer might otherwise
/// expect it, such as application shutdown or dismissal via platform
/// native methods.
///
/// ### Application shutdown
///
/// There is no way to predict when application shutdown will happen. For
/// example, a user's battery could catch fire, or the user could drop the
/// device into a swimming pool, or the operating system could unilaterally
/// terminate the application process due to memory pressure.
///
/// Applications are responsible for ensuring that they are well-behaved
/// even in the face of a rapid unscheduled termination.
@ -1320,6 +1325,10 @@ abstract class State<T extends StatefulWidget> with Diagnosticable {
/// To listen for platform shutdown messages (and other lifecycle changes),
/// consider the [AppLifecycleListener] API.
///
/// ### Dismissing Flutter UI via platform native methods
///
/// {@macro flutter.widgets.runApp.dismissal}
///
/// See also:
///
/// * [deactivate], which is called prior to [dispose].