Add more documentation around keep alive (flutter#168311)

Should
- fix flutter#153860
- fix flutter#146612
- fix flutter#20112


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we expect every widget to implement].
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- [x] I updated/added relevant documentation (doc comments with `///`).
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---------

Co-authored-by: Victor Sanni <[email protected]>

Author: ValentinVignal

packages/flutter/lib/src/widgets/automatic_keep_alive.dart

@@ -27,13 +27,23 @@ import 'sliver.dart';
 ///
 /// To send these notifications, consider using [AutomaticKeepAliveClientMixin].
 ///
+/// The [SliverChildBuilderDelegate] and [SliverChildListDelegate] delegates,
+/// used with [SliverList] and [SliverGrid], as well as the scroll view
+/// counterparts [ListView] and [GridView], have an `addAutomaticKeepAlives`
+/// feature, which is enabled by default. This feature inserts
+/// [AutomaticKeepAlive] widgets around each child, which in turn configure
+/// [KeepAlive] widgets in response to [KeepAliveNotification]s.
+///
+/// The same `addAutomaticKeepAlives` feature is supported by
+/// [TwoDimensionalChildBuilderDelegate] and [TwoDimensionalChildListDelegate].
+///
 /// {@tool dartpad}
 /// This sample demonstrates how to use the [AutomaticKeepAlive] widget in
 /// combination with the [AutomaticKeepAliveClientMixin] to selectively preserve
 /// the state of individual items in a scrollable list.
 ///
 /// Normally, widgets in a lazily built list like [ListView.builder] are
-/// disposed of when they leave the visible area to save resources. This means
+/// disposed of when they leave the visible area to maintain performance. This means
 /// that any state inside a [StatefulWidget] would be lost unless explicitly
 /// preserved.
 ///
@@ -50,6 +60,13 @@ import 'sliver.dart';
 /// ** See code in examples/api/lib/widgets/keep_alive/automatic_keep_alive.0.dart **
 /// {@end-tool}
 ///
+/// See also:
+///
+///  * [AutomaticKeepAliveClientMixin], which is a mixin with convenience
+///    methods for clients of [AutomaticKeepAlive]. Used with [State]
+///    subclasses.
+///  * [KeepAlive] which marks a child as needing to stay alive even when it's
+///    in a lazy list that would otherwise remove it.
 class AutomaticKeepAlive extends StatefulWidget {
   /// Creates a widget that listens to [KeepAliveNotification]s and maintains a
   /// [KeepAlive] widget appropriately.
@@ -354,8 +371,19 @@ class KeepAliveHandle extends ChangeNotifier {
   }
 }
 
-/// A mixin with convenience methods for clients of [AutomaticKeepAlive]. Used
-/// with [State] subclasses.
+/// A mixin with convenience methods for clients of [AutomaticKeepAlive]. It is used
+/// with [State] subclasses to manage keep-alive behavior in lazily built lists.
+///
+/// This mixin simplifies interaction with [AutomaticKeepAlive] by automatically
+/// sending [KeepAliveNotification]s when necessary. Subclasses must implement
+/// [wantKeepAlive] to indicate whether the widget should be kept alive and call
+/// [updateKeepAlive] whenever its value changes.
+///
+/// The mixin internally manages a [KeepAliveHandle], which is used to notify
+/// the nearest [AutomaticKeepAlive] ancestor of changes in keep-alive
+/// requirements. [AutomaticKeepAlive] listens for [KeepAliveNotification]s sent
+/// by this mixin and dynamically wraps the subtree in a [KeepAlive] widget to
+/// preserve its state when it is no longer visible in the viewport.
 ///
 /// Subclasses must implement [wantKeepAlive], and their [build] methods must
 /// call `super.build` (though the return value should be ignored).
@@ -366,9 +394,20 @@ class KeepAliveHandle extends ChangeNotifier {
 /// The type argument `T` is the type of the [StatefulWidget] subclass of the
 /// [State] into which this class is being mixed.
 ///
+/// The [SliverChildBuilderDelegate] and [SliverChildListDelegate] delegates,
+/// used with [SliverList] and [SliverGrid], as well as the scroll view
+/// counterparts [ListView] and [GridView], have an `addAutomaticKeepAlives`
+/// feature, which is enabled by default. This feature inserts
+/// [AutomaticKeepAlive] widgets around each child, which in turn configure
+/// [KeepAlive] widgets in response to [KeepAliveNotification]s.
+///
+/// The same `addAutomaticKeepAlives` feature is supported by
+/// [TwoDimensionalChildBuilderDelegate] and [TwoDimensionalChildListDelegate].
+///
 /// {@tool dartpad}
-/// This example demonstrates how to use the [AutomaticKeepAliveClientMixin]
-/// to keep the state of a widget alive even when it is scrolled out of view.
+/// This example demonstrates how to use the
+/// [AutomaticKeepAliveClientMixin] to keep the state of a widget alive even
+/// when it is scrolled out of view.
 ///
 /// ** See code in examples/api/lib/widgets/keep_alive/automatic_keep_alive_client_mixin.0.dart **
 /// {@end-tool}
@@ -377,6 +416,8 @@ class KeepAliveHandle extends ChangeNotifier {
 ///
 ///  * [AutomaticKeepAlive], which listens to messages from this mixin.
 ///  * [KeepAliveNotification], the notifications sent by this mixin.
+///  * [KeepAlive] which marks a child as needing to stay alive even when it's
+///    in a lazy list that would otherwise remove it.
 @optionalTypeArgs
 mixin AutomaticKeepAliveClientMixin<T extends StatefulWidget> on State<T> {
   KeepAliveHandle? _keepAliveHandle;

packages/flutter/lib/src/widgets/scroll_delegate.dart

@@ -413,6 +413,60 @@ class SliverChildBuilderDelegate extends SliverChildDelegate {
   /// none of the children will ever try to keep themselves alive.
   ///
   /// Defaults to true.
+  ///
+  /// {@tool dartpad}
+  /// This sample demonstrates how to use the [AutomaticKeepAlive] widget in
+  /// combination with the [AutomaticKeepAliveClientMixin] to selectively preserve
+  /// the state of individual items in a scrollable list.
+  ///
+  /// Normally, widgets in a lazily built list like [ListView.builder] are
+  /// disposed of when they leave the visible area to maintain performance. This means
+  /// that any state inside a [StatefulWidget] would be lost unless explicitly
+  /// preserved.
+  ///
+  /// In this example, each list item is a [StatefulWidget] that includes a
+  /// counter and an increment button. To preserve the state of selected items
+  /// (based on their index), the [AutomaticKeepAlive] widget and
+  /// [AutomaticKeepAliveClientMixin] are used:
+  ///
+  /// - The `wantKeepAlive` getter in the item’s state class returns true for
+  ///   even-indexed items, indicating that their state should be preserved.
+  /// - For odd-indexed items, `wantKeepAlive` returns false, so their state is
+  ///   not preserved when scrolled out of view.
+  ///
+  /// ** See code in examples/api/lib/widgets/keep_alive/automatic_keep_alive.0.dart **
+  /// {@end-tool}
+  ///
+  /// {@tool dartpad}
+  /// This sample demonstrates how to use the [KeepAlive] widget
+  /// to preserve the state of individual list items in a [ListView] when they are
+  /// scrolled out of view.
+  ///
+  /// By default, [ListView.builder] only keeps the widgets currently visible in
+  /// the viewport alive. When an item scrolls out of view, it may be disposed to
+  /// free up resources. This can cause the state of [StatefulWidget]s to be lost
+  /// if not explicitly preserved.
+  ///
+  /// In this example, each item in the list is a [StatefulWidget] that maintains
+  /// a counter. Tapping the "+" button increments the counter. To selectively
+  /// preserve the state, each item is wrapped in a [KeepAlive] widget, with the
+  /// keepAlive parameter set based on the item’s index:
+  ///
+  /// - For even-indexed items, `keepAlive: true`, so their state is preserved
+  ///   even when scrolled off-screen.
+  /// - For odd-indexed items, `keepAlive: false`, so their state is discarded
+  ///   when they are no longer visible.
+  ///
+  /// ** See code in examples/api/lib/widgets/keep_alive/keep_alive.0.dart **
+  /// {@end-tool}
+  ///
+  ///  * [AutomaticKeepAlive], which allows subtrees to request to be kept alive
+  ///    in lazy lists.
+  ///  * [AutomaticKeepAliveClientMixin], which is a mixin with convenience
+  ///    methods for clients of [AutomaticKeepAlive]. Used with [State]
+  ///    subclasses.
+  ///  * [KeepAlive] which marks a child as needing to stay alive even when it's
+  ///    in a lazy list that would otherwise remove it.
   /// {@endtemplate}
   final bool addAutomaticKeepAlives;
 

packages/flutter/lib/src/widgets/sliver.dart

@@ -1444,26 +1444,24 @@ class _SliverOffstageElement extends SingleChildRenderObjectElement {
 /// Mark a child as needing to stay alive even when it's in a lazy list that
 /// would otherwise remove it.
 ///
-/// This widget is for use in a [RenderAbstractViewport]s, such as
-/// [Viewport] or [TwoDimensionalViewport].
-///
-/// This widget is rarely used directly. The [SliverChildBuilderDelegate] and
-/// [SliverChildListDelegate] delegates, used with [SliverList] and
-/// [SliverGrid], as well as the scroll view counterparts [ListView] and
-/// [GridView], have an `addAutomaticKeepAlives` feature, which is enabled by
-/// default, and which causes [AutomaticKeepAlive] widgets to be inserted around
-/// each child, causing [KeepAlive] widgets to be automatically added and
-/// configured in response to [KeepAliveNotification]s.
-///
-/// The same `addAutomaticKeepAlives` feature is supported by the
+/// This widget is used in [RenderAbstractViewport]s, such as [Viewport] or
+/// [TwoDimensionalViewport], to manage the lifecycle of widgets that need to
+/// remain alive even when scrolled out of view.
+///
+/// The [SliverChildBuilderDelegate] and [SliverChildListDelegate] delegates,
+/// used with [SliverList] and [SliverGrid], as well as the scroll view
+/// counterparts [ListView] and [GridView], have an `addAutomaticKeepAlives`
+/// feature, which is enabled by default. This feature inserts
+/// [AutomaticKeepAlive] widgets around each child, which in turn configure
+/// [KeepAlive] widgets in response to [KeepAliveNotification]s.
+///
+/// The same `addAutomaticKeepAlives` feature is supported by
 /// [TwoDimensionalChildBuilderDelegate] and [TwoDimensionalChildListDelegate].
 ///
-/// Therefore, to keep a widget alive, it is more common to use those
-/// notifications than to directly deal with [KeepAlive] widgets.
-///
-/// In practice, the simplest way to deal with these notifications is to mix
-/// [AutomaticKeepAliveClientMixin] into one's [State]. See the documentation
-/// for that mixin class for details.
+/// Keep-alive behavior can be managed by using [KeepAlive] directly or by
+/// relying on notifications. For convenience, [AutomaticKeepAliveClientMixin]
+/// may be mixed into a [State] subclass. Further details are available in the
+/// documentation for [AutomaticKeepAliveClientMixin].
 ///
 /// {@tool dartpad}
 /// This sample demonstrates how to use the [KeepAlive] widget
@@ -1488,6 +1486,13 @@ class _SliverOffstageElement extends SingleChildRenderObjectElement {
 /// ** See code in examples/api/lib/widgets/keep_alive/keep_alive.0.dart **
 /// {@end-tool}
 ///
+/// See also:
+///
+///  * [AutomaticKeepAlive], which allows subtrees to request to be kept alive
+///    in lazy lists.
+///  * [AutomaticKeepAliveClientMixin], which is a mixin with convenience
+///    methods for clients of [AutomaticKeepAlive]. Used with [State]
+///    subclasses.
 class KeepAlive extends ParentDataWidget<KeepAliveParentDataMixin> {
   /// Marks a child as needing to remain alive.
   const KeepAlive({super.key, required this.keepAlive, required super.child});