More PageStorage clarity in the documentation (#131954)

Fixes flutter/flutter#10867

Author: Hixie

packages/flutter/lib/src/material/expansion_tile.dart

@@ -184,9 +184,9 @@ class ExpansionTileController {
 /// A single-line [ListTile] with an expansion arrow icon that expands or collapses
 /// the tile to reveal or hide the [children].
 ///
-/// This widget is typically used with [ListView] to create an
-/// "expand / collapse" list entry. When used with scrolling widgets like
-/// [ListView], a unique [PageStorageKey] must be specified to enable the
+/// This widget is typically used with [ListView] to create an "expand /
+/// collapse" list entry. When used with scrolling widgets like [ListView], a
+/// unique [PageStorageKey] must be specified as the [key], to enable the
 /// [ExpansionTile] to save and restore its expanded state when it is scrolled
 /// in and out of view.
 ///

packages/flutter/lib/src/material/paginated_data_table.dart

@@ -28,6 +28,9 @@ import 'theme.dart';
 /// Data is read lazily from a [DataTableSource]. The widget is presented
 /// as a [Card].
 ///
+/// If the [key] is a [PageStorageKey], the [initialFirstRowIndex] is persisted
+/// to [PageStorage].
+///
 /// See also:
 ///
 ///  * [DataTable], which is not paginated.

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

@@ -9,17 +9,19 @@ import 'framework.dart';
 // Examples can assume:
 // late BuildContext context;
 
-/// A key can be used to persist the widget state in storage after
-/// the destruction and will be restored when recreated.
+/// A [Key] that can be used to persist the widget state in storage after the
+/// destruction and will be restored when recreated.
 ///
-/// Each key with its value plus the ancestor chain of other PageStorageKeys need to
-/// be unique within the widget's closest ancestor [PageStorage]. To make it possible for a
-/// saved value to be found when a widget is recreated, the key's value must
-/// not be objects whose identity will change each time the widget is created.
+/// Each key with its value plus the ancestor chain of other [PageStorageKey]s
+/// need to be unique within the widget's closest ancestor [PageStorage]. To
+/// make it possible for a saved value to be found when a widget is recreated,
+/// the key's value must not be objects whose identity will change each time the
+/// widget is created.
 ///
 /// See also:
 ///
-///  * [PageStorage], which is the closet ancestor for [PageStorageKey].
+///  * [PageStorage], which manages the data storage for widgets using
+///    [PageStorageKey]s.
 class PageStorageKey<T> extends ValueKey<T> {
   /// Creates a [ValueKey] that defines where [PageStorage] values will be saved.
   const PageStorageKey(super.value);
@@ -136,12 +138,12 @@ class PageStorageBucket {
 /// included in routes.
 ///
 /// [PageStorageKey] is used by [Scrollable] if [ScrollController.keepScrollOffset]
-/// is enabled to save their [ScrollPosition]s. When more than one
-/// scrollable ([ListView], [SingleChildScrollView], [TextField], etc.) appears
-/// within the widget's closest ancestor [PageStorage] (such as within the same route),
-/// if you want to save all of their positions independently,
-/// you should give each of them unique [PageStorageKey]s, or set some of their
-/// `keepScrollOffset` false to prevent saving.
+/// is enabled to save their [ScrollPosition]s. When more than one scrollable
+/// ([ListView], [SingleChildScrollView], [TextField], etc.) appears within the
+/// widget's closest ancestor [PageStorage] (such as within the same route), to
+/// save all of their positions independently, one must give each of them unique
+/// [PageStorageKey]s, or set the `keepScrollOffset` property of some such
+/// widgets to false to prevent saving.
 ///
 /// {@tool dartpad}
 /// This sample shows how to explicitly use a [PageStorage] to

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

@@ -612,6 +612,14 @@ const PageScrollPhysics _kPagePhysics = PageScrollPhysics();
 /// ** See code in examples/api/lib/widgets/page_view/page_view.0.dart **
 /// {@end-tool}
 ///
+/// ## Persisting the scroll position during a session
+///
+/// Scroll views attempt to persist their scroll position using [PageStorage].
+/// For a [PageView], this can be disabled by setting [PageController.keepPage]
+/// to false on the [controller]. If it is enabled, using a [PageStorageKey] for
+/// the [key] of this widget is recommended to help disambiguate different
+/// scroll views from each other.
+///
 /// See also:
 ///
 ///  * [PageController], which controls which page is visible in the view.

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

@@ -178,8 +178,6 @@ enum ScrollPositionAlignmentPolicy {
 abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   /// Creates an object that determines which portion of the content is visible
   /// in a scroll view.
-  ///
-  /// The [physics], [context], and [keepScrollOffset] parameters must not be null.
   ScrollPosition({
     required this.physics,
     required this.context,

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

@@ -61,6 +61,16 @@ enum ScrollViewKeyboardDismissBehavior {
 /// To control the initial scroll offset of the scroll view, provide a
 /// [controller] with its [ScrollController.initialScrollOffset] property set.
 ///
+/// {@template flutter.widgets.ScrollView.PageStorage}
+/// ## Persisting the scroll position during a session
+///
+/// Scroll views attempt to persist their scroll position using [PageStorage].
+/// This can be disabled by setting [ScrollController.keepScrollOffset] to false
+/// on the [controller]. If it is enabled, using a [PageStorageKey] for the
+/// [key] of this widget is recommended to help disambiguate different scroll
+/// views from each other.
+/// {@endtemplate}
+///
 /// See also:
 ///
 ///  * [ListView], which is a commonly used [ScrollView] that displays a
@@ -619,6 +629,8 @@ abstract class ScrollView extends StatelessWidget {
 /// parameter `semanticChildCount`. This should always be the same as the
 /// number of widgets wrapped in [IndexedSemantics].
 ///
+/// {@macro flutter.widgets.ScrollView.PageStorage}
+///
 /// See also:
 ///
 ///  * [SliverList], which is a sliver that displays linear list of children.
@@ -1165,6 +1177,8 @@ abstract class BoxScrollView extends ScrollView {
 ///
 /// {@macro flutter.widgets.BoxScroll.scrollBehaviour}
 ///
+/// {@macro flutter.widgets.ScrollView.PageStorage}
+///
 /// See also:
 ///
 ///  * [SingleChildScrollView], which is a scrollable widget that has a single
@@ -1810,6 +1824,8 @@ class ListView extends BoxScrollView {
 /// ** See code in examples/api/lib/widgets/scroll_view/list_view.0.dart **
 /// {@end-tool}
 ///
+/// {@macro flutter.widgets.ScrollView.PageStorage}
+///
 /// See also:
 ///
 ///  * [SingleChildScrollView], which is a scrollable widget that has a single

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

@@ -72,6 +72,14 @@ typedef TwoDimensionalViewportBuilder = Widget Function(BuildContext context, Vi
 ///    [PageController], which creates a page-oriented scroll position subclass
 ///    that keeps the same page visible when the [Scrollable] resizes.
 ///
+/// ## Persisting the scroll position during a session
+///
+/// Scrollables attempt to persist their scroll position using [PageStorage].
+/// This can be disabled by setting [ScrollController.keepScrollOffset] to false
+/// on the [controller]. If it is enabled, using a [PageStorageKey] for the
+/// [key] of this widget (or one of its ancestors, e.g. a [ScrollView]) is
+/// recommended to help disambiguate different [Scrollable]s from each other.
+///
 /// See also:
 ///
 ///  * [ListView], which is a commonly used [ScrollView] that displays a

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

@@ -130,6 +130,8 @@ import 'scrollable.dart';
 /// ** See code in examples/api/lib/widgets/single_child_scroll_view/single_child_scroll_view.1.dart **
 /// {@end-tool}
 ///
+/// {@macro flutter.widgets.ScrollView.PageStorage}
+///
 /// See also:
 ///
 ///  * [ListView], which handles multiple children in a scrolling list.