1 framework.dart中BuildConttext源码
/// The [BuildContext] for a particular widget can change location over time as
/// the widget is moved around the tree. Because of this, values returned from
/// the methods on this class should not be cached beyond the execution of a
/// single synchronous function.
///
/// [BuildContext] objects are actually [Element] objects. The [BuildContext]
/// interface is used to discourage direct manipulation of [Element] objects.
abstract class BuildContext {
/// The current configuration of the [Element] that is this [BuildContext].
Widget get widget;
/// The [BuildOwner] for this context. The [BuildOwner] is in charge of
/// managing the rendering pipeline for this context.
BuildOwner? get owner;
/// Whether the [widget] is currently updating the widget or render tree.
///
/// For [StatefulWidget]s and [StatelessWidget]s this flag is true while
/// their respective build methods are executing.
/// [RenderObjectWidget]s set this to true while creating or configuring their
/// associated [RenderObject]s.
/// Other [Widget] types may set this to true for conceptually similar phases
/// of their lifecycle.
///
/// When this is true, it is safe for [widget] to establish a dependency to an
/// [InheritedWidget] by calling [dependOnInheritedElement] or
/// [dependOnInheritedWidgetOfExactType].
///
/// Accessing this flag in release mode is not valid.
bool get debugDoingBuild;
/// The current [RenderObject] for the widget. If the widget is a
/// [RenderObjectWidget], this is the render object that the widget created
/// for itself. Otherwise, it is the render object of the first descendant
/// [RenderObjectWidget].
///
/// This method will only return a valid result after the build phase is
/// complete. It is therefore not valid to call this from a build method.
/// It should only be called from interaction event handlers (e.g.
/// gesture callbacks) or layout or paint callbacks.
///
/// If the render object is a [RenderBox], which is the common case, then the
/// size of the render object can be obtained from the [size] getter. This is
/// only valid after the layout phase, and should therefore only be examined
/// from paint callbacks or interaction event handlers (e.g. gesture
/// callbacks).
///
/// For details on the different phases of a frame, see the discussion at
/// [WidgetsBinding.drawFrame].
///
/// Calling this method is theoretically relatively expensive (O(N) in the
/// depth of the tree), but in practice is usually cheap because the tree
/// usually has many render objects and therefore the distance to the nearest
/// render object is usually short.
RenderObject? findRenderObject();
/// The size of the [RenderBox] returned by [findRenderObject].
///
/// This getter will only return a valid result after the layout phase is
/// complete. It is therefore not valid to call this from a build method.
/// It should only be called from paint callbacks or interaction event
/// handlers (e.g. gesture callbacks).
///
/// For details on the different phases of a frame, see the discussion at
/// [WidgetsBinding.drawFrame].
///
/// This getter will only return a valid result if [findRenderObject] actually
/// returns a [RenderBox]. If [findRenderObject] returns a render object that
/// is not a subtype of [RenderBox] (e.g., [RenderView]), this getter will
/// throw an exception in checked mode and will return null in release mode.
///
/// Calling this getter is theoretically relatively expensive (O(N) in the
/// depth of the tree), but in practice is usually cheap because the tree
/// usually has many render objects and therefore the distance to the nearest
/// render object is usually short.
Size? get size;
/// Registers this build context with [ancestor] such that when
/// [ancestor]'s widget changes this build context is rebuilt.
///
/// Returns `ancestor.widget`.
///
/// This method is rarely called directly. Most applications should use
/// [dependOnInheritedWidgetOfExactType], which calls this method after finding
/// the appropriate [InheritedElement] ancestor.
///
/// All of the qualifications about when [dependOnInheritedWidgetOfExactType] can
/// be called apply to this method as well.
InheritedWidget dependOnInheritedElement(InheritedElement ancestor, { Object aspect });
/// Obtains the nearest widget of the given type `T`, which must be the type of a
/// concrete [InheritedWidget] subclass, and registers this build context with
/// that widget such that when that widget changes (or a new widget of that
/// type is introduced, or the widget goes away), this build context is
/// rebuilt so that it can obtain new values from that widget.
///
/// This is typically called implicitly from `of()` static methods, e.g.
/// [Theme.of].
///
/// This method should not be called from widget constructors or from
/// [State.initState] methods, because those methods would not get called
/// again if the inherited value were to change. To ensure that the widget
/// correctly updates itself when the inherited value changes, only call this
/// (directly or indirectly) from build methods, layout and paint callbacks, or
/// from [State.didChangeDependencies].
///
/// This method should not be called from [State.dispose] because the element
/// tree is no longer stable at that time. To refer to an ancestor from that
/// method, save a reference to the ancestor in [State.didChangeDependencies].
/// It is safe to use this method from [State.deactivate], which is called
/// whenever the widget is removed from the tree.
///
/// It is also possible to call this method from interaction event handlers
/// (e.g. gesture callbacks) or timers, to obtain a value once, if that value
/// is not going to be cached and reused later.
///
/// Calling this method is O(1) with a small constant factor, but will lead to
/// the widget being rebuilt more often.
///
/// Once a widget registers a dependency on a particular type by calling this
/// method, it will be rebuilt, and [State.didChangeDependencies] will be
/// called, whenever changes occur relating to that widget until the next time
/// the widget or one of its ancestors is moved (for example, because an
/// ancestor is added or removed).
///
/// The [aspect] parameter is only used when `T` is an
/// [InheritedWidget] subclasses that supports partial updates, like
/// [InheritedModel]. It specifies what "aspect" of the inherited
/// widget this context depends on.
T? dependOnInheritedWidgetOfExactType<T extends InheritedWidget>({ Object? aspect });
/// Obtains the element corresponding to the nearest widget of the given type `T`,
/// which must be the type of a concrete [InheritedWidget] subclass.
///
/// Returns null if no such element is found.
///
/// Calling this method is O(1) with a small constant factor.
///
/// This method does not establish a relationship with the target in the way
/// that [dependOnInheritedWidgetOfExactType] does.
///
/// This method should not be called from [State.dispose] because the element
/// tree is no longer stable at that time. To refer to an ancestor from that
/// method, save a reference to the ancestor by calling
/// [dependOnInheritedWidgetOfExactType] in [State.didChangeDependencies]. It is
/// safe to use this method from [State.deactivate], which is called whenever
/// the widget is removed from the tree.
InheritedElement? getElementForInheritedWidgetOfExactType<T extends InheritedWidget>();
/// Returns the nearest ancestor widget of the given type `T`, which must be the
/// type of a concrete [Widget] subclass.
///
/// In general, [dependOnInheritedWidgetOfExactType] is more useful, since
/// inherited widgets will trigger consumers to rebuild when they change. This
/// method is appropriate when used in interaction event handlers (e.g.
/// gesture callbacks) or for performing one-off tasks such as asserting that
/// you have or don't have a widget of a specific type as an ancestor. The
/// return value of a Widget's build method should not depend on the value
/// returned by this method, because the build context will not rebuild if the
/// return value of this method changes. This could lead to a situation where
/// data used in the build method changes, but the widget is not rebuilt.
///
/// Calling this method is relatively expensive (O(N) in the depth of the
/// tree). Only call this method if the distance from this widget to the
/// desired ancestor is known to be small and bounded.
///
/// This method should not be called from [State.deactivate] or [State.dispose]
/// because the widget tree is no longer stable at that time. To refer to
/// an ancestor from one of those methods, save a reference to the ancestor
/// by calling [findAncestorWidgetOfExactType] in [State.didChangeDependencies].
///
/// Returns null if a widget of the requested type does not appear in the
/// ancestors of this context.
T? findAncestorWidgetOfExactType<T extends Widget>();
/// Returns the [State] object of the nearest ancestor [StatefulWidget] widget
/// that is an instance of the given type `T`.
///
/// This should not be used from build methods, because the build context will
/// not be rebuilt if the value that would be returned by this method changes.
/// In general, [dependOnInheritedWidgetOfExactType] is more appropriate for such
/// cases. This method is useful for changing the state of an ancestor widget in
/// a one-off manner, for example, to cause an ancestor scrolling list to
/// scroll this build context's widget into view, or to move the focus in
/// response to user interaction.
///
/// In general, though, consider using a callback that triggers a stateful
/// change in the ancestor rather than using the imperative style implied by
/// this method. This will usually lead to more maintainable and reusable code
/// since it decouples widgets from each other.
///
/// Calling this method is relatively expensive (O(N) in the depth of the
/// tree). Only call this method if the distance from this widget to the
/// desired ancestor is known to be small and bounded.
///
/// This method should not be called from [State.deactivate] or [State.dispose]
/// because the widget tree is no longer stable at that time. To refer to
/// an ancestor from one of those methods, save a reference to the ancestor
/// by calling [findAncestorStateOfType] in [State.didChangeDependencies].
///
/// {@tool snippet}
///
/// ```dart
/// ScrollableState? scrollable = context.findAncestorStateOfType<ScrollableState>();
/// ```
/// {@end-tool}
T? findAncestorStateOfType<T extends State>();
/// Returns the [State] object of the furthest ancestor [StatefulWidget] widget
/// that is an instance of the given type `T`.
///
/// Functions the same way as [findAncestorStateOfType] but keeps visiting subsequent
/// ancestors until there are none of the type instance of `T` remaining.
/// Then returns the last one found.
///
/// This operation is O(N) as well though N is the entire widget tree rather than
/// a subtree.
T? findRootAncestorStateOfType<T extends State>();
/// Returns the [RenderObject] object of the nearest ancestor [RenderObjectWidget] widget
/// that is an instance of the given type `T`.
///
/// This should not be used from build methods, because the build context will
/// not be rebuilt if the value that would be returned by this method changes.
/// In general, [dependOnInheritedWidgetOfExactType] is more appropriate for such
/// cases. This method is useful only in esoteric cases where a widget needs
/// to cause an ancestor to change its layout or paint behavior. For example,
/// it is used by [Material] so that [InkWell] widgets can trigger the ink
/// splash on the [Material]'s actual render object.
///
/// Calling this method is relatively expensive (O(N) in the depth of the
/// tree). Only call this method if the distance from this widget to the
/// desired ancestor is known to be small and bounded.
///
/// This method should not be called from [State.deactivate] or [State.dispose]
/// because the widget tree is no longer stable at that time. To refer to
/// an ancestor from one of those methods, save a reference to the ancestor
/// by calling [findAncestorRenderObjectOfType] in [State.didChangeDependencies].
T? findAncestorRenderObjectOfType<T extends RenderObject>();
/// Walks the ancestor chain, starting with the parent of this build context's
/// widget, invoking the argument for each ancestor. The callback is given a
/// reference to the ancestor widget's corresponding [Element] object. The
/// walk stops when it reaches the root widget or when the callback returns
/// false. The callback must not return null.
///
/// This is useful for inspecting the widget tree.
///
/// Calling this method is relatively expensive (O(N) in the depth of the tree).
///
/// This method should not be called from [State.deactivate] or [State.dispose]
/// because the element tree is no longer stable at that time. To refer to
/// an ancestor from one of those methods, save a reference to the ancestor
/// by calling [visitAncestorElements] in [State.didChangeDependencies].
void visitAncestorElements(bool visitor(Element element));
/// Walks the children of this widget.
///
/// This is useful for applying changes to children after they are built
/// without waiting for the next frame, especially if the children are known,
/// and especially if there is exactly one child (as is always the case for
/// [StatefulWidget]s or [StatelessWidget]s).
///
/// Calling this method is very cheap for build contexts that correspond to
/// [StatefulWidget]s or [StatelessWidget]s (O(1), since there's only one
/// child).
///
/// Calling this method is potentially expensive for build contexts that
/// correspond to [RenderObjectWidget]s (O(N) in the number of children).
///
/// Calling this method recursively is extremely expensive (O(N) in the number
/// of descendants), and should be avoided if possible. Generally it is
/// significantly cheaper to use an [InheritedWidget] and have the descendants
/// pull data down, than it is to use [visitChildElements] recursively to push
/// data down to them.
void visitChildElements(ElementVisitor visitor);
/// Returns a description of an [Element] from the current build context.
DiagnosticsNode describeElement(String name, {DiagnosticsTreeStyle style = DiagnosticsTreeStyle.errorProperty});
/// Returns a description of the [Widget] associated with the current build context.
DiagnosticsNode describeWidget(String name, {DiagnosticsTreeStyle style = DiagnosticsTreeStyle.errorProperty});
/// Adds a description of a specific type of widget missing from the current
/// build context's ancestry tree.
///
/// You can find an example of using this method in [debugCheckHasMaterial].
List<DiagnosticsNode> describeMissingAncestor({ required Type expectedAncestorType });
/// Adds a description of the ownership chain from a specific [Element]
/// to the error report.
///
/// The ownership chain is useful for debugging the source of an element.
DiagnosticsNode describeOwnershipChain(String name);
}
从中摘选该行：
/// [BuildContext] objects are actually [Element] objects.
这句备注明确描述了 BuildContext 实际上 就是 Element 对象。
下一句：
///The [BuildContext] interface is used to discourage direct manipulation of [Element] objects.
BuildContext 被设计的原因就是为了不鼓励对 Element 对象进行直接操作。
例如：
在 StatefulWidget 中状态发生改变时，需要调用：
setState(() {
});
更新状态，本质上 context 就是 Element ，因此原理上可以写成：
(context as Element).markNeedsBuild();
创造 BuildContext 接口的原因就是不鼓励上面这种直接对 Element 对象操作的写法。
2 context的其他应用
2.1 Theme调用context
Theme.of(context).textTheme;
打开theme.dart中 of 方法的源码：
/// When the [Theme] is actually created in the same `build` function
/// (possibly indirectly, e.g. as part of a [MaterialApp]), the `context`
/// argument to the `build` function can't be used to find the [Theme] (since
/// it's "above" the widget being returned). In such cases, the following
/// technique with a [Builder] can be used to provide a new scope with a
/// [BuildContext] that is "under" the [Theme]:
///
/// ```dart
/// @override
/// Widget build(BuildContext context) {
/// return MaterialApp(
/// theme: ThemeData.light(),
/// body: Builder(
/// // Create an inner BuildContext so that we can refer to
/// // the Theme with Theme.of().
/// builder: (BuildContext context) {
/// return Center(
/// child: Text(
/// 'Example',
/// style: Theme.of(context).textTheme.headline6,
/// ),
/// );
/// },
/// ),
/// );
/// }
/// ```
static ThemeData of(BuildContext context) {
final _InheritedTheme? inheritedTheme = context.dependOnInheritedWidgetOfExactType<_InheritedTheme>();
final MaterialLocalizations? localizations = Localizations.of<MaterialLocalizations>(context, MaterialLocalizations);
final ScriptCategory category = localizations?.scriptCategory ?? ScriptCategory.englishLike;
final ThemeData theme = inheritedTheme?.theme.data ?? _kFallbackTheme;
return ThemeData.localize(theme, theme.typography.geometryThemeFor(category));
}
从源码中可以看出，对于 ThemeData 调用了context.dependOnInheritedWidgetOfExactType<_InheritedTheme>()方法，也就是从调用该方法的位置出发，向根部查找，找到最近的类型为_InheritedTheme的 Widget 并返回。
在of(context)中的体现就是从调用 of 方法的位置向根部查找，找到最近的 theme 并把 theme 返回。
由于 context 就是 Element，所以也可以写成（不推荐）：
(context as Element).dependOnInheritedWidgetOfExactType();
2.2 Scaffold调用context
Scaffold.of(context).initState();
查看scaffold。dart中 of 方法的源码：
/// Finds the [ScaffoldState] from the closest instance of this class that
/// encloses the given context.
///
/// If no instance of this class encloses the given context, will return null.
/// To throw an exception instead, use [of] instead of this function.
///
/// See also:
///
/// * [of], a similar function to this one that throws if no instance
/// encloses the given context. Also includes some sample code in its
/// documentation.
static ScaffoldState? maybeOf(BuildContext context) {
assert(context != null);
return context.findAncestorStateOfType<ScaffoldState>();
}
与上面 Theme 相同，这里调用context.findAncestorStateOfType<ScaffoldState>()方法，返回一个ScaffoldState类型的Widget。
父 Widget 中没有 Scaffold 情况：
class HomePage extends StatefulWidget {
const HomePage({Key key}) : super(key: key);
@override
_HomePageState createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
@override
Widget build(BuildContext context) {
return Scaffold(
drawer: Drawer(),
floatingActionButton: FloatingActionButton(
onPressed: () {
Scaffold.of(context).openDrawer();
},
),
);
}
}
如上代码在floatingActionButton中通过调用Scaffold.of(context).openDrawer()点击打开抽屉drawer，此处会报错，因为上级 Widget 并没有 Scaffold ，可以通过LayoutBuilder解决该问题：
不考虑 Layout 的 constrain ，可以将 LayoutBuilder 简化成 Builder.
class _HomePageState extends State<HomePage> {
@override
Widget build(BuildContext context) {
return Scaffold(
drawer: Drawer(),
floatingActionButton: Builder(
builder: (BuildContext context) {
return FloatingActionButton(
onPressed: () {
Scaffold.of(context).openDrawer();
}
);
},
),
);
}
}
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