BuildContext class abstract

A handle to the location of a widget in the widget tree.

This class presents a set of methods that can be used from StatelessWidget.build methods and from methods on State objects.

BuildContext objects are passed to WidgetBuilder functions (such as StatelessWidget.build), and are available from the State.context member. Some static functions (e.g. showDialog, Theme.of, and so forth) also take build contexts so that they can act on behalf of the calling widget, or obtain data specifically for the given context.

Each widget has its own BuildContext, which becomes the parent of the widget returned by the StatelessWidget.build or State.build function. (And similarly, the parent of any children for RenderObjectWidgets.)

In particular, this means that within a build method, the build context of the widget of the build method is not the same as the build context of the widgets returned by that build method. This can lead to some tricky cases. For example, Theme.of(context) looks for the nearest enclosing Theme of the given build context. If a build method for a widget Q includes a Theme within its returned widget tree, and attempts to use Theme.of passing its own context, the build method for Q will not find that Theme object. It will instead find whatever Theme was an ancestor to the widget Q. If the build context for a subpart of the returned tree is needed, a Builder widget can be used: the build context passed to the Builder.builder callback will be that of the Builder itself.

For example, in the following snippet, the ScaffoldState.showBottomSheet method is called on the Scaffold widget that the build method itself creates. If a Builder had not been used, and instead the context argument of the build method itself had been used, no Scaffold would have been found, and the Scaffold.of function would have returned null.

@override
Widget build(BuildContext context) {
  // here, Scaffold.of(context) returns null
  return Scaffold(
    appBar: AppBar(title: const Text('Demo')),
    body: Builder(
      builder: (BuildContext context) {
        return TextButton(
          child: const Text('BUTTON'),
          onPressed: () {
            Scaffold.of(context).showBottomSheet(
              (BuildContext context) {
                return Container(
                  alignment: Alignment.center,
                  height: 200,
                  color: Colors.amber,
                  child: Center(
                    child: Column(
                      mainAxisSize: MainAxisSize.min,
                      children: <Widget>[
                        const Text('BottomSheet'),
                        ElevatedButton(
                          child: const Text('Close BottomSheet'),
                          onPressed: () {
                            Navigator.pop(context);
                          },
                        )
                      ],
                    ),
                  ),
                );
              },
            );
          },
        );
      },
    )
  );
}

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.

Avoid storing instances of BuildContexts because they may become invalid if the widget they are associated with is unmounted from the widget tree. If a BuildContext is used across an asynchronous gap (i.e. after performing an asynchronous operation), consider checking mounted to determine whether the context is still valid before interacting with it:

  @override
  Widget build(BuildContext context) {
    return OutlinedButton(
      onPressed: () async {
        await Future<void>.delayed(const Duration(seconds: 1));
        if (context.mounted) {
          Navigator.of(context).pop();
        }
      },
      child: const Text('Delayed pop'),
    );
  }

BuildContext objects are actually Element objects. The BuildContext interface is used to discourage direct manipulation of Element objects.