showDialog<T> function Null safety

Future<T?> showDialog<T>(
  1. {required BuildContext context,
  2. required WidgetBuilder builder,
  3. bool barrierDismissible = true,
  4. Color? barrierColor = Colors.black54,
  5. String? barrierLabel,
  6. bool useSafeArea = true,
  7. bool useRootNavigator = true,
  8. RouteSettings? routeSettings,
  9. Offset? anchorPoint}
)

Displays a Material dialog above the current contents of the app, with Material entrance and exit animations, modal barrier color, and modal barrier behavior (dialog is dismissible with a tap on the barrier).

This function takes a builder which typically builds a Dialog widget. Content below the dialog is dimmed with a ModalBarrier. The widget returned by the builder does not share a context with the location that showDialog is originally called from. Use a StatefulBuilder or a custom StatefulWidget if the dialog needs to update dynamically.

The context argument is used to look up the Navigator and Theme for the dialog. It is only used when the method is called. Its corresponding widget can be safely removed from the tree before the dialog is closed.

The barrierDismissible argument is used to indicate whether tapping on the barrier will dismiss the dialog. It is true by default and can not be null.

The barrierColor argument is used to specify the color of the modal barrier that darkens everything below the dialog. If null the default color Colors.black54 is used.

The useSafeArea argument is used to indicate if the dialog should only display in 'safe' areas of the screen not used by the operating system (see SafeArea for more details). It is true by default, which means the dialog will not overlap operating system areas. If it is set to false the dialog will only be constrained by the screen size. It can not be null.

The useRootNavigator argument is used to determine whether to push the dialog to the Navigator furthest from or nearest to the given context. By default, useRootNavigator is true and the dialog route created by this method is pushed to the root navigator. It can not be null.

The routeSettings argument is passed to showGeneralDialog, see RouteSettings for details.

A DisplayFeature can split the screen into sub-screens. The closest one to anchorPoint is used to render the content.

If no anchorPoint is provided, then Directionality is used:

  • for TextDirection.ltr, anchorPoint is Offset.zero, which will cause the content to appear in the top-left sub-screen.
  • for TextDirection.rtl, anchorPoint is Offset(double.maxFinite, 0), which will cause the content to appear in the top-right sub-screen.

If no anchorPoint is provided, and there is no Directionality ancestor widget in the tree, then the widget asserts during build in debug mode.

If the application has multiple Navigator objects, it may be necessary to call Navigator.of(context, rootNavigator: true).pop(result) to close the dialog rather than just Navigator.pop(context, result).

Returns a Future that resolves to the value (if any) that was passed to Navigator.pop when the dialog was closed.

link
This sample demonstrates how to use showDialog to display a dialog box.
To create a local project with this code sample, run:
flutter create --sample=material.showDialog.1 mysample

link
This sample shows the creation of showDialog, as described in: https://m3.material.io/components/dialogs/overview
To create a local project with this code sample, run:
flutter create --sample=material.showDialog.2 mysample

State Restoration in Dialogs

Using this method will not enable state restoration for the dialog. In order to enable state restoration for a dialog, use Navigator.restorablePush or Navigator.restorablePushNamed with DialogRoute.

For more information about state restoration, see RestorationManager.

link
This sample demonstrates how to create a restorable Material dialog. This is accomplished by enabling state restoration by specifying MaterialApp.restorationScopeId and using Navigator.restorablePush to push DialogRoute when the button is tapped.

To test state restoration on Android:

  1. Turn on "Don't keep activities", which destroys the Android activity as soon as the user leaves it. This option should become available when Developer Options are turned on for the device.
  2. Run the code sample on an Android device.
  3. Create some in-memory state in the app on the phone, e.g. by navigating to a different screen.
  4. Background the Flutter app, then return to it. It will restart and restore its state.

To test state restoration on iOS:

  1. Open ios/Runner.xcworkspace/ in Xcode.
  2. (iOS 14+ only): Switch to build in profile or release mode, as launching an app from the home screen is not supported in debug mode.
  3. Press the Play button in Xcode to build and run the app.
  4. Create some in-memory state in the app on the phone, e.g. by navigating to a different screen.
  5. Background the app on the phone, e.g. by going back to the home screen.
  6. Press the Stop button in Xcode to terminate the app while running in the background.
  7. Open the app again on the phone (not via Xcode). It will restart and restore its state.
To create a local project with this code sample, run:
flutter create --sample=material.showDialog.3 mysample

See also:

Implementation

Future<T?> showDialog<T>({
  required BuildContext context,
  required WidgetBuilder builder,
  bool barrierDismissible = true,
  Color? barrierColor = Colors.black54,
  String? barrierLabel,
  bool useSafeArea = true,
  bool useRootNavigator = true,
  RouteSettings? routeSettings,
  Offset? anchorPoint,
}) {
  assert(builder != null);
  assert(barrierDismissible != null);
  assert(useSafeArea != null);
  assert(useRootNavigator != null);
  assert(_debugIsActive(context));
  assert(debugCheckHasMaterialLocalizations(context));

  final CapturedThemes themes = InheritedTheme.capture(
    from: context,
    to: Navigator.of(
      context,
      rootNavigator: useRootNavigator,
    ).context,
  );

  return Navigator.of(context, rootNavigator: useRootNavigator).push<T>(DialogRoute<T>(
    context: context,
    builder: builder,
    barrierColor: barrierColor,
    barrierDismissible: barrierDismissible,
    barrierLabel: barrierLabel,
    useSafeArea: useSafeArea,
    settings: routeSettings,
    themes: themes,
    anchorPoint: anchorPoint,
  ));
}