builder property Null safety

A builder for inserting widgets above the Navigator or - when the WidgetsApp.router constructor is used - above the Router but below the other widgets created by the WidgetsApp widget, or for replacing the Navigator/Router entirely.

For example, from the BuildContext passed to this method, the Directionality, Localizations, DefaultTextStyle, MediaQuery, etc, are all available. They can also be overridden in a way that impacts all the routes in the Navigator or Router.

This is rarely useful, but can be used in applications that wish to override those defaults, e.g. to force the application into right-to-left mode despite being in English, or to override the MediaQuery metrics (e.g. to leave a gap for advertisements shown by a plugin from OEM code).

For specifically overriding the title with a value based on the Localizations, consider onGenerateTitle instead.

The builder callback is passed two arguments, the BuildContext (as context) and a Navigator or Router widget (as child).

If no routes are provided to the regular WidgetsApp constructor using home, routes, onGenerateRoute, or onUnknownRoute, the child will be null, and it is the responsibility of the builder to provide the application's routing machinery.

If routes are provided to the regular WidgetsApp constructor using one or more of those properties or if the WidgetsApp.router constructor is used, then child is not null, and the returned value should include the child in the widget subtree; if it does not, then the application will have no Navigator or Router and the routing related properties (i.e. navigatorKey, home, routes, onGenerateRoute, onUnknownRoute, initialRoute, navigatorObservers, routeInformationProvider, backButtonDispatcher, routerDelegate, and routeInformationParser) are ignored.

If builder is null, it is as if a builder was specified that returned the child directly. If it is null, routes must be provided using one of the other properties listed above.

Unless a Navigator is provided, either implicitly from builder being null, or by a builder including its child argument, or by a builder explicitly providing a Navigator of its own, or by the routerDelegate building one, widgets and APIs such as Hero, Navigator.push and Navigator.pop, will not function.