navigation_and_routing: a bunch of cleanup (#886)

Fail early on nullable context objects - no code paths allow null
Eliminate superfluous private function
Use a function over an abstract class with one method

navigation_and_routing/lib/src/routing/parser.dart

@@ -8,18 +8,12 @@ import 'package:path_to_regexp/path_to_regexp.dart';
 import 'parsed_route.dart';
 
 /// Used by [TemplateRouteParser] to guard access to routes.
-///
-/// Override this class to change the route that is returned by
-/// [TemplateRouteParser.parseRouteInformation] if a condition is not met, for
-/// example, if the user is not signed in.
-abstract class RouteGuard<T> {
-  Future<T> redirect(T from);
-}
+typedef RouteGuard<T> = Future<T> Function(T from);
 
 /// Parses the URI path into a [ParsedRoute].
 class TemplateRouteParser extends RouteInformationParser<ParsedRoute> {
-  final List<String> _pathTemplates = [];
-  RouteGuard<ParsedRoute>? guard;
+  final List<String> _pathTemplates;
+  final RouteGuard<ParsedRoute>? guard;
   final ParsedRoute initialRoute;
 
   TemplateRouteParser({
@@ -27,27 +21,20 @@ class TemplateRouteParser extends RouteInformationParser<ParsedRoute> {
     required List<String> allowedPaths,
 
     /// The initial route
-    String? initialRoute = '/',
+    String initialRoute = '/',
 
     ///  [RouteGuard] used to redirect.
     this.guard,
-  }) : initialRoute =
-            ParsedRoute(initialRoute ?? '/', initialRoute ?? '/', {}, {}) {
-    for (var template in allowedPaths) {
-      _addRoute(template);
-    }
-  }
-
-  void _addRoute(String pathTemplate) {
-    _pathTemplates.add(pathTemplate);
-  }
+  })  : initialRoute = ParsedRoute(initialRoute, initialRoute, {}, {}),
+        _pathTemplates = [
+          ...allowedPaths,
+        ],
+        assert(allowedPaths.contains(initialRoute));
 
   @override
   Future<ParsedRoute> parseRouteInformation(
-          RouteInformation routeInformation) async =>
-      await _parse(routeInformation);
-
-  Future<ParsedRoute> _parse(RouteInformation routeInformation) async {
+    RouteInformation routeInformation,
+  ) async {
     final path = routeInformation.location!;
     final queryParams = Uri.parse(path).queryParameters;
     var parsedRoute = initialRoute;
@@ -66,7 +53,7 @@ class TemplateRouteParser extends RouteInformationParser<ParsedRoute> {
     // Redirect if a guard is present
     var guard = this.guard;
     if (guard != null) {
-      return guard.redirect(parsedRoute);
+      return guard(parsedRoute);
     }
 
     return parsedRoute;

navigation_and_routing/lib/src/routing/route_state.dart

@@ -44,6 +44,6 @@ class RouteStateScope extends InheritedNotifier<RouteState> {
     Key? key,
   }) : super(key: key, notifier: notifier, child: child);
 
-  static RouteState? of(BuildContext context) =>
-      context.dependOnInheritedWidgetOfExactType<RouteStateScope>()?.notifier;
+  static RouteState of(BuildContext context) =>
+      context.dependOnInheritedWidgetOfExactType<RouteStateScope>()!.notifier!;
 }