docs(example): add /count route (#100)

Author: felangel

example/routes/count/_middleware.dart

@@ -0,0 +1,9 @@
+import 'dart:developer';
+
+import 'package:dart_frog/dart_frog.dart';
+
+final _counter = Counter('counter', 'a simple request counter');
+
+Handler middleware(Handler handler) {
+  return handler.use(provider<Counter>((_) => _counter));
+}

example/routes/count/index.dart

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+import 'dart:developer';
+
+import 'package:dart_frog/dart_frog.dart';
+
+Response onRequest(RequestContext context) {
+  final counter = context.read<Counter>()..value += 1;
+  return Response(
+    body: 'You have requested this route ${counter.value.toInt()} time(s).',
+  );
+}

example/test/routes/count/_middleware_test.dart

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+import 'dart:developer';
+
+import 'package:dart_frog/dart_frog.dart';
+import 'package:mocktail/mocktail.dart';
+import 'package:test/test.dart';
+
+import '../../../routes/count/_middleware.dart';
+
+class _MockRequestContext extends Mock implements RequestContext {}
+
+void main() {
+  group('middleware', () {
+    test('provides count instance', () async {
+      double? count;
+      final handler = middleware(
+        (context) {
+          final counter = context.read<Counter>();
+          count = counter.value += 1;
+          return Response(body: '');
+        },
+      );
+      final request = Request.get(Uri.parse('http://localhost/'));
+      final context = _MockRequestContext();
+      when(() => context.request).thenReturn(request);
+
+      await handler(context);
+      expect(count, equals(1));
+
+      await handler(context);
+      expect(count, equals(2));
+
+      await handler(context);
+      expect(count, equals(3));
+    });
+  });
+}

example/test/routes/count/index_test.dart

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+import 'dart:developer';
+import 'dart:io';
+
+import 'package:dart_frog/dart_frog.dart';
+import 'package:mocktail/mocktail.dart';
+import 'package:test/test.dart';
+
+import '../../../routes/count/index.dart' as route;
+
+class _MockRequestContext extends Mock implements RequestContext {}
+
+class _MockCounter extends Mock implements Counter {}
+
+void main() {
+  group('GET /', () {
+    test('responds with a 200 and count.', () async {
+      const count = 42;
+      final counter = _MockCounter();
+      when(() => counter.value).thenReturn(count.toDouble());
+      final context = _MockRequestContext();
+      when(() => context.read<Counter>()).thenReturn(counter);
+      final response = route.onRequest(context);
+      expect(response.statusCode, equals(HttpStatus.ok));
+      expect(
+        response.body(),
+        completion(
+          equals('You have requested this route $count time(s).'),
+        ),
+      );
+    });
+  });
+}