docs: fix/improve example for `singleton-services` guide (#21589)

PR Close #21589

aio/content/examples/ngmodules/src/app/app.module.ts

@@ -4,22 +4,20 @@ import { FormsModule } from '@angular/forms';
 import { HttpModule } from '@angular/http';
 
 /* App Root */
-import { AppComponent }   from './app.component';
+import { AppComponent } from './app.component';
 
 /* Feature Modules */
-import { ContactModule }    from './contact/contact.module';
-import { CoreModule }       from './core/core.module';
+import { ContactModule } from './contact/contact.module';
+// #docregion import-for-root
+import { CoreModule } from './core/core.module';
+// #enddocregion import-for-root
 
 /* Routing Module */
 import { AppRoutingModule } from './app-routing.module';
 
 
-
+// #docregion import-for-root
 @NgModule({
-  declarations: [
-    AppComponent
-  ],
-  // #docregion import-for-root
   imports: [
     BrowserModule,
     ContactModule,
@@ -28,6 +26,11 @@ import { AppRoutingModule } from './app-routing.module';
   ],
   // #enddocregion import-for-root
   providers: [],
+  declarations: [
+    AppComponent
+  ],
   bootstrap: [AppComponent]
+  // #docregion import-for-root
 })
 export class AppModule { }
+// #enddocregion import-for-root

aio/content/examples/ngmodules/src/app/core/core.module.ts

@@ -1,31 +1,34 @@
-/* tslint:disable:member-ordering no-unused-variable */
- // #docregion whole-core-module
-import {
-  ModuleWithProviders, NgModule,
-  Optional, SkipSelf }       from '@angular/core';
+// #docregion whole-core-module
+import { ModuleWithProviders, NgModule, Optional, SkipSelf } from '@angular/core';
 
-import { CommonModule }      from '@angular/common';
+import { CommonModule } from '@angular/common';
 
-import { TitleComponent }    from './title.component';
-import { UserService }       from './user.service';
+import { TitleComponent } from './title.component';
+// #docregion user-service
+import { UserService } from './user.service';
+// #enddocregion user-service
 import { UserServiceConfig } from './user.service';
 
+
+// #docregion user-service
 @NgModule({
+  // #enddocregion user-service
   imports:      [ CommonModule ],
   declarations: [ TitleComponent ],
   exports:      [ TitleComponent ],
+  // #docregion user-service
   providers:    [ UserService ]
 })
 export class CoreModule {
- // #docregion ctor
+  // #enddocregion user-service
+  // #docregion ctor
   constructor (@Optional() @SkipSelf() parentModule: CoreModule) {
     if (parentModule) {
       throw new Error(
         'CoreModule is already loaded. Import it in the AppModule only');
     }
   }
-// #enddocregion ctor
-
+  // #enddocregion ctor
 
   // #docregion for-root
   static forRoot(config: UserServiceConfig): ModuleWithProviders {
@@ -36,9 +39,8 @@ export class CoreModule {
       ]
     };
   }
-
   // #enddocregion for-root
+  // #docregion user-service
 }
-
+// #enddocregion user-service
 // #enddocregion whole-core-module
-

aio/content/guide/singleton-services.md

@@ -5,10 +5,8 @@
 * A basic understanding of [Bootstrapping](guide/bootstrapping).
 * Familiarity with [Providers](guide/providers).
 
-For a sample app using the app-wide singleton service
-that this page describes, see the
-<live-example name="ngmodules">live example</live-example>
-showcasing all the documented features of NgModules.
+For a sample app using the app-wide singleton service that this page describes, see the
+<live-example name="ngmodules"></live-example> showcasing all the documented features of NgModules.
 
 <hr />
 
@@ -25,7 +23,7 @@ have the same lifetime of the application, hence singleton.
 The following example module is called, as a convention, `CoreModule`. This use of `@NgModule` creates organizational infrastructure and gives you
 a way of providing services from a designated NgModule.
 
-<code-example path="providers/src/app/core/core.module.ts" region="" title="src/app/core/core.module.ts" linenums="false">
+<code-example path="ngmodules/src/app/core/core.module.ts" region="user-service" title="src/app/core/core.module.ts" linenums="false">
 </code-example>
 
 Here, `CoreModule` provides the `UserService`, and because `AppModule`
@@ -66,7 +64,7 @@ If a module provides both providers and declarations (components, directives, pi
 
 <!-- MH: show a simple example how to do that without going to deep into it. -->
 
-To make this more concrete, consider the `RouterModule` as an example. `RouterModule` needs to provide the `Router` service, as well as the `RouterOutlet` directive. `RouterModule` has to be imported by the root application module so that the application has a `Router` and the application has at least one `RouterOutlet`. It also must be imported by the individual route components so that they can place `RouterOutlet` directives into their template for sub-routes. 
+To make this more concrete, consider the `RouterModule` as an example. `RouterModule` needs to provide the `Router` service, as well as the `RouterOutlet` directive. `RouterModule` has to be imported by the root application module so that the application has a `Router` and the application has at least one `RouterOutlet`. It also must be imported by the individual route components so that they can place `RouterOutlet` directives into their template for sub-routes.
 
 If the `RouterModule` didn’t have `forRoot()` then each route component would instantiate a new `Router` instance, which would break the application as there can only be one `Router`. For this reason, the `RouterModule` has the `RouterOutlet` declaration so that it is available everywhere, but the `Router` provider is only in the `forRoot()`. The result is that the root application module imports `RouterModule.forRoot(...)` and gets a `Router`, whereas all route components import `RouterModule` which does not include the `Router`.