feat(core): Make ConfigModule importable in every NgModule (#157)

* Updated config.module.ts

Removed the check in the constructor, this way every module can import ConfigModule and have access to ConfigPipe.

* Update README.md

* Provide a forChild() method

This change makes ConfigModule importable in lazy loaded modules with forChild() but also preserves the forRoot() check.

* Updated README to reflect last commit

* Update config.module.ts

Removed the generic from InjectionToken

* Update config.module.ts (tslint)

Co-authored-by: Burak Tasci <burak.tasci@deliveryhero.com>

Author: UserGalileo

packages/@ngx-config/core/README.md

@@ -211,6 +211,8 @@ export class AnyClass {
 <span id="property">{{['some', 'setting'] | config}}</span>
 ```
 
+In order to use this pipe in lazy-loaded modules, you must import `ConfigModule.forChild()`.
+
 ## <a name="license"></a> License
 
 The MIT License (MIT)

packages/@ngx-config/core/src/config.module.ts

@@ -1,4 +1,4 @@
-import { APP_INITIALIZER, ModuleWithProviders, NgModule, Optional, SkipSelf } from '@angular/core';
+import { APP_INITIALIZER, Inject, InjectionToken, ModuleWithProviders, NgModule, Optional, SkipSelf } from '@angular/core';
 
 import { ConfigLoader, ConfigStaticLoader } from './config.loader';
 import { ConfigPipe } from './config.pipe';
@@ -13,17 +13,30 @@ export const initializerFactory = (config: ConfigService) => {
   return res;
 };
 
+export const CONFIG_FORROOT_GUARD = new InjectionToken('CONFIG_FORROOT_GUARD');
+
+// tslint:disable-next-line:only-arrow-functions
+export function provideForRootGuard(config: ConfigService): any {
+  if (config) {
+    throw new Error(
+        `ConfigModule.forRoot() called twice. Lazy loaded modules should use ConfigModule.forChild() instead.`);
+  }
+
+  return 'guarded';
+}
+
 @NgModule({
   declarations: [ConfigPipe],
   exports: [ConfigPipe]
 })
 export class ConfigModule {
+  
   static forRoot(
     configuredProvider: any = {
       provide: ConfigLoader,
       useFactory: configFactory
     }
-  ): ModuleWithProviders {
+  ): ModuleWithProviders<ConfigModule> {
     return {
       ngModule: ConfigModule,
       providers: [
@@ -34,14 +47,22 @@ export class ConfigModule {
           useFactory: initializerFactory,
           deps: [ConfigService],
           multi: true
+        },
+        {
+          provide: CONFIG_FORROOT_GUARD,
+          useFactory: provideForRootGuard,
+          deps: [[ConfigService, new Optional(), new SkipSelf()]]
         }
       ]
     };
   }
-
-  constructor(@Optional() @SkipSelf() parentModule?: ConfigModule) {
-    if (parentModule) {
-      throw new Error('ConfigModule already loaded; import in root module only.');
-    }
+  
+  static forChild(): ModuleWithProviders<ConfigModule> {
+    return {
+      ngModule: ConfigModule
+    };
   }
+  
+  // tslint:disable-next-line:no-empty
+  constructor(@Optional() @Inject(CONFIG_FORROOT_GUARD) guard: any) {}
 }