refactor: reconstruct module and reorganize rxjs

src/app/app.module.ts

@@ -2,19 +2,19 @@ import { NgModule } from '@angular/core';
 import { BrowserModule } from '@angular/platform-browser';
 import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
 
-import { SharedModule } from 'app/shared/shared.module';
 import { AppRoutingModule } from './app-routing.module';
 import { AppComponent } from './app.component';
+import { CoreModule } from './core/core.module';
 
 @NgModule({
-  declarations: [
-    AppComponent
-  ],
   imports: [
     BrowserModule,
     BrowserAnimationsModule,
-    SharedModule.forRoot(),
     AppRoutingModule,
+    CoreModule,
+  ],
+  declarations: [
+    AppComponent,
   ],
   providers: [],
   bootstrap: [AppComponent]

src/app/core/core.module.ts

@@ -0,0 +1,16 @@
+import { NgModule, Optional, SkipSelf } from '@angular/core';
+import { throwIfAlreadyLoaded } from './module-import-guard';
+
+@NgModule({
+  imports: [],
+  declarations: [],
+})
+export class CoreModule {
+  constructor(
+    @Optional()
+    @SkipSelf()
+    parentModule: CoreModule,
+  ) {
+    throwIfAlreadyLoaded(parentModule, 'CoreModule');
+  }
+}

src/app/core/module-import-guard.ts

@@ -0,0 +1,7 @@
+export function throwIfAlreadyLoaded(parentModule: any, moduleName: string) {
+  if (parentModule) {
+    throw new Error(
+      `${moduleName} has already been loaded. Import Core modules in the AppModule only.`,
+    );
+  }
+}

src/app/shared/shared.module.ts

@@ -1,7 +1,6 @@
 import { NgModule, ModuleWithProviders } from '@angular/core';
 import { CommonModule } from '@angular/common';
 import { FormsModule } from '@angular/forms';
-import 'app/shared/rxjs-operators';
 
 import { MaterialModule } from './material.module';
 
@@ -13,16 +12,10 @@ import { MaterialModule } from './material.module';
   ],
   declarations: [],
   exports: [
+    CommonModule,
     FormsModule,
     MaterialModule,
   ]
 })
 export class SharedModule {
-  static forRoot(): ModuleWithProviders {
-    return {
-      ngModule: SharedModule,
-      providers: [
-      ]
-    };
-  }
 }