Angular 2 Use component from another module

Question

I have Angular 2 (version 2.0.0 - final) app generated with angular-cli.

When I create a component and add it to AppModule's declarations array it's all good, it works.

I decided to separate the components, so I created a TaskModule and a component TaskCard. Now I want to use the TaskCard in one of the components of the AppModule (the Board component).

AppModule:

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { HttpModule } from '@angular/http';

import { AppComponent } from './app.component';
import { BoardComponent } from './board/board.component';
import { LoginComponent } from './login/login.component';

import { MdButtonModule } from '@angular2-material/button';
import { MdInputModule } from '@angular2-material/input';
import { MdToolbarModule } from '@angular2-material/toolbar';

import { routing, appRoutingProviders} from './app.routing';
import { PageNotFoundComponent } from './page-not-found/page-not-found.component';

import { UserService  } from './services/user/user.service';
import { TaskModule } from './task/task.module';


@NgModule({
  declarations: [
    AppComponent,
    BoardComponent,// I want to use TaskCard in this component
    LoginComponent,
    PageNotFoundComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
    HttpModule,
    MdButtonModule,
    MdInputModule,
    MdToolbarModule,
    routing,
    TaskModule // TaskCard is in this module
  ],
  providers: [UserService],
  bootstrap: [AppComponent]
})
export class AppModule { }

TaskModule:

import { NgModule } from '@angular/core';
import { TaskCardComponent } from './task-card/task-card.component';

import { MdCardModule } from '@angular2-material/card';

@NgModule({
  declarations: [TaskCardComponent],
  imports: [MdCardModule],
  providers: []
})
export class TaskModule{}

The whole project is available on https://github.com/evgdim/angular2 (kanban-board folder)

What I'm missing? What I have to do to use TaskCardComponent in BoardComponent?

Answer 1

Whatever you want to use from another module, just put it in the export array. Like this-

 @NgModule({
  declarations: [TaskCardComponent],
  exports: [TaskCardComponent],
  imports: [MdCardModule]
})

Answer 2

You have to export it from your NgModule:

@NgModule({
  declarations: [TaskCardComponent],
  exports: [TaskCardComponent],
  imports: [MdCardModule],
  providers: []
})
export class TaskModule{}

Answer 3

Note that in order to create a so called "feature module", you need to import CommonModule inside it. So, your module initialization code will look like this:

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';

import { TaskCardComponent } from './task-card/task-card.component';
import { MdCardModule } from '@angular2-material/card';

@NgModule({
  imports: [
    CommonModule,
    MdCardModule 
  ],
  declarations: [
    TaskCardComponent
  ],
  exports: [
    TaskCardComponent
  ]
})
export class TaskModule { }

More information available here: https://angular.io/guide/ngmodule#create-the-feature-module

Answer 4

SOLVED HOW TO USE A COMPONENT DECLARED IN A MODULE IN OTHER MODULE.

Based on Royi Namir explanation (Thank you so much). There is a missing part to reuse a component declared in a Module in any other module while lazy loading is used.

1st: Export the component in the module which contains it:

@NgModule({
  declarations: [TaskCardComponent],
  imports: [MdCardModule],
  exports: [TaskCardComponent] <== this line
})
export class TaskModule{}

2nd: In the module where you want to use TaskCardComponent:

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { MdCardModule } from '@angular2-material/card';

@NgModule({
  imports: [
   CommonModule,
   MdCardModule
   ],
  providers: [],
  exports:[ MdCardModule ] <== this line
})
export class TaskModule{}

Like this the second module imports the first module which imports and exports the component.

When we import the module in the second module we need to export it again. Now we can use the first component in the second module.

Answer 5

One big and great approach is to load the module from a NgModuleFactory, you can load a module inside another module by calling this:

constructor(private loader: NgModuleFactoryLoader, private injector: Injector) {}

loadModule(path: string) {
    this.loader.load(path).then((moduleFactory: NgModuleFactory<any>) => {
        const entryComponent = (<any>moduleFactory.moduleType).entry;
        const moduleRef = moduleFactory.create(this.injector);
        const compFactory = moduleRef.componentFactoryResolver.resolveComponentFactory(entryComponent);
        this.lazyOutlet.createComponent(compFactory);
    });
}

I got this from here.

Answer 6

The main rule here is that:

The selectors which are applicable during compilation of a component template are determined by the module that declares that component, and the transitive closure of the exports of that module's imports.

So, try to export it:

@NgModule({
  declarations: [TaskCardComponent],
  imports: [MdCardModule],
  exports: [TaskCardComponent] <== this line
})
export class TaskModule{}

What should I export?

Export declarable classes that components in other modules should be able to reference in their templates. These are your public classes. If you don't export a class, it stays private, visible only to other component declared in this module.

The minute you create a new module, lazy or not, any new module and you declare anything into it, that new module has a clean state(as Ward Bell said in https://devchat.tv/adv-in-angular/119-aia-avoiding-common-pitfalls-in-angular2)

Angular creates transitive module for each of @NgModules.

This module collects directives that either imported from another module(if transitive module of imported module has exported directives) or declared in current module.

When angular compiles template that belongs to module X it is used those directives that had been collected in X.transitiveModule.directives.

compiledTemplate = new CompiledTemplate(
    false, compMeta.type, compMeta, ngModule, ngModule.transitiveModule.directives);

https://github.com/angular/angular/blob/4.2.x/packages/compiler/src/jit/compiler.ts#L250-L251

This way according to the picture above

• YComponent can't use ZComponent in its template because directives array of Transitive module Y doesn't contain ZComponent because YModule has not imported ZModule whose transitive module contains ZComponent in exportedDirectives array.

• Within XComponent template we can use ZComponent because Transitive module X has directives array that contains ZComponent because XModule imports module (YModule) that exports module (ZModule) that exports directive ZComponent

• Within AppComponent template we can't use XComponent because AppModule imports XModule but XModule doesn't exports XComponent.

See also

• why lazy loaded module has to import commonModule? Angular 2

• Angular Module FAQs

• What is difference between declarations, providers and import in NgModule