Angular 2 How to make singleton service available

问题:

According to my understanding of Angular 2 rc5, to make a service from another module (not AppModule) available as a singleton to every component, even those lazy-loaded, we don't include the service in the providers array of that other module. We instead export it with RouterModule.forRoot() and import the result in AppModule

According to the docs:

The SharedModule should only provide the UserService when imported by the root AppModule. The SharedModule.forRoot method helps us meet this challenge...the SharedModule does not have providers...When we add the SharedModule to the imports of the AppModule, we call forRoot. In doing so, the AppModule gains the exported classes and the SharedModule delivers the singleton UserService provider at the same time

I'm really struggling with how to make a 3rd-party service (a service used by a module in the imports array of my AppModule) available to lazy loaded routes. I have no control over this 3rd-party module, so I cannot just remove that service from the NgModule.providers array of that module and place it inside RouterModule.forRoot() as I would with one of my services.

The specific service is MdIconRegistry, which is in providers for the MdIconModule of Angular Material 2 alpha 7-3. This service is used to register svg icons that can then be displayed on the page with the <md-icon svgIcon='iconName'> tag. So:

• I imported MdIconModule in my root AppModule
• I used the service in question to register svg icons in my AppComponent

The icon is visible and works well, but only in the modules that were loaded at launch. Lazy-loaded modules cannot see these icons, so I suspect that the Angular injector is not injecting the same instance of the MdIconRegistry service.

tl;dr: How can I make the service from a 3rd-party module a singleton available to my lazy-loaded components?

Here is a plunker that demonstrates the problem (coded in typescript).

PS: This just got the attention of the MdIconModule developer on github.

回答1:

I do not think it has anything to do with the component being lazy-loaded.

LazyLoadedComponent is not part of the AppModule – it is part of the LazyModule. According to the docs, a component can only be part of one module. If you try adding LazyLoadedComponent to AppModule also, you would get an error to that effect. So LazyLoadedComponent is not even seeing MdIconModule at all. You can confirm this by looking at the template output in the debugger – it is unchanged.

<md-icon svgIcon="play"></md-icon>

The solution appears to be adding the MdIconModule to the LazyModule, and while this alone does not fix the problem, it does add an error to the output.

Error retrieving icon: Error: Unable to find icon with the name ":play"

And the template output now looks like this, so we know it is loading.

<md-icon role="img" svgicon="play" ng-reflect-svg-icon="play" aria-label="play"></md-icon>

I added the call to addSvgIconSet from LazyLoadedComponent, and that got it working… so this proves there is an instance of the MdIconRegistry service per component – not what you want, but may point you in the right direction.

Here’s the new plunk - http://plnkr.co/edit/YDyJYu?p=preview

After further review, I found this in the docs:

Why is a service provided in a lazy loaded module visible only to that module?

Unlike providers of the modules loaded at launch, providers of lazy loaded modules are module-scoped.

Final Update! Here is the answer. MdIconModule is not properly setup for lazy loaded components... but we can easily create our own module that IS properly set up and use that instead.

import { NgModule } from '@angular/core';
import { HttpModule } from '@angular/http';

import { MdIcon } from '@angular2-material/icon';
import { MdIconRegistry } from '@angular2-material/icon';

@NgModule({
  imports: [HttpModule],
  exports: [MdIcon],
  declarations: [MdIcon]
})
export class MdIconModuleWithProviders {
  static forRoot(): ModuleWithProviders {
    return {
      ngModule: MdIconModuleWithProviders,
      providers: [ MdIconRegistry ]
    };
  }
}

Plunk updated and fully working. (sorry, updated the same one) -> http://plnkr.co/edit/YDyJYu?p=preview

One might submit a pull request such that Angular Material exports modules of both styles.

回答2:

New to Angular 6 there is a new way to register a provider as a singleton. Inside the @Injectable() decorator for a service, use the providedIn attribute. Set its value to 'root'. Then you won't need to add it to the providers list of the root module, or in this case you could also set it to your MdIconModuleWithProviders module like this:

@Injectable({
  providedIn: MdIconModuleWithProviders // or 'root' for singleton
})
export class MdIconRegistry {
...