angular - 服务在 APP_INITIALIZER 之后实例化了两次

Question

问题是:我需要进行 http 调用并存储生成动态路由所需的对象。所以，我正在利用 APP_INITIALIZER。

// app.module.ts
import { ApplicationService } from './application.service';


providers: [
  ApplicationService,
  { 
  provide: APP_INITIALIZER, useFactory: appServiceFactory, deps: 
  [Injector, ApplicationService], multi: true 
  },
],

function appServiceFactory(injector: Injector, appService: ApplicationService): Function {
  return () => {
    return appService.loadApplication().then((app: Application) => {
      /custom logic
      });
    });
  };
}


// application.service.ts
@Injectable({
  providedIn: 'root'
})

// navigation.component.ts
import { ApplicationService } from './application.service';

export class NavigationComponent implements OnInit {

    constructor(private _applicationService: ApplicationService) {
  }
}    

但是在navigation.component里面，applicationservice又被初始化了。我确信这一点，因为如果我记录或放置调试器语句，服务的construct() 方法将被调用两次。
为什么即使 Service 被声明为单例并且 providedIn: root 正在被重新实例化？

Answer 1

这样做的原因是，一旦您将 Router 包含在您的 APP_INITIALIZER 工厂的依赖项中，您就会获得循环依赖 ( https://github.com/angular/angular/blob/4c2ce4e8ba4c5ac5ce8754d67bc6603eaad4564a/packages/router/src/router_module.ts#L61-L64 )。

ApplicationService
       |
    TestService
       |
     Router
       |
  ApplicationRef
       |
ApplicationInitStatus
       |
 APP_INITIALIZER
       |
ApplicationService

为了解决这个问题，你可以懒惰地获取路由器:

export class TestService {

  get router() {
    return this.injector.get(Router)
  }

  constructor(private _http: HttpClient, private injector: Injector ) {
  }
}

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