如何在 Angular 5 中的应用程序范围启动时加载列表（http 调用）？

我希望在启动时加载数据，例如使用 LocationsService 的国家/地区。

我已经实现了当前的服务：

...

@Injectable()
export class LocationsService {

  public countries: Country[];

  constructor(private http: HttpClient) {
  }

  public getCountries() {
    if (this.countries.length == 0) {
        this.http.get<Country[]>(`${this.url}`)
        .subscribe(data => { this.countries = data; },
            err => console.error(err),
            () => console.log(this.countries)
        )
    };
    return this.countries;
  }

}

我尝试将服务放入引导程序中：

bootstrap: [AppComponent, LocationsService]

但它不起作用（实际上引发错误）。我需要这种类型的列表从启动时就可用（仅加载 1 次）。谢谢！

使用APP_初始化器

首先更改您的服务而不是返回数据，而是返回 Observable

@Injectable()
export class LocationsService {

  public countries: Country[];

  constructor(private http: HttpClient) {
  }

  public getCountries() {
    if (this.countries.length == 0) {
        return this.http.get<Country[]>(`${this.url}`);
    };
    return new Observable( c => {
      c.next(this.countries);
      c.complete();
   });
  }

}

创建一个用于设置的服务

@Injectable()
export class SetupLocations {



  constructor(private loc: LocationsService ) {
  }

  public initliaze(): Promise<any> {
    return new Promise((resolve, reject) =>{
        this.loc.getCountries().subscribe((response) =>{

         //Do whatever you want here.

         //always call resolve method, because it will freeze your app.   
         resolve(true);

       }, (err) =>{});
     })
  }

}

接下来在主模块中初始化它

import { NgModule, APP_INITIALIZER } from "@angular/core";



//create a function outside in the class module
export function SetupApp(setup: SetupLocations) {
    return () => setup.initliaze();
}



 @NgModule({
   providers: [
     SetupLocations,
     { 
        provide: APP_INITIALIZER,
        useFactory: SetupApp,
        deps: [SetupLocations],
        multi: true
     }]
})
export class AppModule {}