Angularjs2 - preload server configuration before the application starts

Question

I am working with Angular2 RC5. I would like to be able to load my server IP address and some other configuration parameters before my app boots up. How can I do this in the latest RC5?

I have seen other articles, but they are not helpful:

1. How to preload a config file in angular2: The answer provided here is specific to webpack. I am using systemjs.
2. https://medium.com/@hasan.hameed/reading-configuration-files-in-angular-2-9d18b7a6aa4#.m8lontnas: This is for the older version of Angular2. Some of the classes used are deprecated.

Any help would be greatly appreciated.

Edit

I tried to use the APP_INITIALIZER as follows in my app.module.ts:

    import { NgModule, provide, APP_INITIALIZER }       from "@angular/core";
    import { ConfigService } from "./shared/services/config.service";
    @NgModule({
        declarations: [AppComponent],
        imports: [BrowserModule,
            routes,
            FormsModule,
            HttpModule],
        providers: [AuthService,
            Title,
            appRoutingProviders,
            ConfigService],
        bootstrap: [AppComponent
            , provide(APP_INITIALIZER,
                {
                    useFactory: (config: ConfigService) => () => config.load(),
                    deps: [ConfigService], multi: true
                })]
    })
    export class AppModule { }

This is my config.service.ts file:

    import { Config } from "../model/config";

    export class ConfigService {
        constructor() {
        }

        load(): Promise<Config> {
            let config: Config = new Config("localhost", "5050");
            return Promise.resolve(config);
        }
    }

Note that I will eventually re-write load to actually read a property or some such file to load configuration.

This is what config.ts looks like:

export class Config {

    constructor(
        private machine: string,
        private port: string
    ) {
    }

    private base_url: string = "http://" + this.machine +
    this.port + "/";

    public loginURL: string = this.base_url + "login";
}

I am getting a compile error

public/app/app.module.ts(27,11): error TS2345: Argument of type '{ declarations: (typeof AppComponent ...' is not assignable to parameter of type 'NgModuleMetadataType'.
  Types of property 'bootstrap' are incompatible.
    Type '(typeof AppComponent | Provider)[]' is not assignable to type '(Type | any[])[]'.
      Type 'typeof AppComponent | Provider' is not assignable to type 'Type | any[]'.
        Type 'Provider' is not assignable to type 'Type | any[]'.
          Type 'Provider' is not assignable to type 'any[]'.
            Property '[Symbol.iterator]' is missing in type 'Provider'.

Edit 2

I move on to the next step. Update ConfigService to read a json file using http.get. Now I get an error. Here are the updated files:

export class ConfigService {
    private config: Config;
        constructor(private http: Http) {
    }

    load(): Promise<Config> {
            this.http.get("/blah/config.json")
                 .map(res => res.json())
                 .subscribe((env_data) => {
                     console.log(env_data);
                  });
        this.config = new Config("localhost", "5050");
        return Promise.resolve(this.config);
    }

    getConfig(): Config {
        return this.config;
    }
}

Here is the providers section in NgModule class

    providers: [AuthService,
    Title,
    appRoutingProviders,
    ConfigService,
    {
        provide: APP_INITIALIZER,
        useFactory: (config: ConfigService) => () => config.load(),
        deps: [ConfigService, Http],
        multi: true
    }],

Note that in RC6 I am not able to import HTTP_PROVIDERS since I get the error that these are not defined in the http module anymore. This is why I tried Http.

On runtime, I get following error

(index):27 Error: Error: Can't resolve all parameters for ConfigService: (?).(…)(anonymous function) @ (index):27
ZoneDelegate.invoke @ zone.js:332
Zone.run @ zone.js:225(anonymous function) @ zone.js:591
ZoneDelegate.invokeTask @ zone.js:365
Zone.runTask @ zone.js:265
drainMicroTaskQueue @ zone.js:497
ZoneTask.invoke @ zone.js:437

Answer 1

Provide this in your root module

{provide: APP_INITIALIZER, useValue: () => promise, multi: true}]}

where () => promise is some call that returns a Promise that resolves to the desired value.

See also How to pass parameters rendered from backend to angular2 bootstrap method

You can pass a service as dependency where you can store the result.

update

export function loadConfig(config: ConfigService) => () => config.load()

@NgModule({
        declarations: [AppComponent],
        imports: [BrowserModule,
            routes,
            FormsModule,
            HttpModule],
        providers: [AuthService,
            Title,
            appRoutingProviders,
            ConfigService,
            { provide: APP_INITIALIZER,
              useFactory: loadConfig,
              deps: [ConfigService], 
              multi: true }
        ],
        bootstrap: [AppComponent]
    })
    export class AppModule { }

update 2

Plunker example