APP_INITIALIZER not work in angular6 library

Question

I create an angular6 library call mapModule, and be imported into app module, now I want to do something before load mapModule, so I try to add APP_INITIALIZER in this library :

export function StartupServiceFactory2(mapConfigService: MapConfigService) {
    const x = 2 + 2; 
    console.log(x);
    return () => mapConfigService.readConfig();
}

but it doesn't work

Answer 1

Do you provide it?

// app.module
providers: [
  { provide: APP_INITIALIZER,
   useFactory: StartupServiceFactory2,
   deps: [MapConfigService], multi: true }
]

Does mapConfigService.readConfig() return Promise ?

Take a look at example

Answer 2

I have run into it as well, where APP_INITIALIZER wouldn't completely initialize the app before triggering CanActivate route guard.

Few things to look out for:

1. mapConfigService should return Promise
2. When converting Observable to promise via toPromise() and subsequently calling .then() - the promise is released before tapping into .then() :

This will have issue:

let promise = this.mapConfigService.map(...).toPromise().then(()=>{
 ///  this part of the code will NOT block the APP_INITIALIZER
 ///  meaning when toPromise() is called the promise is released 
 ///  right away before having a chance to fully execute the .then() part
       ....
});

Instead wrap the Observable into Promise and have the resolve() release the promise, like following:

readConfig(): Promise<any> {                                    
    const promise = new Promise((resolve, reject) => {                                    
            this.mapConfigService.subscribe((data)=> {
                // additional stuff

                resolve();
                return data;                
    });

    return promise;
}   

Hope that helps.