Injecting services in custom ErrorHandler causes "Cannot instantiate cyclic dependency!" error, how can I fix this?

Question

I have the following code :

app.module.ts:

NgModule({
    declarations: [
        AppComponent
    ],
    imports: [
        RouterModule,
        BrowserModule,
        ReactiveFormsModule,
        FormsModule,
        HttpModule,
        AppRoutingModule // Routes
    ],
    providers: [ // services
        AppLog,
        {provide: ErrorHandler, useClass: MyErrorHandler}
    ],
    bootstrap: [AppComponent]
})
export class AppModule {}

MyErrorHandler.ts:

@Injectable()
export class MyErrorHandler implements ErrorHandler {
  constructor (private _appLog: AppLog) {}

  handleError(error:any):void {
    let errorMessage: string = "" + error
    this._appLog.logMessageAsJson(errorMessage, "error")
            .subscribe(
              ...
            )
  }
}

appLog.ts

@Injectable()
export class AppLog {
    constructor (private _http: Http) {}

    logMessageAsJson(message: string, type: string) {
        let headers = new Headers({ "Content-Type": "application/json" })
        let jsonMessage = {"type": type, "message": message}

        return this._http.post(JSON.stringify(jsonMessage), headers)
    }
}

However, when my app bootstrap, it fails if I have an injection in MyErrorHandler with the following error :

Unhandled Promise rejection: Provider parse errors:
Cannot instantiate cyclic dependency!

If I do remove constructor (private _appLog: AppLog) {} and then do something else in handleError it works just fine and the ErrorHandler is called.

I guess it doesn't work as AppLog and MyErrorHandler are instantiated at the same time

Answer 1

You can use this workaround to break up cyclic dependencies with DI

@Injectable()
export class MyErrorHandler implements ErrorHandler {
  private _appLog: AppLog;
  constructor (injector:Injector) {
    setTimeout(() => this._appLog = injector.get(AppLog));
  }
  ...
}

Angulars DI itself just doesn't support cyclic dependencies.

Answer 2

The ErrorHandler is created before the providers. So we need Injector for dependency injection in our custom error handler class.

@Injectable()
export class MyErrorHandler implements ErrorHandler{
   constructor(private injector: Injector) { } 
  handleError(error: any) {
      let router = this.injector.get(ServiceName);

    }
}

Create a default service via Angular cli and check the first part see providedIn: 'root',

@Injectable({
  providedIn: 'root',
})
export class CustomService{
  constructor(private otherService: OtherService) { // ok not failed } 
}

For more detail check angular @Injectable-level configuration

@NgModule-level injectors You can configure a provider at the module level using the providers metadata option for a non-root NgModule, in order to limit the scope of the provider to that module. This is the equivalent of specifying the non-root module in the @Injectable() metadata, except that the service provided via providers is not tree-shakable.