Angular 6+ :ProvidedIn a non root module is causing a circular dependency

问题

I'm trying to provide a resolve service via the new providedIn attribute.

This is a translations resolver which I use in a protected module:

import { Injectable } from '@angular/core';

import { Observable , pipe } from 'rxjs';
import {map} from "rxjs/operators";

//This is causing: "WARNING in Circular dependency detected:"
import {ProtectedModule} from "../../../protected/protected.module";

import { HttpHandlerService } from '../../http/http-handler.service';

@Injectable({
  providedIn: ProtectedModule //Over here (I need the import for this line)
})
export class TranslationsResolverService {
  constructor(private _httpHandlerService : HttpHandlerService) { }
    resolve(): any {
      //Do Something...
    }
}

I declared the translations resolver service in the protected routing module:

import { NgModule }           from '@angular/core';
import {RouterModule, Routes} from '@angular/router';

import {AuthGuard} from "../core/resolvers/auth/auth.guard";
import {TranslationsResolverService} from "./../core/resolvers/translations/translations-resolver.service";

const routes: Routes = [
  {
    path : 'app' ,
    component: ProtectedComponent,
    resolve : {
      translations : TranslationsResolverService // <---- Over here - i can't remove that of course
    },
    canActivate: [AuthGuard],
    ]
  }
];


@NgModule({
  imports : [RouterModule.forChild(routes)],
  exports : [RouterModule]
})
export class ProtectedRoutingModule { }

Because of the fact that I import (typescript import) the protected.module in the translations-resolver.service.ts in order to use it in the providedIn attribute I get a WARNING in Circular dependency detected:

path/to/translations-resolver.service.ts -> 

protected/protected.module.ts ->

protected/protected-routing.module.ts -> 

path to translations-resolver.service.ts

The 2nd path (protected/protected.module.ts) is added due to the providedIn attribute.

I can fix this by just providing the translationsResolver as a NgModule provider (in the providers array) but I prefer it to be an injectable provider.

Any suggestions for solving this?

回答1:

This isn't an Angular dependencies problem.

The circular reference is generated by the TypeScript compiler when it tries to resolve the circular imports.

First Solution

Create a new module named ProtectedResolversModule and use providedIn: ProtectedResolversModule and move the resolvers there.

Now you can import that module into ProtectedModule and you won't get a circular dependency error when loading ProtectedRoutingModule.

Second Solution

Use the providers array of ProtectedModule.

回答2:

I ran into the same problem. Turns out the solution is "don't do it", as explained in this thread by one of the Angular guys: https://github.com/angular/angular-cli/issues/10170#issuecomment-380673276

It boils down to services being easier to tree shake when they are provided by the root module, as I gather.

I'm as disappointed as you are.

回答3:

I think Angular have made a bit of a mess of the providedIn syntax. It seems to have confused a lot of people. E.g. see these two github threads:

https://github.com/angular/angular-cli/issues/10170
https://github.com/angular/angular/issues/25784

The providedIn syntax seems to have 2 main benefits:

It supports tree-shaking of unused services
providedIn: 'root' ensures that you only ever get one instance of the service

But you only really need (1) if you're writing a library rather than an application (because why would you include a service that you didn't need in your application), and you can avoid multiple service instances (2) just by making sure you don't import the service module more than once.

The problems with the providedIn syntax are:

providedIn: 'root' breaks the link between the service and the module it "lives in" (or "with") - because the service doesn't know about the module and the module doesn't know about the service. This means the service no longer really "belongs" to that module, and will just get bundled with whatever references it. This in turn means that it is now up to the service consumer to make sure the service's injectable dependencies (if it has any) are available before it is used, which is confusing and quite counter-intuitive.
The circular reference problem described above. It is actually not possible - via this syntax - for the link between the service and its module to be preserved, if the service is actually used by any components within the same module.

This is contrary to official Angular guidance, but my advice would be: Don't use providedIn, unless you are writing a third party library which requires tree-shaking - use the old (not deprecated) providers syntax on the module instead, i.e.:

@NgModule({ providers: [MyService], })

Addendum

I've received 5 up-votes now for this answer, so I feel as though I ought to come clean and say that I'm no longer actually following my own advice on this!

Since official (and widely followed) Angular policy is to use providedIn: 'root', I decided that on the whole it would be less confusing for other developers if I just stuck with this. So far it hasn't caused me any problems, but the caveats above still remain and I believe it's important to remain aware of this.

回答4:

Check forwardRef() function from angular/core. It allows to refer to references which are not yet defined.

import {MyService} from './service';

constructor(@Inject(forwardRef(() => MyService)) public myService: MyService) {
}

来源：https://stackoverflow.com/questions/51062235/angular-6-providedin-a-non-root-module-is-causing-a-circular-dependency

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