Angular 2 - service - dependency injection from another service

Question

I have written two services in Angular 2. One of those is a basic, customised class of Http with some custom functionality in (it looks basic for now, but it will be expanding):

ServerComms.ts

import {Injectable} from 'angular2/core';
import {Http} from 'angular2/http';

@Injectable()
export class ServerComms {
    private url = 'myservice.com/service/';

    constructor (public http: Http) {
        // do nothing
    }

    get(options) {
        var req = http.get('https://' + options.name + url);

        if (options.timout) {
            req.timeout(options.timeout);
        }

        return req;
    }
}

Another class, TicketService utilises this class above, and calls one of the methods in the service. This is defined below:

TicketService.ts

import {Injectable} from 'angular2/core';
import {ServerComms} from './ServerComms';

@Injectable()
export class TicketService {
    constructor (private serverComms: ServerComms) {
        // do nothing
    }

    getTickets() {
        serverComms.get({
            name: 'mycompany',
            timeout: 15000
        })
            .subscribe(data => console.log(data));
    }
}

However, I receive the following error whenever I try this:

"No provider for ServerComms! (App -> TicketService -> ServerComms)"

I do not understand why? Surely I do not need to inject every service that each other service relies upon? This can grow pretty tedious? This was achievable in Angular 1.x - how do I achieve the same in Angular 2?

Is this the right way to do it?

Thierry Templier · Accepted Answer · 2016-04-15 17:55:52Z

In short since injectors are defined at component level, the component that initiates the call services must see the corresponding providers. The first one (directly called) but also the other indirectly called (called by the previous service).

Let's take a sample. I have the following application:

Component AppComponent: the main component of my application that is provided when creating the Angular2 application in the bootstrap function

@Component({
  selector: 'my-app', 
    template: `
      <child></child>
    `,
    (...)
    directives: [ ChildComponent ]
})
export class AppComponent {
}

Component ChildComponent: a sub component that will be used within the AppComponent component

@Component({
    selector: 'child', 
    template: `
      {{data | json}}<br/>
      <a href="#" (click)="getData()">Get data</a>
    `,
    (...)
})
export class ChildComponent {
  constructor(service1:Service1) {
    this.service1 = service1;
  }

  getData() {
    this.data = this.service1.getData();
      return false; 
  }
}

Two services, Service1 and Service2: Service1 is used by the ChildComponent and Service2 by Service1

@Injectable()
export class Service1 {
  constructor(service2:Service2) {
    this.service2 = service2;
  }

  getData() {
    return this.service2.getData();
  }
}

@Injectable()
export class Service2 {

  getData() {
    return [
      { message: 'message1' },
      { message: 'message2' }
    ];
  }
}

Here is an overview of all these elements and there relations:

Application
     |
AppComponent
     |
ChildComponent
  getData()     --- Service1 --- Service2

In such application, we have three injectors:

The application injector that can be configured using the second parameter of the bootstrap function
The AppComponent injector that can be configured using the providers attribute of this component. It can "see" elements defined in the application injector. This means if a provider isn't found in this provider, it will be automatically look for into this parent injector. If not found in the latter, a "provider not found" error will be thrown.
The ChildComponent injector that will follow the same rules than the AppComponent one. To inject elements involved in the injection chain executed forr the component, providers will be looked for first in this injector, then in the AppComponent one and finally in the application one.

This means that when trying to inject the Service1 into the ChildComponent constructor, Angular2 will look into the ChildComponent injector, then into the AppComponent one and finally into the application one.

Since Service2 needs to be injected into Service1, the same resolution processing will be done: ChildComponent injector, AppComponent one and application one.

This means that both Service1 and Service2 can be specified at each level according to your needs using the providers attribute for components and the second parameter of the bootstrap function for the application injector.

This allows to share instances of dependencies for a set of elements:

If you define a provider at the application level, the correspoding created instance will be shared by the whole application (all components, all services, ...).
If you define a provider at a component level, the instance will be shared by the component itself, its sub components and all the "services" involved in the dependency chain.

So it's very powerful and you're free to organize as you want and for your needs.

Here is the corresponding plunkr so you can play with it: https://plnkr.co/edit/PsySVcX6OKtD3A9TuAEw?p=preview.

This link from the Angular2 documentation could help you: https://angular.io/docs/ts/latest/guide/hierarchical-dependency-injection.html.

Mark Rajcok · Accepted Answer · 2016-04-15 16:54:48Z

Surely you do.

In Angular2, there are multiple injectors. Injectors are configured using the providers array of components. When a component has a providers array, an injector is created at that point in the tree. When components, directives, and services need to resolve their dependencies, they look up the injector tree to find them. So, we need to configure that tree with providers.

Conceptually, I like to think that there is an injector tree that overlays the component tree, but it is sparser than the component tree.

Again, conceptually, we have to configure this injector tree so that dependencies are "provided" at the appropriate places in the tree. Instead of creating a separate tree, Angular 2 reuses the component tree to do this. So even though it feels like we are configuring dependencies on the component tree, I like to think that I am configuring dependencies on the injector tree (which happens to overlay the component tree, so I have to use the components to configure it).

Clear as mud?

The reason Angular two has an injector tree is to allow for non-singleton services – i.e., the ability to create multiple instances of a particular service at different points in the injector tree. If you want Angular 1-like functionality (only singleton services), provide all of your services in your root component.

Architect your app, then go back and configure the injector tree (using components). That's how I like to think of it. If you reuse components in another project, it is very likely that the providers arrays will need to be changed, because the new project may require a different injector tree.

kemsky · Accepted Answer · 2016-04-15 18:46:16Z

1

Well, i guess you should provide both services globally:

bootstrap(App, [service1, service2]);

or provide to component that uses them:

@Component({providers: [service1, service2]})

@Injectable decorator adds necessary metadata to track dependecies, but does not provide them.

answered Apr 15, 2016 at 18:46

kemsky

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