AngularJs 2 - multiple instance of service created

Question

I have created AngularJS 2 service and use it in 2 differents components : App-Component & Sub-Component. Each one output property 'log' (a string) of my service.

StateService class :

@Injectable ()
class StateService {

    public log : string;
    static count : number = 0;

    constructor () {
        this.log = '';
        StateService.count++;
        this.writeToLog ('CREATED '+StateService.count+' at ' + new Date().toString());
    }

    public writeToLog (text : string) : void {
        this.log += text + '\n';
    }
}  

Component :

@Component ({
    selector : 'Sub-Component',
    template : `<hr>
            This is the Sub-Component !
            <BR>
            StateService Log : 
            <pre>{{ _stateService.log }}</pre>
            <button (click)="WriteToLog ()">Write to log</button>
            `,
    providers : [StateService]
})

export class SubComponent {
    constructor (private _stateService : StateService) {
    }

    public WriteToLog () : void {
        this._stateService.writeToLog ('From Sub-Component - This is '+new Date().toString());
    }
}

Live example of code here

I except that service is created once and when each component call WriteToLog method, the output is the same in each component but it's not.

Example of output :

The App-Component can output this :

Instance 1 - Created at Thu Jan 21 2016 11:43:51

From App-Component - This is Thu Jan 21 2016 11:43:54

From App-Component - This is Thu Jan 21 2016 11:43:55

and the Sub-Component can output this :

Instance 2 - Created at Thu Jan 21 2016 11:43:51

From Sub-Component - This is Thu Jan 21 2016 11:43:57

From Sub-Component - This is Thu Jan 21 2016 11:43:58

So it appear that 2 instance of service is created (instance 1 + instance 2)

I only want one instance ;) and when I append string in log, this must appear in both component.

Thank you for your help

Answer 1

update Angular >= 2.0.0-RC.6

Don't add the service to the providers of the component. Instead add it to

@NgModule({ providers: [...], ...

(of a module that is not lazy loaded because lazy loaded modules introduce their own scope)

@Component ({
    selector : 'Sub-Component',
    template : `<hr>
            This is the Sub-Component !
            <BR>
            StateService Log : 
            <pre>{{ _stateService.log }}</pre>
            <button (click)="WriteToLog ()">Write to log</button>
            `,
    // providers : [StateService] <== remove
})

Angular <=2.0.0-RC.5

If you add it on a component you get a new service instance for each component instance. Instead add it to

bootstrap(AppComponent, [StateService]);

You can have more fine-grained control by adding it to a single component, then this component and all children get the same instance injected but otherwise the application works with the instance created by bootstrap(). This is the "hierarchical" in Angulars DI.

See also
- http://blog.thoughtram.io/angular/2015/05/18/dependency-injection-in-angular-2.html
- http://blog.thoughtram.io/angular/2015/09/17/resolve-service-dependencies-in-angular-2.html

Answer 2

In addition to the Günter's great answer, this link could perhaps give more details about how the hierarchical dependency injection of Angular2 works:

• What's the best way to inject one service into another in angular 2 (Beta)?.