我应该在其构造函数中还是在 app.component 的 ngOnInit 方法中初始化 Angular 服务？

Question

I'm new to Angular. I have a service that needs to subscribe to a subject provided by another service. If this was a component, I would create the subscription in the ngOnInit method of the component. NgOnInit isn't called for Services, though. I have created an initialise method to create the subscription but I'm not sure where best to call it from. I have seen it done two ways:

1) Call the initialise method in the service constructor

2) Inject the service into the app.component and call the service initialise method in the app.component's ngOnInit method eg

method 1:

export class MyService{

    constructor(private myRequiredService: RequiredService) {
        this.initialiseSubs();
    }

    initaliseSubs(){
        //code to set up subscriptions
    }
}

---

Method 2

---

export class MyService{

    constructor(private myRequiredService: RequiredService) {}

    initaliseSubs(){
        //code to set up subscriptions
    }
}

---

export class AppComponent implements OnInit{
  title = 'my-app';

  constructor(private myService:MyService){}

  ngOnInit(){
    this.myService.initialiseSubs();

  }
}

Both methods seem to work but I would welcome advice as to which is preferable. Thanks in advance.

Answer 1

The service is the place where you define your observables, for example, getAllEmployees() , The components are the place where you use those services ( inject the service into the constructor).

    export class MyService{
    
        constructor(private myRequiredService: RequiredService) {}
    
        getAllEmployees():Observable<Employee[]>{
           
           // return Observables of type Employee[]
        }
    }

Suppose now you have a component called EmployeesList that's responsible for showing all the employees data, first, you need to inject the service into the constructor, then you need to call the getAllEmployees service method and subscribe in order to get the data.

    export class EmployeesListimplements OnInit{
      private employees: Employee[];
    
      constructor(private myService:MyService){}
    
      ngOnInit(){
         // it's better to do the subscription inside ngOnInit.
        this.myService.getAllEmployees().subscribe(
          employees => {
             this.employees = employees; 
          }

        );
  
      }
    }

That's basically how you should do it, but for better development and better user experience I advice using angular resolvers , resolvers are basically methods that can be attached to a specific route and angular will call them before rendering the component, inside these methods you need to subscribe to the getAllEmployees service method and get the values inside the component, the main goal of using resolvers is to avoid rendering the view then waiting for the data to be fetched from the resources.
But that's considered as an advanced topic, I will not post any codes for that unless you asked me for.

Please mark it as correct if the answer did help you. All the best!

Edit After discussing below in the comments, I understood that you have different back-ends to communicate with depending on the current route, I would handle this case just like this:

    // inside the component 
    constructor(private router: Router) {}
    ngOnInit(){
            // You need to find 
            this.myService.getAllEmployees(this.router.url).subscribe(
              employees => {
                 this.employees = employees; 
              }
    
            );
      
    }

// inside the service file:

    export class MyService{
        
            constructor(private myRequiredService: RequiredService) {}
        
            getAllEmployees(URL: string):Observable<Employee[]>{
               // decide which back-end to connect with depending on the URL
               switch(url){
                case "example1":
                                URL = "url1";
                         break;
                case "example2":
                                URL = "url2";
                         break;
               }
               // Connect to the resource using URL param
               // return Observables of type Employee[]
            }
    }