Ionic2/angular2 correct way of consuming service

I am leaing to build Ionic-2 app, I have a few components which consume services that make an http call and fetch some data, which in tu will be set in component and will finally be displayed in template. I overall understood the flow but I am making some logical mistake while coding it.

My example component:

export class FarmList {

  items: Object;

  constructor(private testService: TestService, public nav: NavController){}

  getData(): any {

    this.items = this.testService.fetchData()

  }

  nextView(){

    this.nav.push(Farm)

  }

  showDetails(id: Number){

    this.nav.push(Farm, {
      param1: id
    })

  }
}

My corresponding service:

@Injectable()
export class TestService{

    loading: boolean;
    data: Object;

    constructor(private http: Http){

        let myUrl = 'http://jsonplaceholder.typicode.com/users';

        this.loading = true;
        this.http.request(myUrl)
          .subscribe(
            (res: Response) => {

            this.loading=false;
            this.data=res.json();

          });

    }

    public fetchData(){

        retu this.data;

      }
}

Screenshot of the app:

So the problem here is:

• Unless I click Fetch button it will not load the data, somehow in the constuctor of the component the data must be set and not when I call getData() function. I tried writing this.items = this.testService.fetchData() this line in the constructor but it doesn't work.

• This problem further becomes worse when in another component and service I have to append navparam sent from this FarmList component: let myUrl = 'http://jsonplaceholder.typicode.com/users/' + this.id ; I try to append this.id which is set to navparam as received in it's constructor and I get undefined

I am simply trying to have a list on one page and then clicking on one of the list items will open new page with it's some more details. I am hitting this publically available API: http://jsonplaceholder.typicode.com/users/ and then appending some number to it to get only one object.

What is the correct way to do it?