typescript - Angular2: How to load data before rendering the component?

Question

I am trying to load an event from my API before the component gets rendered. Currently I am using my API service which I call from the ngOnInit function of the component.

My EventRegister component:

import {Component, OnInit, ElementRef} from "angular2/core";
import {ApiService} from "../../services/api.service";
import {EventModel} from '../../models/EventModel';
import {Router, ROUTER_DIRECTIVES, ROUTER_PROVIDERS, RouteConfig, RouteParams, RouterLink} from 'angular2/router';
import {FORM_PROVIDERS, FORM_DIRECTIVES, Control} from 'angular2/common';

@Component({
    selector: "register",
    templateUrl: "/events/register"
    // provider array is added in parent component
})

export class EventRegister implements OnInit {
    eventId: string;
    ev: EventModel;

    constructor(private _apiService: ApiService, 
                        params: RouteParams) {
        this.eventId = params.get('id');
    }

    fetchEvent(): void {
        this._apiService.get.event(this.eventId).then(event => {
            this.ev = event;
            console.log(event); // Has a value
            console.log(this.ev); // Has a value
        });
    }

    ngOnInit() {
        this.fetchEvent();
        console.log(this.ev); // Has NO value
    }
}

My EventRegister template

<register>
    Hi this sentence is always visible, even if `ev property` is not loaded yet    
    <div *ngIf="ev">
        I should be visible as soon the `ev property` is loaded. Currently I am never shown.
        <span>{{event.id }}</span>
    </div>
</register>

My API service

import "rxjs/Rx"
import {Http} from "angular2/http";
import {Injectable} from "angular2/core";
import {EventModel} from '../models/EventModel';

@Injectable()
export class ApiService {
    constructor(private http: Http) { }
    get = {
        event: (eventId: string): Promise<EventModel> => {
            return this.http.get("api/events/" + eventId).map(response => {
                return response.json(); // Has a value
            }).toPromise();
        }     
    }     
}

The component gets rendered before the API call in the ngOnInit function is done fetching the data. So I never get to see the event id in my view template. So it looks like this is a ASYNC problem. I expected the binding of the ev (EventRegister component) to do some work after the ev property was set. Sadly it does not show the div marked with *ngIf="ev" when the property gets set.

Question: Am I using a good approach? If not; What is the best way to load data before the component is starting to render?

NOTE: The ngOnInit approach is used in this angular2 tutorial.

EDIT:

Two possible solutions. First was to ditch the fetchEvent and just use the API service in the ngOnInit function.

ngOnInit() {
    this._apiService.get.event(this.eventId).then(event => this.ev = event);
}

Second solution. Like the answer given.

fetchEvent(): Promise<EventModel> {
    return this._apiService.get.event(this.eventId);
}

ngOnInit() {
    this.fetchEvent().then(event => this.ev = event);
}

Answer 1

update

• If you use the router you can use lifecycle hooks or resolvers to delay navigation until the data arrived. https://angular.io/guide/router#milestone-5-route-guards

• To load data before the initial rendering of the root component APP_INITIALIZER can be used How to pass parameters rendered from backend to angular2 bootstrap method

original

When console.log(this.ev) is executed after this.fetchEvent();, this doesn't mean the fetchEvent() call is done, this only means that it is scheduled. When console.log(this.ev) is executed, the call to the server is not even made and of course has not yet returned a value.

Change fetchEvent() to return a Promise

     fetchEvent(){
        return  this._apiService.get.event(this.eventId).then(event => {
            this.ev = event;
            console.log(event); // Has a value
            console.log(this.ev); // Has a value
        });
     }

change ngOnInit() to wait for the Promise to complete

    ngOnInit() {
        this.fetchEvent().then(() =>
        console.log(this.ev)); // Now has value;
    }

This actually won't buy you much for your use case.

My suggestion: Wrap your entire template in an <div *ngIf="isDataAvailable"> (template content) </div>

and in ngOnInit()

    isDataAvailable:boolean = false;

    ngOnInit() {
        this.fetchEvent().then(() =>
        this.isDataAvailable = true); // Now has value;
    }