Link: https://angular.io/docs/js/latest/api/core/OnInit-interface.html, https://www.youtube.com/watch?v=bY92HFyaBvo

 

export interface OnInit

exported from  defined in 

Implement this interface to execute custom initialization logic after your directive's data-bound properties have been initialized.

ngOnInit is called right after the directive's data-bound properties have been checked for the first time, and before any of its children have been checked. It is invoked only once when the directive is instantiated.

 

Keep it as a best pratices that implements OnInit, OnDestroy. Then inside ngOnInit, we call service to fetch data back. And in ngOnDestroy, we unsubscribe the Observable.

 

import {Component, ElementRef, EventEmitter, Output, OnInit, OnDestroy} from 'angular2/core';import {Observable} from 'rxjs/Rx';import {SearchServcie} from './SearchServcie';@Component({    selector: 'search-input',    template: ``})export class SearchInput implements OnInit, OnDestroy{    @Output() loading = new EventEmitter();    @Output() results = new EventEmitter();    private _subscription;    constructor(        public youtube: SearchServcie,        private el:ElementRef) {}    ngOnDestroy() {        // Unsubscribe the Observabler        console.log('ngOnDestroy'); 　　　　　if(this._subscription)　            this._subscription.unsubscribe();    }    ngOnInit() {        // Fetch the data from server        console.log("init");        const keyup$ = Observable.fromEvent(this.el.nativeElement, 'keyup');        const searchInput$ = keyup$            .debounceTime(500)            .map((e:any) => {                return e.target.value.trim();            })            .distinctUntilChanged()            .filter((val:string) => {                return val.length > 1;            })            .do(() => {                this.loading.emit(true);            });        const query$ = searchInput$            .switchMap( (query) => this.youtube.doSearch(query));        this._subscription = query$.subscribe((videos) => {                this.results.emit(videos);                this.loading.emit(false);            }, (err) => {                console.log(err);                this.loading.emit(false);            }, () => {                this.loading.emit(false);            });    }}