ngOnInit not being called when Injectable class is Instantiated

Question

Why isn't ngOnInit() called when an Injectable class is resolved?

Code

import {Injectable, OnInit} from 'angular2/core';
import { RestApiService, RestRequest } from './rest-api.service';

@Injectable()
export class MovieDbService implements OnInit {

    constructor(private _movieDbRest: RestApiService){
        window.console.log('FROM constructor()');
    }

    ngOnInit() {
         window.console.log('FROM ngOnInit()');
    }

}

Console Output

FROM constructor()

Answer 1

Lifecycle hooks, like OnInit() work with Directives and Components. They do not work with other types, like a service in your case. From docs:

A Component has a lifecycle managed by Angular itself. Angular creates it, renders it, creates and renders its children, checks it when its data-bound properties change and destroy it before removing it from the DOM.

Directive and component instances have a lifecycle as Angular creates, updates, and destroys them.

Answer 2

I don't know about all the lifecycle hooks, but as for destruction, ngOnDestroy actually get called on Injectable when it's provider is destroyed (for example an Injectable supplied by a component).

From the docs :

Lifecycle hook that is called when a directive, pipe or service is destroyed.

Just in case anyone is interested in destruction check this question:

Answer 3

Note: this answer applies only to Angular components and directives, NOT services.

I had this same issue when ngOnInit (and other lifecycle hooks) were not firing for my components, and most searches led me here.

The issue is that I was using the arrow function syntax (=>) like this:

class MyComponent implements OnInit {
    // Bad: do not use arrow function
    public ngOnInit = () => {
        console.log("ngOnInit");
    }
}

Apparently that does not work in Angular 6. Using non-arrow function syntax fixes the issue:

class MyComponent implements OnInit {
    public ngOnInit() {
        console.log("ngOnInit");
    }
}

Answer 4

I had to call a function once my dataService was initialized, instead, I called it inside the constructor, that worked for me.

Answer 5

Adding to answer by @Sasxa,

In Injectables you can use class normally that is putting initial code in constructor instead of using ngOnInit(), it works fine.

Answer 6

import {Injectable, OnInit} from 'angular2/core';
import { RestApiService, RestRequest } from './rest-api.service';
import {Service} from "path/to/service/";

@Injectable()
export class MovieDbService implements OnInit {

userId:number=null;

constructor(private _movieDbRest: RestApiService,
            private instanceMyService : Service ){

   // do evreything like OnInit just on services

   this._movieDbRest.callAnyMethod();

   this.userId = this.instanceMyService.getUserId()


}