Why ngOnInit is Not Called in Injectable Classes in Angular

Scope of Lifecycle Hooks

In Angular development, a common question arises: why is the ngOnInit method not called when implemented in a class decorated with @Injectable()? To address this, it is essential to understand the design purpose and applicable scope of Angular's lifecycle hooks.

According to the Angular official documentation, lifecycle hooks (such as OnInit, OnDestroy, etc.) are specifically designed for directives and components. Instances of these types are fully managed by the Angular framework throughout their lifecycle, including creation, rendering, checking of data-bound property changes, and destruction.

Differences Between Component and Service Lifecycles

Components, as core building blocks of Angular applications, undergo complete lifecycle management. Angular handles component instance creation, template rendering, creation and rendering of child components, monitoring of data-bound property changes, and execution of cleanup upon component disposal. During this process, Angular automatically invokes corresponding lifecycle hook methods at specific stages.

In contrast, services, although managed via the dependency injection system, have a much simpler lifecycle. Service instances typically remain as singletons throughout the application's lifecycle (unless configured otherwise) or are managed according to the provider's scope. Angular does not maintain complex lifecycle states for service instances and therefore does not automatically call lifecycle hooks like ngOnInit.

Code Example Analysis

Consider the following typical service implementation:

import {Injectable, OnInit} from '@angular/core';

@Injectable()
export class MovieDbService implements OnInit {
    constructor(private _movieDbRest: RestApiService) {
        window.console.log('FROM constructor()');
    }

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

When this service is injected into other components or services, the console will only output FROM constructor(), not FROM ngOnInit(). This occurs because during service instantiation, Angular only calls the constructor and does not trigger any lifecycle hooks.

Proper Initialization Approaches

If initialization logic is required in a service, it is recommended to execute it directly within the constructor or provide a dedicated initialization method for explicit invocation by consumers. For example:

@Injectable()
export class MovieDbService {
    constructor(private _movieDbRest: RestApiService) {
        this.initializeService();
    }

    private initializeService() {
        // Perform initialization logic
        window.console.log('Service initialized');
    }
}

This approach ensures that initialization logic is executed immediately upon service instantiation, avoiding reliance on lifecycle hooks that will not be called.

Framework Design Principles

Angular's design choice reflects its adherence to the separation of concerns principle. Components handle view-related logic and user interactions, necessitating full lifecycle management, whereas services primarily provide business logic and data operations with simpler lifecycles. This separation promotes modular, testable, and maintainable code.

Modern web development platforms like Angular offer clear architectural guidance through such distinct responsibility divisions. Understanding these underlying principles aids in building more robust and maintainable applications.