Injectors and Providers in Angular

In Angular, injectors and providers are key components of the Dependency Injection (DI) system, enabling efficient creation and injection of services.

Injectors

Injectors are responsible for instantiating dependencies required by components, directives, and services.

Hierarchy of Injectors

Angular's injectors are hierarchical, meaning child injectors inherit providers from their parent injectors:

Root Injector: Created during application bootstrap, typically providing singleton services.
Module Injector: Specific to Angular modules, allowing for module-level service instances.
Component Injector: Specific to components, providing component-level service instances.

Providers

Providers tell the Angular injector how to create instances of services. They define how dependencies are instantiated and injected.

Types of Providers

Class Provider: Uses a class to provide the service.
Value Provider: Uses a specific value as the service.
Factory Provider: Uses a factory function to create the service.
Alias Provider: Provides an alias to another provider.

Using Providers

Providers can be defined in different places within an Angular application:

@Injectable() decorator: Specify the provider directly within the service itself.
@NgModule() decorator: Register providers at the module level.
@Component() decorator: Register providers at the component level.

Example: Defining a Provider in @NgModule()

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';
import { MyService } from './my-service.service';

@NgModule({
  declarations: [AppComponent],
  imports: [BrowserModule],
  providers: [MyService], // Registering the provider
  bootstrap: [AppComponent]
})
export class AppModule { }

Key Points

Injectors create instances of dependencies and inject them where needed.
Providers define how to create instances of services.
Providers can be defined using various methods such as class, value, factory, and alias providers.
Providers can be registered at different levels: root, module, or component.

Conclusion

Understanding injectors and providers in Angular is crucial for effective Dependency Injection, leading to more modular and maintainable applications.