Scope and LifeTime of Services

The providedIn property in the @Injectable decorator determines where an Angular service is made available and how it's instantiated. It controls the scope and lifetime of the service.

Understanding ProvidedIn

providedIn specifies the injector that will provide the service. It essentially tells Angular where to register the service so that it can be injected into components or other services.

Possible Values of ProvidedIn

There are three main values for providedIn:

• 'root': The service is available throughout the entire application.
• 'any': The service is available in any module that injects it.
• NgModule: The service is available within a specific NgModule.

1. providedIn: 'root'

When you use providedIn: 'root', the service is registered with the root injector. This makes it a singleton, meaning only one instance of the service is created and shared across the entire application.

Example

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

    @Injectable({
      providedIn: 'root'
    })
    export class RootService {
      getValue(): string {
        return 'Value from RootService';
      }
    }
    

Explanation:

• The RootService is available to all components and services in the application.
• It's a singleton, so all components using it share the same instance.

2. providedIn: 'any'

Using providedIn: 'any' allows the service to be available in any module that imports it. This can be useful for library authors who want to provide services that can be used in any application.

Example

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

    @Injectable({
      providedIn: 'any'
    })
    export class AnyService {
      getValue(): string {
        return 'Value from AnyService';
      }
    }
    

Explanation:

• The AnyService is available in any module that imports it.
• It creates a separate instance for each module that imports it, unlike 'root'.

3. providedIn: NgModule

You can also provide a service within a specific NgModule. This makes the service available only to components and services within that module.

Example

First, create the service:

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

    @Injectable() // No providedIn here
    export class ModuleService {
      getValue(): string {
        return 'Value from ModuleService';
      }
    }
    

Then, provide it in your NgModule:

    import { NgModule } from '@angular/core';
    import { ModuleService } from './module.service';

    @NgModule({
      providers: [ModuleService]
    })
    export class MyModule { }
    

Explanation:

• The ModuleService is only available to components and services within MyModule.
• It creates a new instance of the service for each instance of the module.

Choosing the Right ProvidedIn Value

• Use 'root' for services that should be shared across the entire application (e.g., data services, logging services).
• Use 'any' for library services that should be available in any module.
• Use NgModule for services that should be scoped to a specific module.

If you have any questions, feel free to ask. Thank you for reading!