A provider is an instruction to the Dependency Injection system on how to obtain a value for a dependency. Most of the time, these dependencies are services that you create and provide.
Providing a service
If you already have an application that was created with the Angular CLI, you can create a service using the ng generate CLI command in the root project directory.
Replace User with the name of your service.
ng generate service User
This command creates the following UserService skeleton:
src/app/user.service.ts
import { Injectable } from '@angular/core';@Injectable({ providedIn: 'root',})export class UserService {}
You can now inject UserService anywhere in your application.
The service itself is a class that the CLI generated and that's decorated with @Injectable().
By default, this decorator has a providedIn property, which creates a provider for the service.
In this case, providedIn: 'root' specifies that Angular should provide the service in the root injector.
Provider scope
When you add a service provider to the root application injector, it's available throughout the application. Additionally, these providers are also available to all the classes in the application as long they have the lookup token.
You should always provide your service in the root injector unless there is a case where you want the service to be available only if the consumer imports a particular @NgModule.
Limiting provider scope by lazy loading modules
In the basic CLI-generated app, modules are eagerly loaded which means that they are all loaded when the application launches. Angular uses an injector system to make things available between modules. In an eagerly loaded app, the root application injector makes all of the providers in all of the modules available throughout the application.
This behavior necessarily changes when you use lazy loading. Lazy loading is when you load modules only when you need them; for example, when routing. They aren't loaded right away like with eagerly loaded modules. This means that any services listed in their provider arrays aren't available because the root injector doesn't know about these modules.
When the Angular router lazy-loads a module, it creates a new injector. This injector is a child of the root application injector. Imagine a tree of injectors; there is a single root injector and then a child injector for each lazy loaded module. This child injector gets populated with all the module-specific providers, if any. Look up resolution for every provider follows the rules of dependency injection hierarchy.
Any component created within a lazy loaded module's context, such as by router navigation, gets its own local instance of child provided services, not the instance in the root application injector. Components in external modules continue to receive the instances created for the application root injector.
Though you can provide services by lazy loading modules, not all services can be lazy loaded. For instance, some modules only work in the root module, such as the Router. The Router works with the global location object in the browser.
As of Angular version 9, you can provide a new instance of a service with each lazy loaded module.
The following code adds this functionality to UserService.
src/app/user.service.ts
import { Injectable } from '@angular/core';@Injectable({ providedIn: 'any',})export class UserService {}
With providedIn: 'any', all eagerly loaded modules share a singleton instance; however, lazy loaded modules each get their own unique instance, as shown in the following diagram.
Limiting provider scope with components
Another way to limit provider scope is by adding the service you want to limit to the component's providers array.
Component providers and NgModule providers are independent of each other.
This method is helpful when you want to eagerly load a module that needs a service all to itself.
Providing a service in the component limits the service only to that component and its descendants.
Other components in the same module can't access it.
src/app/app.component.ts
@Component({ // ... providers: [UserService]})export class AppComponent {}
Providing services in modules vs. components
Generally, provide services the whole application needs in the root module and scope services by providing them in lazy loaded modules.
The router works at the root level so if you put providers in a component, even AppComponent, lazy loaded modules, which rely on the router, can't see them.
Register a provider with a component when you must limit a service instance to a component and its component tree, that is, its child components.
For example, a user editing component, UserEditorComponent, that needs a private copy of a caching UserService should register the UserService with the UserEditorComponent.
Then each new instance of the UserEditorComponent gets its own cached service instance.
Injector hierarchy and service instances
Services are singletons within the scope of an injector, which means there is at most one instance of a service in a given injector.
Angular DI has a hierarchical injection system, which means that nested injectors can create their own service instances.
Whenever Angular creates a new instance of a component that has providers specified in @Component(), it also creates a new child injector for that instance.
Similarly, when a new NgModule is lazy-loaded at run time, Angular can create an injector for it with its own providers.
Child modules and component injectors are independent of each other, and create their own separate instances of the provided services. When Angular destroys an NgModule or component instance, it also destroys that injector and that injector's service instances.
For more information, see Hierarchical injectors.