Dependency Injection

Question 1

What is Dependency Injection?

Answer 1

• It is an important application design pattern.
• This increases the efficiency and modularity.
• They are services or objects that a class needs to perform its function.
• It is a coding pattern in which a class asks for dependencies from external sources rather than creating them itself.

Question 2

@Injectable decorator

Answer 2

@Injectable({ providedIn: ‘root’ })

export class xyzService{

getXyz() { return abc; }

}

• The class is a service.
• The injectable decorator markts it as a service that can be injected.
• The injector is responsible for creating service instances and injecting them into classes.
• A provider tells an injector how to create the service.
• Injector has to be configured in the provider.
• It can be configured at different levels
  
  • @Injectable() decorator
  • @NgModule() decorator
  • @Component() decorator
• When generating services, it is adding extra metadata with a ‘provided in’ property with a default of ‘root’ (it can be any module of the application)
• If ‘root’ is used then injectable will be registered as a singleton in the application and it is not needed to be added to the providers of the root module.
• If ‘UserModule’ is used then the injectable is registered as a provider of the userModule without adding it to the providers in the module.
• The better option is to provide it in @NgModule() - otherwise it will be plauged with circular dependencies.

Question 3

Configure injectors

Answer 3

1. By using providedIn metadata option - with root injector or the injector for a specific NgModule. @Injectable({ providedIn:’root’}) – provides this service in the root
2. The @NgModule() and @Component() decorators have the providers metadata option, where you can configure providers for NgModule-level or component-level injectors. @Component({ providers: [{provide: abcService, useValue: { name:’lmp’} }] }) —This is visible only at component.
3. @NgModule()

Question 4

Injector hierarchy

Answer 4

There is only one root injector for an app.