Services and Dependency Injection

Question 1

What are services?

Answer 1

Services are:

• Class with focused purpose
• Used for features that are independent from any particular component
• Used for providing shared data or logic across components
• Encapsulate external interactions

Question 2

How many ways for using service?

Answer 2

1 Way: (bad)
Component can create instance of service class and use it. But instance is local to the component and it's difficult to mock the service

2 Way:
Register service with angular. Angular creates a singleton service instance. Angular has a built in injector. It maintains a container of services.
If component needs service, it define the service as a dependency.

Question 3

Dependency injection is

Answer 3

A coding pattern in which a class receives the instance of objects it needs (called dependencies) from an external source rather than creating them itself.

In angular the external source is the angular injector.

Question 4

What to do in order to create a service?

Answer 4

1. Create class

2. Add @Injectable() decorator

Question 5

How many ways for registering a service

Answer 5

Two. Root injector or Component injector.

Question 6

What Root injector does?

Answer 6

Service is available throughout the application.

Recommended for most scenarios

Question 7

What Component injector does?

Answer 7

Service is only available to that component and its child (nested) components.
Isolates a service used by only one component.
Provides multiple instances of the service.

Question 8

How to register service in root application

Answer 8

In @Injectable decorator set {
providedIn: ‘root’
}

Question 9

How to register service for specific component

Answer 9

In @Component decorator in the component itself add {…
providers: [ProductService]
}

Question 10

How the service is injected?

Answer 10

Define it as a depndency.

export class ProductionListComponent {
private _productService;
constructor(productService: ProductService)  {
this._productService = productService;
}
}

A sort cut of this is

constructor(private productService: ProductService)