What is/are service(s)

Services & DI (in aspect of angular)

• What is service?
• Why we need?
• How to use service in angular?
• What is DI (Dependency Injection)
• What is Injector in Angular Services 
• Hands On Services & DI
• More on Angular Services (How they actually works)

A plain type script class for well-defined purpose that’s it, nothing else.

Export class EmployeeeService{

constructor (){

        }

}

Why Services?

• DRY
• Separation of concerns
• To Increase modularity & reusability 
• https://angular.io/guide/architecture-services - Angular docs also ask for same

How to use services in Angular?

• Create a service class (best to use Robot )
• Register service with angular (Injector).
• Inject/use service using (Dependency Injection )

Dependency Injection

• Code without DI - Drawbacks
• DI as Design Pattern
• DI as Framework in context of Angular

Code without  DI - Drawbacks

DI as Design Pattern

• DI is a coding pattern in which a class receives it dependencies from external sources rather than creating it from itself

DI as Design Pattern Cont.

Dependency Injection as Framework using Injector

• Injector is nothing but just like a container in angular framework which keeps track or list of all dependencies.
• When any components or elements would required any dependency then injector is responsible to provide that dependency at the time of initialization.

Let’s get back to hands on Services

• Create a service class (best to use robot) - done
• Register service with angular (Injector). - done
• Inject/use service using (Dependency Injection )

More on Services : Ways of Registering services 

Module Injector

• With-in provider section of @NgModule decorator 
• Using @Injectable decorator with meta data 

Element Injector

• created implicitly at each DOM element (@Component, @Directive)

Note : Registration of services in angular follows the hierarchy system and it would be responsibility of  Injector (by whom it can be injected & by whom can be not). 

Register at Child component level

Register at Parent component level

Register at module/root level

More on Services : ‘root’ || ‘platform’ || any

More on Services : Deep Dive

• Which one is better Module Injector (@NgModule or @Injectable)
• @Injectable annotation (what else this annotation do apart from registration)
• Is Angular services are Singleton?  
• What is Sandboxing ?