README.md

Day 05 - Dependency Injection

Projects

fun-with-di	Deep dive into Dependency Injection in angular

Introduction to Dependency Injection

• We talked about the motivation to use a dependency injection infrastructure
• We saw how to create a service - an object encapsulating a bit of logic and perhaps data
• We defined the terms:
  • Consumer - the object that requires dependency
  • Injection request / token - The item that the consumer asks for, to be provided using dependency in the constructor
  • Injector - An object responsible for resolving (providing, injecting) the injection request
  • Provider - The algorithm, or logic, used to resolve the injected object
• We saw how to define injecter and provider using the providers property of a Module, or Component
• We understood the effect of defining a component as injector, and how to use the component hierarchy as injector hierarchy
• We saw how to define a provider that used another class instead of the requested class
• We saw other types of providers:
  • useClass - defines a class provider, essentialy calling new to instantiate an object
  • useValue - provide an already created value
  • useExisting - provides the value of a different request / token
  • useFactory - calls a function to calculated the provided value
• We saw how to define a service as Injectable so it can also consume dependencies
  • We talked about the Tree Shaking Algorithm and how to define "tree shakable" services
• We created an Injector on our own and saw how to
  • Create it using the Injector.create factory
  • Define the injector providers
  • Define the parent injector
  • Use it to get instances of objects per token.
• We saw how to define an Injection Token
  • We used the useValue and useFactory providers to set the value of the token
  • We consumed the token using the @Inject decorator
• We saw how to inject APP_INITIALIZER to initialize a service