What is dependency injection (DI) in Angular?

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Dependency injection (DI) is an important application design pattern. In DI, a class asks for dependencies from external sources rather than creating them itself. Angular has its own dependency injection framework to resolve dependencies. So, your services depend on other services throughout your application.

In Angular, Dependency Injection (DI) is a design pattern used to manage the dependencies of components and services. It allows components and services to declare their dependencies, without having to create or manage them directly. Instead, Angular’s dependency injection system takes care of providing the necessary dependencies when a component or service is instantiated.

Here’s a more detailed breakdown of how dependency injection works in Angular:

  1. Declaration: Dependencies are declared in the constructor of a component or service.
    typescript
    constructor(private dependency: DependencyService) {}
  2. Provider Registration: Angular’s injector needs to know how to create these dependencies. Providers are used to register dependencies with the Angular injector. Providers can be registered at various levels, including at the module level, component level, or in a service.
    typescript
    @NgModule({
    providers: [DependencyService]
    })
  3. Injection: When Angular creates an instance of a component or service, it checks the constructor parameters for any dependencies and provides them using the registered providers.
    typescript
    constructor(private dependency: DependencyService) {}
  4. Hierarchical Injector: Angular’s DI system is hierarchical, meaning that components and services inherit the injectors of their parent components or modules. This allows for easy sharing of dependencies throughout an application.

Dependency Injection in Angular offers several benefits, including:

  • Modularity: Components and services become more modular since they rely on interfaces rather than concrete implementations.
  • Testability: It becomes easier to mock dependencies for unit testing.
  • Reusability: Components and services can be reused more easily in different parts of the application since their dependencies are externalized.
  • Loose Coupling: Components and services are loosely coupled with their dependencies, making it easier to replace or update them without affecting other parts of the application.

In summary, Dependency Injection in Angular simplifies the management of dependencies by externalizing them and providing them when needed, promoting modularity, testability, and maintainability in Angular applications.