Dependency injection in Angular

Dependencies are services or objects that a class needs to perform its function. Dependency injection, or DI, is a design pattern in which a class requests dependencies from external sources rather than creating them.

What are the services?

A Service is a broad category encompassing any value, function, or feature that an app needs. A service is typically a class with a narrow, well-defined purpose. It should do something specific and do it well. Angular distinguishes components from services to increase modularity and reusability. By separating a component's view-related functionality from other kinds of processing, you can make your component classes lean and efficient.

Service examples

Here's an example of a service class that logs to the browser console.src/app/logger.service.ts (class)

content_copyexport class Logger {
  log(msg: any)   { console.log(msg); }
  error(msg: any) { console.error(msg); }
  warn(msg: any)  { console.warn(msg); }
}

Services can depend on other services. For example, here's a HeroService that depends on the Logger service, and also uses BackendService to get heroes. That service in turn might depend on the HttpClient service to fetch heroes asynchronously from a server.src/app/hero.service.ts (class)

content_copyexport class HeroService {
  private heroes: Hero[] = [];

  constructor(
    private backend: BackendService,
    private logger: Logger) { }

  getHeroes() {
    this.backend.getAll(Hero).then( (heroes: Hero[]) => {
      this.logger.log(`Fetched ${heroes.length} heroes.`);
      this.heroes.push(...heroes); // fill cache
    });
    return this.heroes;
  }
}