Understanding Services and Dependency Injection in Angular

In Angular, Services and Dependency Injection are used effectively for communication between non-related components. In most cases, Angular applications comprise of numerous components, not all of which are directly related. Therefore, a means for these components to exchange data becomes necessary. This is where Services and Dependency Injection (also known as DI) comes in to play.

What are Angular Services?

Services in Angular are essentially classes that encapsulate specific functionality or logic. They are designed to keep the components of an application light and focused on their designated purpose. By outsourcing common tasks and data management to Services, the components themselves can stay slim and focused on presenting data and responding to user interactions. This design principle is part of a larger concept known as separation of concerns (SoC), where different parts of an application are responsible for different functions.

For instance, you might have a ShoppingCartService handling all issues relating to the shopping cart, like adding items, removing items, and checking out. Any component that needs to deal with these cart-related actions could then inject this service and use its methods rather than implementing similar logic in multiple places.

How does Dependency Injection work?

Dependency Injection is a design pattern that Angular uses to provide new instances of services or other dependencies that a requestor class needs to perform its role. The requestor (usually a component or another service) doesn't need to know how these dependencies are created or where they come from; it just asks for them, and the DI system provides them.

When a component depends on a service, it asks the Angular DI system for that service in its constructor. Something along the lines of constructor(private cartService: ShoppingCartService) {}. Angular's DI system then supplies an instance of ShoppingCartService to the component when it creates it.

Angular’s DI system is hierarchical, which means that when a component requests a service, Angular searches for that service in the component’s own injector and, if it doesn’t find it there, in the injectors of its ancestor components, all the way up to the root injector. This hierarchy of injectors makes services configured at higher levels available to their descendant components, which is advantageous for sharing configuration and state across those elements.

Best Practices

A recommended practice when working with Services and DI in Angular is to ensure that you adhere to the Single Responsibility Principle. Make sure services do one thing, and do it well. This way, you avoid ending up with large, complex Services that are hard to maintain.

Ensure services are stateless i.e. they don't store data. They should only contain functions. If you need to share data across components, consider using a state management library like NgRx or Akita.

Following these practices, you can use Services and Dependency Injection in Angular to promote code reuse and separation of concerns, whilst aiding in efficient communication between related and non-related components.