What is Dependency Injection

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Introduction

Dependency Injection (DI) is a software design pattern that deals with how components get hold of their dependencies. The Angular framework uses dependency injection to provide new components with the services or objects that they need to perform their tasks. It is a crucial part of Angular and quite integral to understand, especially for beginners.

Understanding Dependency Injection

In general, a 'dependency' means a service or an object that a class needs to perform its function. 'Injection' means the passing of a dependency to a dependent object (a client) that would use it.

Let's consider a simple example. Suppose we have a Car class. This Car class has a dependency on an engine. Without an engine, the Car won't function. In traditional programming, you might create an engine using the new keyword within the Car class, but this approach makes your Car class dependent on the Engine class.

Instead, the Car class should only be concerned with using the engine, not creating it. This separation is where Dependency Injection comes in. It gives the Car class its engine, so the Car doesn't have to create one. This approach makes our Car class more flexible, more testable, and easier to manage.

Dependency Injection in Angular

In Angular, objects are wired together by an "Injector." The injector is responsible for creating components, resolving their dependencies, and providing them to other components as requested.

When Angular creates a new instance of a component, it first checks to see if any dependencies are required. If any are needed, the Angular Injector is responsible for fulfilling those dependencies. If the Injector doesn't know how to fulfill a dependency, it throws an error.

Let's consider the previous Car example in the context of Angular. If CarComponent required EngineService as a dependency, and we registered EngineService in the Injector, Angular would automatically create an instance of EngineService when creating CarComponent.

import { Component } from '@angular/core';
import { EngineService } from './engine.service';

@Component({
    selector: 'car',
    template: `
      <h2>Car</h2>
    `,
    providers: [EngineService]
})
export class CarComponent {
    constructor(private engineService: EngineService) { }
}

Here, EngineService is injected into CarComponent via the constructor. Angular recognizes the engineService argument in the CarComponent constructor and provides an instance of EngineService.

Benefits of Dependency Injection

Dependency Injection design pattern offers several benefits:

• Decoupling: The objects do not need to know how their dependencies are created, which leads to a high degree of decoupling.
• Easy to Test: DI allows us to replace dependencies with mock objects easily and thus makes unit testing easier.
• Code Reusability: Dependency Injection can help make parts of our application reusable, as the components are decoupled and can be reused in different scenarios.
• Code Maintainability: Since the components are decoupled, they can be easily maintained and developed independently.

Conclusion

Dependency Injection is a powerful pattern for managing code dependencies. It promotes code reusability, testability, and maintainability. In Angular, the DI pattern is used extensively and is a critical concept to understand. It not only makes your application development easier but also helps to produce cleaner and more readable code.

Hopefully, this article has helped you to understand what Dependency Injection is and how it is used in Angular. Happy Learning!

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