Mastering Angular Dependency Injection in a Large Scale Project

Angular is a robust and comprehensive framework that helps web developers build scalable applications. One of its most distinguishing features is Dependency Injection (DI) – a concept that enables developers to write decoupled, maintainable, and testable code. However, DI in Angular can be challenging, especially in large-scale projects. In this article, we will explore the concepts of DI in Angular and how to master it in a large-scale project.

What is Dependency Injection?

Dependency Injection is a design pattern that allows developers to invert control of object creation and management. Rather than having to hard-code dependencies on other objects or services, the injector (DI) provides the necessary dependencies as required. The purpose of DI is to promote loose coupling between application components, improve reusability and testability of code, and facilitate better code maintenance.

DI in Angular takes a slightly different approach than other Java-based applications. In Angular, DI is generally done through the @Injectable() decorator, which is used to declare dependencies within a service, directive, or component.

Understanding the Injector Hierarchy in Angular

Dependency Injection in Angular is organized around an injector hierarchy that consists of providers (or services) that can be injected into other components, services, or directives. The root injector is created when the application boots up and is responsible for providing the services to the app.

Angular Modules are also injectors, and each module has its own provider hierarchy. Whenever modules are imported into the root module, they become part of the provider hierarchy of the root injector. This hierarchy allows you to create and organize services in a structured way that makes it easy to manage even in large applications.

Injecting Services in Angular

When using Angular, injecting a service is as easy as adding it to the component’s constructor. The service will be resolved automatically by the injector and injected into the component.

import { Component } from '@angular/core';
import { MyService } from './my-service';

@Component({
  selector: 'my-component',
    template: `<h1>{{ title }}</h1>`,
})
export class MyComponent {
  constructor(private myService: MyService) { }

  title = this.myService.getTitle();
}

In the example above, the MyComponent constructor expects to receive an instance of the MyService class. The @Injectable() decorator on the MyService class indicates to the Injector that this service should be provided to any component that needs it.

Creating Custom Providers in Angular

In larger Angular applications, it is common to have multiple instances of the same service with different configurations or dependencies. Custom providers allow you to configure and customize instances of services.

To create a custom provider in Angular, you can use the provide function provided by Angular. The provide function takes two arguments: the token of the service and a configuration object.

Here’s an example of a custom provider that allows you to create different instances of the MyService class:

import { Component, Provider } from '@angular/core';
import { MyService } from './my-service';

const myServiceConfig: Provider = {
  provide: MyService,
  useFactory: (param: string) => new MyService(param),
  deps: ['param']
};

@Component({
  selector: 'my-component',
    template: `<h1>{{ title }}</h1>`,
    providers: [myServiceConfig],
})
export class MyComponent {
  constructor(private myService: MyService) { }
  
  title = this.myService.getTitle();
}

In the example above, we create a myServiceConfig provider that has a useFactory method. This method takes a param argument and creates a new instance of MyService using this parameter.

Using this custom provider in the MyComponent class requires adding it to the providers array of the component decorator.

Injecting Services into Child Components

In complex Angular applications, it is common to have components nested inside other components. You may need to inject services from a parent component or a module into the child components.

To do this, you can use the @Host() decorator to find and inject the service from a higher-level component or module. The @Host()` decorator looks for a parent injector and retrieves the closest matching provider from that injector.

Here’s an example of injecting a service from a higher-level component:

import { Component, Host } from '@angular/core';
import { ParentService } from './parent-service';

@Component({
  selector: 'parent-component',
  template: `
    <h1>Parent Component</h1>
    <child-component></child-component>
  `,
  providers: [ParentService],
})
export class ParentComponent {}

@Component({
  selector: 'child-component',
  template: `<h2>{{ childTitle }}</h2>`,
})
export class ChildComponent {
  constructor(@Host() private parentService: ParentService) {}

  childTitle = this.parentService.getTitle();
}

In the example above, we define a ParentComponent and a ParentService provider. The ChildComponent constructor accepts the parent service using the @Host() decorator.

Conclusion

Dependency Injection is an essential aspect of building scalable and maintainable applications. Angular provides robust support for DI through its injector hierarchy, provider system, and @Injectable() decorator. It’s essential to master DI in Angular to build large-scale applications easily and efficiently. Hopefully, this article has provided some insight into how to use DI in Angular effectively.