Dependency Injection in Angular: A Comprehensive Guide

Angular is an open source framework for building modern web applications. One of the key principles of Angular is dependency injection. It is a design pattern that allows the creation of efficient and scalable applications.

In this comprehensive guide, we will explore what dependency injection is in an Angular development company's workflow, the benefits, implementation, and best practices. Whether you are new to working at an Angular development company or an experienced developer, this guide will help you master the art of creating efficient and scalable applications using Angular dependency injection.

Let's dive deeper into the world of dependency injection!

Angular Dependency Injection Overview

What is dependency injection?

Dependency injection (DI) is a design pattern. Angular dependency injection is a mechanism where dependency injections of components or services are created. It is then injected into it at runtime rather than being created within the service or component. This allows the creation of modular, loosely coupled code. This means that components and services only need to worry about what the dependencies are, rather than creating them.

There are three types of dependency injection methods in Angular that can be used to provide dependencies to components and services. They are as follows.

The use of the above methods varies depending on the scenario and also depends on the application requirements. Let's look at the benefits of dependency injection in Angular.

The Benefits of Using Dependency Injection in Angular

Dependency injection offers many critical and important benefits for Angular applications. Let's look at some of them.

1. The component and service class are more modular.
2. Complex configuration and mockup are unnecessary; we can test in isolation as it is much easier.
3. Due to the modularity of the code, it can be reused very easily.
4. Codebase is now easier to manage.

Understanding Inversion of Control (IoC) and its role in DI

In Inversion of Control (IoC), the component or service is not responsible for managing its dependencies. Therefore, it must be injected into a separate container or structure. It is a design principle closely linked to dependency injection. The responsibility should be reversed. This means it must be injected into a separate container or framework.

An example of IoC and DI in Angular can be seen in the following code.

 import { Component } from '@angular/core';
 import { ProductService } from './product.service';

 @Component({
 selector: 'app-product-list',
 templateUrl: './product-list.component.html',
 })
 export class ProductListComponent {
 products: any;

 constructor(private productService: ProductService) {}

 ngOnInit {
  this.products = this.productService.getProducts;
 }
 }

In this example, the ProductListComponent depends on the ProductServiceProductService export class. Instead of creating an instance of the ProductService inside the ProductListComponent. The constructor function is used to inject ProductService into the component through the constructor.

The ProductService itself may have dependencies on other services, but you don't need to worry about creating these dependencies because the DI framework is responsible for creating the instances of these services and injecting them into the ProductService.

Basic concepts in Angular dependency injection

Dependency injection is facilitated by several basic concepts. It is important to understand these concepts to effectively utilize Angular dependency injection.

Providers

Providers are objects responsible for creating and managing instances of dependencies that can be injected into components and services. It can be defined at the component, module, or application level. You need to use the Provider property to implement it in Angular. Let's look at an example to see how it is implemented.

 import { Component, Injectable } from '@angular/core';

 @Injectable
 export class MyService {
 getData {
  return "Data from MyService";
 }
 }

 @Component({
 selector: 'my-component',
 providers: (MyService),
 template: ' {{ data }} '
 })
 export class MyComponent {
 constructor(private myService: MyService) {}

 data = this.myService.getData;
 }

The example above defines a MyService class with an @Injectable decorator. The MyService class must be injected as a dependency by Angular.

useClass property

It is also important to specify which classes will be used as dependencies. Let's look at the useClass property.

import { Component, Injectable } from '@angular/core';

 @Injectable
 export class MyService {
 getData {
  return "Data from MyService";
 }
 }

 @Injectable
 export class MyOtherService {
 getData {
  return "Data from MyOtherService";
 }
 }

 @Component({
 selector: 'my-component',
 providers: ({ provide: MyService, useClass: MyOtherService }),
 template: ' {{ data }} '
 })
 export class MyComponent {
 constructor(private myService: MyService) {}

 data = this.myService.getData;
 }

useValue property

This property is used to specify a value that should be used as a dependency. An example implementation is as follows:

 import { Component } from '@angular/core';

 const myValue = "Data from useValue";

 @Component({
 selector: 'my-component',
 providers: ({ provide: 'MyValue', useValue: myValue }),
 template: ' {{ data }} '
 })
 export class MyComponent {
 constructor(@Inject('MyValue') private data: string) {}
 }

In the example above, the useValue Providers property specifies and provides a string value. The value is injected into the data property using the @Inject decorator.

useFactory property

This property is used to specify a factory function that should be used to create a dependency. Let's look at an example.

 import { Component } from '@angular/core';

 export function myFactory {
 return "Data from useFactory";
 }

 @Component({
 selector: 'my-component',
 providers: ({ provide: 'MyValue', useFactory: myFactory }),
 template: ' {{ data }} '
 })
 export class MyComponent {
 constructor(@Inject('MyValue') private data: string) {}
 }

The useFactory attribute is used to define and provide a factory function in the previous example. The @Inject decorator instructs the application to set the data property to the value returned by the factory function.

useExisting property

This property is used to specify an existing dependency that should be used as the value of the new dependency. Let's look at an example.

 import { Component, Injectable } from '@angular/core';

 @Injectable
 export class MyService {
 getData {
  return "Data from MyService";
 }
 }

 @Injectable
 export class MyOtherService {
 getData {
  return "Data from MyOtherService";
 }
 }

 @Component({
 selector: 'my-component',
 providers: (MyService, { provide: MyOtherService, useExisting: MyService }),
 template: ' {{ data }} '
 })
 export class MyComponent {
 constructor(private myOtherService: MyOtherService) {}

 data = this.myOtherService.getData;
 }

The @Injectable decorator is used in defining MyService and MyOtherService in the previous example. Using the useExisting property, the MyComponent class declares that MyService should be used as the value of MyOtherService.

Injectors

The injector is responsible for creating and managing dependencies. An entire application has a root injector that Angular creates automatically. All other injectors created later are children of the root injector. Let's look at an example.

 import { Component, Injectable, Injector } from '@angular/core';

 @Injectable
 export class MyService {
 getData {
  return "Data from MyService";
 }
 }

 @Component({
 selector: 'my-component',
 template: ' {{ data }} '
 })
 export class MyComponent {
 constructor(private injector: Injector) {}

 data = this.injector.get(MyService).getData ;
 }

In the code above, the MyService class is defined with an @Injectable decorator. This means the class can be injected as a dependency. The MyComponent class defines an Injector dependency using the constructor to inject an Injector instance. The data property is then set to the result of calling the getData method on an instance of MyService which is retrieved using the get method on the injector instance.

Files

Dependency injection token in Angular is used to identify a dependency represented by a string or class. It is important to note that the injection token is used with the provided property to specify which dependency should be used for the provided token. Let's look at the implementation example as follows:

 import { Component, Injectable, Inject } from '@angular/core';

 export const MY_TOKEN = 'myToken';

 @Injectable
 export class MyService {
 getData {
  return "Data from MyService";
 }
 }

 @Component({ 
selector: 'my-component',
 providers: ({ provide: MY_TOKEN, useClass: MyService }),
 template: ' {{ data }} '
 })
 export class MyComponent {
 constructor(@Inject(MY_TOKEN) private myService: MyService) {}

 data = this.myService.getData;
 }

In the code example above, the MyService class tells Angular that this class can be injected as a dependency. The MyComponent class defines a dependency on MyService by specifying it as the value of the property provided using the MY_TOKEN token. The @Inject decorator is used to inject an instance of MyService into the myService property of the MyComponent class. The data property is then set to the result of calling the getData method on myService.

Implementing dependency injection in Angular

This section of the article will focus on implementing Angular dependency injection.

Prerequisites

You must have the following:

• Node.js: The latest version of Node.js on your machine.
• A code editor: Any IDE that supports Angular.
• npm: Node Package Manager to install necessary dependencies.
• Angular CLI: The latest version that provides Angular core.

Install necessary dependencies

If you don't have Angular pre-installed on your machine. Use the following command in the terminal to install Angular CLI:

 npm install -g @angular/cli

Create a new project

To create a new Angular project and starter app, run the CLI command ng new and provide the name my-app.

 ng new my-app

Your package.json file should look like this:

Note that the dependency versions are specified in the file above. These were the versions at the time of creating this guide.

Implementation

Next, a service needs to be created so that injection can be configured for the components. Services are classes that provide functionality for the entire application. You can create a service using Angular CLI by running the following command.

 ng generate service my-service

Once the service is created, you will need to register it with Angular's dependency injection system. You can do this by adding it to the providers array in the app module. The root component is added here by default. Open the app.module.ts file and add the following line.

 import { MyServiceService } from './my-service.service';

 @NgModule({
 declarations: (
  AppComponent
 ),
 imports: (
  BrowserModule,
  AppRoutingModule
 ),
 providers: (MyServiceService), // add the service here
 bootstrap: (AppComponent)
 })
 export class AppModule { }

Injecting Dependencies into Components and Services

Now that the service is configured and registered with the dependency injection system. It can be injected into the parent component or child components. To do this, you must add a constructor to the component and specify the service as a parameter. For example, think of a MyComponent component that needs to use the MyService service. You can inject the service into the component like this. You should add this in my component's ts file.

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

 @Component({
 selector: 'my-component',
 template: '<p> {{ getMessage }} </p>'
 })
 export class MyComponent {

 constructor(private myService: MyServiceService) {}

 getMessage: string {
  return this.myService.getMessage;
 }
 }

In the component constructor, MyServiceService is specified as a parameter. A getMessage method is also defined that fetches data from the service.

Next, instead of registering a service with a module, we can use provideIn to register the service with the root injector. We can do this by specifying provideIn: 'root' in our service's @Injectable decorator.

 import { Injectable } from '@angular/core';

 @Injectable({
 providedIn: 'root'
 })
 export class MyServiceService {
 getMessage: string {
  return 'Hello from MyService!';
 }
 }

This will register the service with the root injector. This makes it available to any component in our application. You can also fetch data from services from external sources. This is how you register services in Angular.

Dependency Injection in Directives and Pipes

To inject services and other dependencies into directives and channels, you need to add a constructor and specify the dependencies as parameters.

For example, let's say you have a custom directive that needs to use the MyService service. You can inject the service into the directive like this.

First, create a new custom.directive.ts directive file in the application folder. Add the following code.

 import { Directive, Input, ElementRef } from '@angular/core';

 @Directive({
 selector: '(customDirective)'
 })
 export class CustomDirective {
 @Input customDirective: string;

 constructor(private el: ElementRef) {
  this.customDirective="";
 }

 ngOnInit {
  this.el.nativeElement.style.color = this.customDirective;
 }
 }

This directive takes an input of type string and uses it to change the color of the element to which it is applied.

Next, in the app.module.ts file, import the CustomDirective and add it to the export declarations and arrays. This file is the root module of the application. In the declarations you can see all the components and the directive.

 import { NgModule } from '@angular/core';
 import { BrowserModule } from '@angular/platform-browser';

 import { AppComponent } from './app.component';
 import { MyServiceService } from './my-service.service'; 
import { MyComponent } from './my-component/my-component.component';
 import { CustomDirective } from './custom.directive';

 @NgModule({
 declarations: (
 AppComponent,
 MyComponent,
 CustomDirective // ​​add here
 ),
 imports: (
 BrowserModule,
 ),
 providers: (MyServiceService),
 bootstrap: (AppComponent)
 })
 export class AppModule { }

Finally, in the app.component.html file, apply the directive to an element and pass the desired color. Add my-component too.

 <h1 customDirective="red">Hello World!</h1>
 <my-component></my-component>

Run the application

Open a terminal window and navigate to your project's root directory. Run the ng serve command to start the development server.

Open a web browser and navigate to view the application.

This is how the web page should look.

Hierarchical Dependency Injection

Dependency injection creates a tree-like structure of injectors. Each component injector can access the service provided by the parent and root injectors. Whenever a component class requests a service, Angular looks for it in the current injector. If not found, it will search the injector tree until a provider is found.

Testing with dependency injection

TestBed is a utility provided by Angular for testing. It allows you to configure and create a test module. You can use the TestBed.configureTestingModule method to configure the test module with the required dependencies and providers.

Let's see how the tests are done.

Testing a component with an injected service.

 import { ComponentFixture, TestBed } from '@angular/core/testing';
 import { MyComponent } from './my.component';
 import { MyService } from './my.service';

 describe('MyComponent', => {
 let component: MyComponent; 
let fixture: ComponentFixture<MyComponent>;

 beforeEach(async => {
 await TestBed.configureTestingModule({
 declarations: (MyComponent),
 providers: (MyService)
 })
 .compileComponents;
 });

 beforeEach( => {
 fixture = TestBed.createComponent(MyComponent);
 component = fixture.componentInstance;
 fixture.detectChanges;
 });

 it('should create', => {
 expect(component).toBeTruthy;
 });

 it('should use injected service', => {
 const service = TestBed.inject(MyService);
 spyOn(service, 'getData').and.returnValue('test data');
 expect(component.getDataFromService ).toEqual('test data');
 });
 });

Testing a service with injected dependencies.

 import { TestBed } from '@angular/core/testing';
 import { MyService } from './my.service';
 import { HttpClient } from '@angular/common/http';
 import { of } from 'rxjs';

 describe('MyService', => {
 let service: MyService;
 let httpSpy: jasmine.SpyObj<HttpClient>;
 
beforeEach( => {
 httpSpy = jasmine.createSpyObj('HttpClient', ('get'));
 TestBed.configureTestingModule({
 providers: (
 MyService,
 { provide: HttpClient, useValue: httpSpy }
 )
 });
 service = TestBed.inject(MyService);
 });

 it('should be created', => {
 expect(service).toBeTruthy;
 });

 it('should return expected data', => {
 const expectedData = { message: 'test message' };
 httpSpy.get.and.returnValue(of(expectedData));
 service.getData .subscribe((data) => {
 expect(data).toEqual(expectedData);
 });
 });
 });

In some cases, you may want to test a component or service in isolation from its dependencies. To do this, you can use spies or simulated services. A spy function records all calls made to it and is then used to assert them in tests. A mock service is a fake service implementation that allows you to control its behavior and return values.

Best practices and common pitfalls

It's important to address some best practices and common pitfalls when working in Angular.

• Distinguishing between services and their providers is essential. This has the potential to simplify code testing and maintenance.
• Each service should only be created once and shared across the application, which can significantly decrease overhead and increase speed.
• Circular dependencies are a typical pitfall that should be avoided when using dependency injection. This can happen if two or more services depend on each other in some way. As a result, a vicious circle of dependency is created, which can be difficult to control and lead to errors. To avoid this, it is crucial to examine the interdependencies between services and organize them so that there are no loops.

If you liked this article, check out:

• Mastering Angular Routing: A Comprehensive Guide
• Angular Project Structure: Best Practices for Files and Folders
• Mastering Angular Data Binding: A Comprehensive Guide for Experts
• Top Angular UI Component Libraries and Frameworks
• What is Angular and why should your company consider it for development?
• Today's best Javascript frameworks
• Angular for business
• What is the best framework for web development?

Conclusion

Understanding and implementing dependency injection in Angular is critical to building scalable and maintainable applications, whether you are directly involved or outsourcing Angular software development. Through this guide, we cover the basics of Angular dependency injection and an example implementation to clarify the understanding.

By following the step-by-step guide and example, you should understand how to use dependency injection in your own Angular projects or when outsourcing Angular software development. With this knowledge, developers can create applications that are more efficient, flexible, and easier to maintain.