Angular is an open-source framework for building web 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 in an Angular development company‘s workflow is, the benefits, implementation, and best practices. Whether you are new to working in 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!
Overview of Angular Dependency Injection
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 a component. This enables the creation of a loosely coupled and modular 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 utilized to provide dependencies to components and services. They are as follows.
| Injection Type | Description |
| Constructor Injection | It provides dependencies using class constructor function. |
| Setter Injection | A setter method is used to inject the dependency by the injector. |
| Interface Injection | The dependency provides an injector method that injects dependency into any client passed to it. The client also needs to have a setter method that is ready to accept the Angular dependency injection. |
The usage of the above methods varies on the scenario and is also dependent on the requirements of the application. Let’s look into the benefits of dependency injection in Angular.
The Benefits of Using Dependency Injection in Angular
Dependency injection offers a lot of critical and important benefits to Angular applications. Let’s look a few of them.
- The component and service class are more modular.
- Complex setup and mockup are unnecessary; we can test in isolation since it’s much easier.
- Due to the modularity of code, it can be reused very easily.
- 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 should be injected into a separate container or framework. It is a design principle that is closely linked to dependency injection. The responsibility should be inverted. This means that it should 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 has a dependency on the export class ProductServiceProductService. Rather than creating an instance of the ProductService within 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 it doesn’t need to worry about creating those dependencies because the DI framework is responsible for creating the instances of those services and injecting them into the ProductService.
Core Concepts in Angular’s Dependency Injection
Dependency injection is facilitated through several core concepts. It’s important to understand these concepts to effectively utilize Angular dependency injection.
Providers
Providers are objects that are responsible for creating and managing instances of dependencies that can be injected into components and services. It can be defined at either component, module, or application level. You have to use 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 above example defines a MyService class with an @Injectable decorator. The MyService class is to be injected as a dependency by Angular.
useClass Property
It is also important to specify what classes are to be used as a dependency. 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 above instance, the useValue providers property specifies and supplies 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 give a factory function in the preceding example. The @Inject decorator instructs the app 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 the definition of both MyService and MyOtherService in the preceding 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 subsequently are the 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 above code, the MyService class is defined with an @Injectable decorator. This means that the class can be injected as a dependency. The MyComponent class defines a dependency on Injector using the constructor to inject an instance of Injector. The data property is then set to the result of calling the getData() method on an instance of MyService that is retrieved using the get() method on the injector instance.
Tokens
Dependency injection token in Angular is used to identify a dependency represented by a string or a 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 given token. Let’s look at the sample implementation 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 above code example, 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 for the provided property 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 the required dependencies.
- Angular CLI: The latest version which provides Angular core.
Install Required 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 initial app run the CLI command ng new and provide the name my-app.
ng new my-app
Your package.json file should look something like this:
Please note that the versions of dependencies are specified in the above file. These were the versions at the time of the creation of 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 to the entire application. You can create a service using the Angular CLI by running the following command.
ng generate service my-service
Once the service is created you 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. 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 set up and registered with the dependency injection system. It can be injected into the parent component or child components. To do this, you have to add a constructor to the component and specify the service as a parameter. For example, think of a MyComponent component that needs to use MyService service. You can inject the service into the component like this. You have to add this in my-component 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’s constructor, MyServiceService is specified as a parameter. A getMessage method is also defined which fetches the data from the service.
Next, instead of registering a service with a module, we can use providedIn to register the service with the root injector. We can do this by specifying providedIn: ‘root’ in the @Injectable() decorator of our service.
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 in the 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 the directives and pipes 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 directive file custom.directive.ts in the app 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 it is applied to.
Next, in the app.module.ts file import the CustomDirective and add it to the declarations and exports arrays. This file is the root module of the app. In 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 { }
At last in the app.component.html file, apply the directive to an element and pass in the desired color. Add my-component as well.
<h1 customDirective="red">Hello World!</h1> <my-component></my-component>
Run the App
Open a terminal window and navigate to the root directory of your project. Run the ng serve command to start the development server.
Open a web browser and navigate to http://localhost:4200/ to view the app.
This is how the web page should look like.
Hierarchical Dependency Injection
Dependency injection creates a tree-like structure of injectors similar to a component tree. Every component injector can access the service provided by the parent and root injectors. Whenever a component class requests for a service, Angular searches for it in the current injector. If it’s not found, then it searches up 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 testing module. You can use the TestBed.configureTestingModule method to configure the testing module with the necessary dependencies and providers.
Let’s see how testing is done.
Testing a Component with 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 mock services. A spy function records all calls made to it and then is 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
Covering a few best practices and common pitfalls when working in Angular is important.
- 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 could significantly decrease overhead and boost speed.
- Circular dependencies are a typical trap that must be avoided when using dependency injection. This can happen if two or more services rely on one another in some way. As a result, a vicious circle of dependence is created, which can be difficult to control and can lead to mistakes. To prevent this, it is crucial to examine the interdependencies between services and arrange them such that there are no loops.
If you enjoyed this article, check out:
- Mastering Angular Routing: A Comprehensive Guide
- Angular Project Structure: Best Practices for Files & Folders
- Mastering Angular Data Binding: A Comprehensive Guide for Expert
- Top Angular UI Component Libraries & Frameworks
- What Is Angular and Why Should Your Business Consider It for Development?
- The Best Javascript Frameworks Today
- Angular for Business
- What Is the Best Framework for Web Development?
Conclusion
Understanding and implementing dependency injection in Angular is critical for building scalable and maintainable applications, whether you’re directly involved or outsourcing Angular software development. Through this guide, we have covered the core concepts of Angular’s dependency injection and a sample implementation to clarify the understanding.
Following the step-by-step guide and example, you should understand how to use dependency injection in your own Angular projects, or while outsourcing Angular software development. With this knowledge, developers can build more efficient and flexible applications that are easier to maintain.
