Introduction
As an Angular developer, you must be familiar with dependency injection. It is a powerful feature in Angular that enables the injection of dependencies into components, directives, services, and more. However, there could be scenarios where you might want to inject custom services that are not available via classes or interfaces. This is where injection tokens come into play.
Injection tokens in Angular allow you to inject values that are not represented by a class or interface. Instead, they are represented by a token, which is a string or an instance of the InjectionToken class. In this article, we will explore injection tokens and how they can be used in an Angular application.
What are Injection Tokens?
Injection tokens are a way to provide a custom value to a dependency injection system in Angular. So, what is a dependency injection system? In Angular, the dependency injection system is responsible for providing the components, directives, and services with the dependencies they need to function correctly.
The injection token is just an object that acts like a key. When an injector looks up an injection token, it returns the instance of the service associated with that token. It is the job of the token to point to the actual provider of a particular dependency.
Injection tokens can be defined using a string or an instance of the InjectionToken class. The InjectionToken class provides a way to create unique tokens that can be reused across an application.
Benefits of Injection Tokens
There are several benefits of using injection tokens in Angular:
1. Reusability: Injection tokens can be defined once and used multiple times across an application.
2. Customization: Injection tokens allow you to inject custom services that are not available via classes or interfaces.
3. Decoupling: Injection tokens enable the injection of services without coupling them to the specific implementation. This makes it easier to switch implementations later.
How to Use Injection Tokens in Angular
Let’s take a look at how to use injection tokens in Angular.
Step 1: Define the Injection Token
The first step is to define the injection token. This can be done using a string or an instance of the InjectionToken class. Here’s an example of creating an injection token using a string:
“`
import { InjectionToken } from ‘@angular/core’;
export const MY_TOKEN = new InjectionToken
“`
In the example above, we have defined an injection token called MY_TOKEN, which is a string. We have also used the InjectionToken class to create a token and provide a description of the token.
Step 2: Provide the Dependency
The next step is to provide the dependency associated with the injection token. This can be done using the providers array in the module file. Here’s an example:
“`
import { NgModule } from ‘@angular/core’;
import { MY_TOKEN } from ‘./my-token’;
import { MyService } from ‘./my-service’;
@NgModule({
providers: [
{ provide: MY_TOKEN, useClass: MyService }
]
})
export class AppModule {}
“`
In the example above, we have provided the MyService class as the dependency for the MY_TOKEN injection token.
Step 3: Inject the Dependency
The final step is to inject the dependency in the component or service where it is needed. Here’s an example:
“`
import { Component, Inject } from ‘@angular/core’;
import { MY_TOKEN } from ‘./my-token’;
@Component({
selector: ‘app-my-component’,
template: ‘
‘
})
export class MyComponent {
constructor(@Inject(MY_TOKEN) private myValue: string) {}
}
“`
In the example above, we are injecting the MY_TOKEN injection token into the MyComponent class using the @Inject decorator. The string value of the token is passed as an argument to the decorator.
FAQs
Q. What is the difference between InjectionToken and useClass?
A. InjectionToken is used to define the token that represents the dependency that will be injected. useClass is used to provide the implementation of the dependency that will be injected.
Q. When should I use InjectionToken?
A. InjectionToken should be used when you want to inject custom services or values that are not represented by a class or an interface.
Q. Can I use InjectionToken with @Injectable decorator?
A. Yes, you can use InjectionToken with @Injectable decorator to inject custom services or values.
Q. How can I use multiple injection tokens in a component?
A. You can inject multiple injection tokens using the constructor and the @Inject decorator. Simply pass each injection token as an argument to the @Inject decorator.
Conclusion
Injection tokens in Angular are a powerful feature that enables the injection of custom services and values into components and services. They provide reusability, customization, and decoupling benefits, making it easier to switch implementations later. By following the steps outlined in this article, you can start using injection tokens in your Angular application today.