Dependency Injection

When you need to share logic between components, Angular leverages the design pattern of dependency injection that allows you to create a “service” which allows you to inject code into components while managing it from a single source of truth.

What are services?

Services are reusable pieces of code that can be injected.

Similar to defining a component, services are comprised of the following:

• A TypeScript decorator that declares the class as an Angular service via @Service and allows you to define a service that can be accessed anywhere in your application.
• A TypeScript class that defines the desired code that will be accessible when the service is injected

Here is an example of a Calculator service.

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

@Service()
export class Calculator {
  add(x: number, y: number) {
    return x + y;
  }
}

How to use a service

When you want to use a service in a component, you need to:

1. Import the service
2. Declare a class field where the service is injected. Assign the class field to the result of the call of the built-in function inject which creates the service

Here’s what it might look like in the Receipt component:

import {Component, inject} from '@angular/core';
import {Calculator} from './calculator';

@Component({
  selector: 'app-receipt',
  template: `<h1>The total is {{ totalCost }}</h1>`,
})
export class Receipt {
  private calculator = inject(Calculator);
  totalCost = this.calculator.add(50, 25);
}

In this example, the Calculator is being used by calling the Angular function inject and passing in the service to it.

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