Exploring useFactory and useExisting in Angular

Angular is a powerful framework that provides developers with a wide range of tools and functionalities to create complex web applications. Two of the most essential tools in Angular are the useFactory and useExisting APIs.

These APIs help in creating dynamic and efficient applications with better performance, extensibility, and readability. In this article, we will delve into the useFactory and useExisting APIs, how they work, and their use cases in Angular.

Understanding useFactory in Angular

The Angular useFactory API is a powerful tool that helps in creating and returning dependencies in a more dynamic way than the standard providers. UseFactory is a type of provider that allows developers to define a factory function that returns a dependency. This API takes a factory function that returns the instance of a service or an object.

Using useFactory with a provider provides us with various benefits over creating a provider directly. Consider the following:

  • It allows us to create dynamic dependencies by providing a different implementation for the same provider based on the context of the application.

  • It allows us to implement different providers based on the target environment like production or development.

  • It helps reduce boilerplate code and improves the maintainability of the application.

Let’s explore how to use useFactory in Angular:

// Import the necessary dependencies

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



// Define the factory function 

export function myDependencyFactory() {

  return new MyDepService();

}



// Define the Injectable & provide myDependency

@Injectable({

  providedIn: 'root',

  useFactory: myDependencyFactory

})

export class MyService {

  constructor(private myDependency: MyDepService) {

  }

}

In the code block above, we first defined a factory function that returns an instance of a service. Then we used the useFactory provider property to provide the definition of our myDependencyProvider. Now whenever the MyService is requested, the myDependencyProvider will return the instance returned by the factory function.

Understanding useExisting in Angular

The Angular useExisting API is used to provide a reference to an already existing provider. It enables us to reuse an existing provider definition instead of creating a new one. It takes a reference to an already available provider and uses that to consume the service.

Consider the following code block:

// Import the necessary dependencies

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



// Define a new provider

@Injectable({

  providedIn: 'root'

})

export class MyService {

  constructor() {}

}



// Reuse MyService provider using useExisting provider property

@Injectable({

  providedIn: 'root',

  useExisting: MyService

})

export class MyNewService {

  constructor(private myService: MyService) {}

}

In the code block above, we first defined a provider MyService. Then we used the useExisting provider reference to reuse that same provider in our application. Thus, whenever the MyNewService is requested, it will receive an instance of MyService. This helps us cleanly abstract the dependency injection layer.

FAQs

What is a provider in Angular?

A provider in Angular is a declarative object that tells Angular how to create a dependency to be consumed by another component or service. Providers define how the service is constructed, how it can be injected into other components and services, and how it can be managed or disposed of as required by the application.

What are the benefits of using useFactory over direct providers?

The benefits of using useFactory over direct providers include improved maintainability of the application, reduction of boilerplate code, and dynamic creation of dependencies based on context. It allows us to create dynamic dependencies by providing a different implementation for the same provider based on the context of the application. For instance, we may need to switch implementations during development or production, which can be easily achieved using the useFactory API.

When to use useExisting over useFactory?

Use useExisting over useFactory when we already have a provider defined and we want to reuse the same provider. It provides a reference to an already available provider and uses that to consume the service. It’s helpful when we want to cleanly abstract the dependency injection layer and reuse functions with existing services.

Is it possible to combine useExisting and useFactory?

Yes. It’s possible to use both useExisting and useFactory when defining providers in an Angular application. Combining both allows for more efficient and dynamic creation of dependencies.

How does Angular handle circular dependencies?

Angular provides an error when it detects circular dependencies in Angular services. When a circular dependency is detected, it throws an error at runtime, either in the browser log or on the console. The error message notifies the developer of the circular dependency and provides the necessary information to fix the problem.

What is the difference between providedIn and useClass?

The providedIn and useClass properties are both used in defining dependency providers in Angular. The providedIn property specifies the module or injector that will provide the service, while the useClass property specifies the class that will provide the service. UseProvideIn is newer and recommended over useClass for performance reasons.

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