Detailed explanation of Angular's dependency injection method based on Constructor Parameter

Angular Dependency Injection

In Angular, Dependency Injection (DI) is a design pattern that handles how to create and pass dependent objects in different parts of the code. In Angular, we usually rely on the features of TypeScript, such as constructor parameters, to perform dependency injection.

Dependency injection of constructor parameters is an important feature of Angular DI system. In Angular, any class (such as services, components, directives, etc.) can be used as a dependency for other classes. When Angular creates an instance of a class, it first looks at the class's constructor to determine the dependencies required by the class. Angular then looks for these dependencies and if found, pass them as parameters to the constructor, thus completing dependency injection.

Here is a simple example, we create aLoggerServiceService, it has alogmethod. Then inAppComponentInject this service into the component:

import { Injectable } from `@angular/core`;
@Injectable({
  providedIn: `root`,
})
export class LoggerService {
  log(message: string) {
    (`LoggerService: ${message}`);
  }
}
import { Component } from `@angular/core`;
import { LoggerService } from `./`;
@Component({
  selector: `app-root`,
  templateUrl: `./`,
  styleUrls: [`./`],
})
export class AppComponent {
  title = `app`;
  constructor(private logger: LoggerService) {
    (`Hello!`);
  }
}

In the above code,AppComponentThe component constructor has a parameterlogger, its type isLoggerService. Angular knows through this constructor parameterAppComponentNeed oneLoggerServiceExamples. Therefore, when Angular createsAppComponentWhen an instance of  , it will first create aLoggerServiceInstance (if not already), then pass this instance toAppComponentconstructor.

Advantages of dependency injection of constructor parameters:

• Code decoupling: With dependency injection, we can easily share the same service instances between different classes. This makes the code more modular and reusable.
• Convenient testing: When testing, we can easily provide mock objects for dependencies, which makes unit testing easier.
• Object creation and lifecycle management are processed by the Angular framework, and developers can focus more on the implementation of business logic.

Disadvantages of dependency injection of constructor parameters:

• If the dependencies are too complex, it may cause difficulties in reading and maintaining the code.
• Dependency injection errors are usually found only at runtime and may make debugging difficult.

summary

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