Take you to understand component communication and dependency injection in Angular

AngularHow to communicate between components? What is dependency injection? The following article will give you a brief understanding of the component communication method and introduce dependency injection. I hope it will be helpful to you!

1. Component communication

##1.1 Passing data to the inside of the component

<app-favorite [isFavorite]="true"></app-favorite>

// favorite.component.ts
import { Input } from '@angular/core';

export class FavoriteComponent {
    @Input() isFavorite: boolean = false;
}

Note: Add

[] outside the attribute to indicate binding the dynamic value. After receiving it in the component, it will be a Boolean type. Do not add [] to indicate Bind a normal value, which is a string type after being received in the component. [Related tutorial recommendations: "angular tutorial"]

1.2 Components transmit data to the outside

Requirements : Pass data to the parent component by clicking the button in the child component

<!-- 子组件模板 -->
<button (click)="onClick()">click</button>

// 子组件类
import { EventEmitter, Output } from "@angular/core"

export class FavoriteComponent {
  @Output() change = new EventEmitter()
  onClick() {
    this.change.emit({ name: "张三" })
  }
}

<!-- 父组件模板 -->
<app-favorite (change)="onChange($event)"></app-favorite>

// 父组件类
export class AppComponent {
  onChange(event: { name: string }) {
    console.log(event)
  }
}

---

2. Dependency Injection

2.1 Overview

Dependency injection (

Dependency Injection), referred to as DI, is a design principle in object-oriented programming that is used to reduce the coupling between codes Degree

class MailService {
  constructor(APIKEY) {}
}

class EmailSender {
  mailService: MailService
  constructor() {
    this.mailService = new MailService("APIKEY1234567890")
  }

  sendMail(mail) {
    this.mailService.sendMail(mail)
  }
}

const emailSender = new EmailSender()
emailSender.sendMail(mail)

EmailSender class must use the MailService class when running, EmailSender class depends on the MailService class, The MailService class is a dependency of the EmailSender class.

The coupling degree of the above writing method is too high, and the code is not robust. If the

MailService class changes the parameter delivery method, the writing method in the EmailSender class will also change

class EmailSender {
  mailService: MailService
  constructor(mailService: MailService) {
    this.mailService = mailService;
  }
}
const mailService = new MailService("APIKEY1234567890")
const emailSender = new EmailSender(mailService)

when instantiating the

EmailSender class When injecting its dependencies into the interior of the class in the form of constructor constructor parameters, this way of writing is dependency injection.

Through dependency injection, the coupling between codes is reduced and the maintainability of the code is increased.

MailService Changes to the code in the class will no longer affect the EmailSender class

2.2 DI Framework

Angular has its own DI framework, which hides the process of implementing dependency injection. For developers, only Complex dependency injection functionality can be used with very simple code.

There are four core concepts in

Angular's DI framework:

• Dependency: The instance object on which the component depends, service instance object

• Token: Get the identifier of the service instance object

• Injector: Injector, responsible for creating and maintaining instance objects of service classes and injecting service instance objects into components.

• Provider: Configure the object of the injector, specify the service class to create the service instance object and obtain the identifier of the instance object.

2.2.1 InjectorsInjectors

The injector is responsible for creating service class instance objects and converting service class instances to Inject the object into the required component

• Create the injector

  import { ReflectiveInjector } from "@angular/core"
  // 服务类
  class MailService {}
  // 创建注入器并传入服务类
  const injector = ReflectiveInjector.resolveAndCreate([MailService])

• Get the service class instance object in the injector

  const mailService = injector.get(MailService)

• The service instance object is in singleton mode, and the injector will cache it after creating the service instance.

  const mailService1 = injector.get(MailService)
  const mailService2 = injector.get(MailService)
  
  console.log(mailService1 === mailService2) // true

• Different injectors return different service instance objects

  const injector = ReflectiveInjector.resolveAndCreate([MailService])
  const childInjector = injector.resolveAndCreateChild([MailService])
  
  const mailService1 = injector.get(MailService)
  const mailService2 = childInjector.get(MailService)
  
  console.log(mailService1 === mailService2) // false

• The search for service instances is similar to the function scope chain. If the current level can be found, use the current level. If the current level cannot be found, go to the parent to find it

  const injector = ReflectiveInjector.resolveAndCreate([MailService])
  const childInjector = injector.resolveAndCreateChild([])
  
  const mailService1 = injector.get(MailService)
  const mailService2 = childInjector.get(MailService)
  
  console.log(mailService1 === mailService2) // true

2.2.2 ProviderProvider

• Configure the object of the injector, specifying the service class and creation instance object The identifier of the access service instance object

  const injector = ReflectiveInjector.resolveAndCreate([
    { provide: MailService, useClass: MailService }
  ])

• The identifier of the access dependent object can also be a string type

  const injector = ReflectiveInjector.resolveAndCreate([
    { provide: "mail", useClass: MailService }
  ])
  const mailService = injector.get("mail")

• useValue

  const injector = ReflectiveInjector.resolveAndCreate([
    {
      provide: "Config",
      useValue: Object.freeze({
        APIKEY: "API1234567890",
        APISCRET: "500-400-300"
      })
    }
  ])
  const Config = injector.get("Config")

  Establishes a loose coupling relationship between the instance object and the external reference. The external object obtains the instance object through the identifier. As long as the identifier remains unchanged, no matter how the internal code changes, it will not affect the outside world
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