Controller負責處理Request及產生Response，資料庫存取或是商業邏輯(business logic)通常寫在Service class裡

商業邏輯以最簡單hello world為例

export class AppService {
  sayHello(): string {
    return 'Hello World! in App Service';
  }
}

在app.controller.ts裡

...

import { AppService } from './app.service';

@Controller()
export class AppController {
  
  // 宣告變數，型別為AppService
  appService: AppService;
    
  // 建立AppController實例時，一併建立AppService實例
  constructor(){
    this.appService = new AppService();
  }

  @Get()
  sayHello(){
    // 呼叫AppService的sayHello方法，回傳client
    return this.appService.sayHello();
  }
  ...
}

Postman結果

nest.js提供Dependency Injection(DI, 相依注入)，可以不用每次要用AppService的時候都要new，讓nest.js DI機制幫我們處理相依class

nest.js的DI幾乎是從Angular整套借來用。

在app.module.ts中providers註冊AppService

import { Module } from '@nestjs/common';
import { AppController } from './app.controller';
import { AppService } from './app.service';

@Module({
  imports: [],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

到app.controller.ts，把要注入的AppService當作contructor參數

@Controller()
export class AppController {
  
  
  appService: AppService;
    
  // 將要注入的AppService當作constructor傳入參數
  constructor(_appService: AppService){
    // 把注入的AppService指定給controller變數
    this.appService = _appService;
  }

  @Get()
  sayHello(){
    return this.appService.sayHello();
  }
  ...
}

Postman測試得到一樣的結果

Typescript提供一種complier sugar，可以再簡化程式碼

@Controller()
export class AppController {
  
  
  // appService: AppService;
    
  // 以private 修飾 appService
  // 等同註解掉的程式碼
  constructor(
      private appService: AppService
      // _appService:AppService
  ){
    //this.appService = appService;
  }

  @Get()
  sayHello(){
    return this.appService.sayHello();
  }
  ...
}

Providers有三種

• Value
• Class
• Factory

明天再繼續