[Angular 依賴注入詳談] Angular Module Providers幾種類型的實作源代碼具體位置

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MessageServiceFactory = () => {

 if (environment.production) {

   return new MessageService();

 } else {

   return new NewMessageService();

 }

};

{

 // ...

 providers: [

   {

     provide: MessageService,

     useFactory: MessageServiceFactory

   }

 ]

}