为什么注入服务后每次都调用构造函数？

Question

我使用Angular 2.我在section providers中使用单例服务@NgModule({})：['MenuService']我使用MenuService.

MenuService 看起来像： @Injectable()

export class MenuService {

  constructor(private userService: UserService, private i18nService: I18nService) { console.log('Called'); }
}

我在其中注入了两个组件：

export class HeaderComponent {
  constructor(private menuService: MenuService) {}
}

export class HomeComponent {
  constructor(private menuService: MenuService) {}
}

我看到console.log('Called');两次，为什么重复调用？

Answer 1

服务总是运行它的构造函数，当注入一个组件时，它必须设置服务的fields/props。所以这根本不是奇怪的行为。注入接口 'act' 作为 2 个对象。

Answer 2

There are two ways to make a service a singleton in Angular:

Declare that the service should be provided in the application root.

Include the service in the AppModule or in a module that is only imported by the AppModule.

Beginning with Angular 6.0, the preferred way to create a singleton services is to specify on the service that it should be provided in the application root. This is done by setting providedIn to root on the service's @Injectable decorator
import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class UserService {
}

取自Angular official documentation。

Answer 3

是的，注入的服务 constructors 在初始化时被组件立即调用。

export class HomeComponent {
  constructor(private menuService: MenuService) {}
}

相当于做

private menuService: MenuService;
export class HomeComponent {
  constructor() {
      this.menuService = new MenuService(params...);
  }
}

所以基本上，DI 消除了显式创建对象的需要，如第二种方法所示。

DI 在 MenuService 构造函数需要一些参数而您不确定要发送什么时很有用。

为什么注入服务后每次都调用构造函数？

Why is constructor call each time after injection of service?

angular

angular6