未捕获错误：无法解析所有参数

Question

我正在学习Angular5以及如何使用useFactory和deps。目前在我的Angular应用程序中，我有一个组件和三个服务。

这是组件

// email.component.ts
import { Component, OnInit, Inject } from '@angular/core';

@Component({
  ...
})
export class EmailComponent implements OnInit {
  private currentEmail: string;
  private companyName: string;

  constructor(@Inject('EMS') private emailService) { }

  ngOnInit() {
    this.currentEmail = this.emailService.emailServiceName;
  }
}

而模板是

// email.component.html
<p>Your Current Email Service is : {{ currentEmail }}</p>

这是app.module.ts：

...
import { EmailComponent } from './components/email/email.component';
import { EmailService } from './email-service';
import { HotMail } from './hot-mail';
import { YahooMail } from './yahoo-mail';


function HotMailOrYahooMail(x) {
  if (x.isHotMail) {
    return new HotMail();
  } else {
    return new YahooMail();
  }
}
@NgModule({
  ...
  providers: [
    EmailService,
    {
      provide: 'EMS',
      useFactory: HotMailOrYahooMail,
      deps: [EmailService]
    }
  ],
  ...
})
export class AppModule { }

逻辑是EmailComponent应该渲染从HotMail服务或YahooMail服务返回的值，并且这个逻辑位于EmailService，即EmailService有一个名为isHotMail的属性，如果它的值为true则'EMS'应该指向HotMail服务，否则它应该指向朝YahooMail服务。

这是EmailService

export class EmailService {
    constructor(public isHotMail: boolean) {
        this.isHotMail = true;
    }
}

和HotMail服务：

export class HotMail {
  public emailServiceName = 'HotMail by Microsoft';
}

和YahooMail服务：

export class YahooMail {
  public emailServiceName = 'YahooMail by Yahoo';
}

但是，当运行ng serve时，我收到以下错误：

Uncaught Error: Can't resolve all parameters for EmailService: (?).
        at syntaxError (compiler.js:485)
        at CompileMetadataResolver._getDependenciesMetadata (compiler.js:15699)
        at CompileMetadataResolver._getTypeMetadata (compiler.js:15534)
        at CompileMetadataResolver._getInjectableMetadata (compiler.js:15514)
        at CompileMetadataResolver.getProviderMetadata (compiler.js:15874)
        at eval (compiler.js:15785)
        at Array.forEach (<anonymous>)
        at CompileMetadataResolver._getProvidersMetadata (compiler.js:15745)
        at CompileMetadataResolver.getNgModuleMetadata (compiler.js:15313)
        at JitCompiler._loadModules (compiler.js:34404)

我在这里失踪了什么？

Answer 1

我的坏，我应该用这个

export class EmailService {
    public isHotMail = true;
}