在AppComponent中，我在HTML代碼中使用了nav組件。用戶(hù)界面看起來(lái)不錯(cuò)。服務(wù)時(shí)沒(méi)有錯(cuò)誤。當(dāng)我查看該應(yīng)用程序時(shí)，控制臺(tái)中也沒(méi)有錯(cuò)誤。但是當(dāng)我為我的項(xiàng)目運(yùn)行Karma時(shí)，出現(xiàn)了一個(gè)錯(cuò)誤：Failed: Template parse errors: 'app-nav' is not a known element:1. If 'app-nav' is an Angular component, then verify that it is part of this module.2. If 'app-nav' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.在我的app.module.ts中：有：import { NavComponent } from './nav/nav.component';它也在NgModule的聲明部分中@NgModule({  declarations: [    AppComponent,    CafeComponent,    ModalComponent,    NavComponent,    NewsFeedComponent  ],  imports: [    BrowserModule,    FormsModule,    HttpModule,    JsonpModule,    ModalModule.forRoot(),    ModalModule,    NgbModule.forRoot(),    BootstrapModalModule,    AppRoutingModule  ],  providers: [],  bootstrap: [AppComponent]})我NavComponent在我的AppComponentapp.component.tsimport { Component, ViewContainerRef } from '@angular/core';import { Overlay } from 'angular2-modal';import { Modal } from 'angular2-modal/plugins/bootstrap';import { NavComponent } from './nav/nav.component';@Component({  selector: 'app-root',  templateUrl: './app.component.html',  styleUrls: ['./app.component.css']})export class AppComponent {  title = 'Angela';}app.component.html<app-nav></app-nav><div class="container-fluid"></div>我已經(jīng)看到了類(lèi)似的問(wèn)題，但是該問(wèn)題的答案表明我們應(yīng)該在具有導(dǎo)出功能的nav組件中添加NgModule，但是這樣做時(shí)會(huì)出現(xiàn)編譯錯(cuò)誤。還有：app.component.spec.tsimport {NavComponent} from './nav/nav.component';import { TestBed, async } from '@angular/core/testing';import { AppComponent } from './app.component';