我是Angular的新手，所以请耐心等待 .
我想从firebase firestore获取当前登录用户的firstName，并在仪表板页面上显示它 . 但是当我尝试加载仪表板时，出现以下错误：
AdminOverviewComponent_Host.ngfactory.js? [sm]:1 ERROR TypeError: Cannot read property 'firstName' of undefined
at AdminOverviewComponent.push../src/app/admin/admin-overview/admin-overview.component.ts.AdminOverviewComponent.ngOnInit (admin-overview.component.ts:19)
at checkAndUpdateDirectiveInline (core.js:10097)
at checkAndUpdateNodeInline (core.js:11363)
at checkAndUpdateNode (core.js:11325)
at debugCheckAndUpdateNode (core.js:11962)
at debugCheckDirectivesFn (core.js:11922)
at Object.eval [as updateDirectives] (AdminOverviewComponent_Host.ngfactory.js? [sm]:1)
at Object.debugUpdateDirectives [as updateDirectives] (core.js:11914)
at checkAndUpdateView (core.js:11307)
at callViewAction (core.js:11548)
出现此错误后，将从firebase中检索firstName(因为我是console.log) . 我知道页面加载时，由于滞后提取，firstName为null，但是如何解决这个问题或者最好的方法是什么？
相关的HTML代码：
{{user}}
admin-overview-component.ts
import { Component, OnInit } from '@angular/core';
import { AuthService } from '../../services/auth.service';
import { UserService } from '../../services/user.service';
@Component({
selector: 'app-admin-overview',
templateUrl: './admin-overview.component.html',
styleUrls: ['./admin-overview.component.css']
})
export class AdminOverviewComponent implements OnInit {
user: string;
constructor(private authService: AuthService, private userService: UserService) {
}
ngOnInit() {
this.user = this.userService.userInfo.firstName;
}
logoutUser() {
this.authService.logout()
}
}
user.service.ts
import { Injectable } from '@angular/core';
import { AngularFirestore, AngularFirestoreDocument } from 'angularfire2/firestore';
import { Observable } from 'rxjs/Observable';
import { AuthService } from './auth.service';
export class UserInfo {
Country: string;
age: string;
firstName: string;
lastName: string;
}
@Injectable()
export class UserService {
private userDoc: AngularFirestoreDocument;
public userInfo: UserInfo;
public gotData: boolean;
constructor(private db: AngularFirestore,private authService: AuthService) {
this.userDoc = db.doc('users/' + authService.getUserUID);
this.userDoc.valueChanges()
.take(1)
.subscribe(user => {
if (user) {
this.userInfo = user;
this.gotData = true;
console.log("Got Data: " + user.firstName);
}
else {
this.userInfo = null;
}
})
}
}