角8 @ViewChild返回未定义

Question

我正在尝试使用8号角@ViewChild来将变量从一个组件转换为另一个组件，但它会产生未定义的

import { Component, OnInit } from '@angular/core';

@Component({
  selector: 'app-signin',
  templateUrl: './signin.component.html',
  styleUrls: ['../styles/css/main.css']
})
export class SigninComponent implements OnInit {
  isAuthenticated: any = false;


  constructor() { }

  ngOnInit() {
  }

}

我想在signinComponent的navbar组件中使用isAuthenticated变量

import { Component, OnInit, ViewChild, AfterViewInit } from '@angular/core';
import { SigninComponent } from "../signin/signin.component";
@Component({
  selector: 'app-navbar',
  templateUrl: './navbar.component.html',
  styleUrls: ['../styles/css/main.css']
})
export class NavbarComponent implements OnInit, AfterViewInit {
  isAuthenticated: any;
  @ViewChild(SigninComponent, {static: false}) signReference !: SigninComponent;



  constructor() { }

  ngOnInit() {

  }

  ngAfterViewInit() {
    this.isAuthenticated = this.signReference.isAuthenticated
    console.log(this.isAuthenticated)
  }

}

这是navbar.component.html，其中包含isAuthenticated变量，用于验证导航栏的某些项目

<nav class="navbar navbar-expand-md navbar-dark bg-primary">
    <div class="container">
        <a href="#" class="navbar-brand">Mango App</a>

        <button class="navbar-toggler" data-toggle="collapse" data-target="#navbarToggle">
            <i class="navbar-toggler-icon"></i>
        </button>

        <div class="collapse navbar-collapse" id="navbarToggle">
            <ul class="navbar-nav mr-auto" *ngIf="isAuthenticated">
                <li class="nav-item">
                    <a href="" class="nav-link">Posts</a>
                </li>
                <li class="nav-item">
                    <a href="" class="nav-link">Messages</a>
                </li>
            </ul>


            <ul class="navbar-nav ml-auto" *ngIf="!isAuthenticated">
                <li class="nav-item">
                    <a routerLink="/signup" class="nav-link">SignUp</a>
                </li>
                <li class="nav-item">
                    <a routerLink="/signin" class="nav-link">Login</a>
                </li>

            </ul>

            <ul class="navbar-nav ml-auto" *ngIf="isAuthenticated">
                <li class="nav-item">
                    <a href="" class="nav-link">Change Password</a>
                </li>
                <li class="nav-item">
                    <a routerLink="/signup" class="nav-link">Logout</a>
                </li>

            </ul>
        </div>

    </div>
</nav>

如何处理角度8中的错误