@viewChild return 在 Angular 中未定义

Question

我已经看过其他类似的帖子，但它们并不完全相关，所以我问一下我的具体情况。当我尝试在控制台中获取 child.message 的值或打印 child 时，出现 'child is undefined' 错误。

app.component.ts

@Component({
  selector: 'app-root',
  template: `
    <h1>Example of uses of @ViewChild with child component</h1>
    <app-child-component></app-child-component>
    <p>This is the message value of the child component selected with @ViewChild: '{{childMessage}}' </p>
  `
})

export class AppComponent implements AfterViewInit{
  
    @ViewChild(ChildComponent, { read: true }) child!: ChildComponent

    childMessage:string
    
    ngAfterViewInit(){
        console.log(this.child)
        this.childMessage = this.child.message
    }

}

child.component.ts

@Component({
  selector: 'app-child-component',
  template: `
    <div class="container">
        <h2>This is the child component!</h2>
        <p> The message value is '{{message}}'</p>
    </div>
    `,
    styles: ['.container{ background-color: rgb(154, 218, 226); border: 1px solid black; }']
})
export class ChildComponent {

    message:string = 'Ave Caesar!'

}

我按照官方angular文档，怎么了？谢谢！

Answer 1

read 属性用于区分几个可用的可注射项目（顺便说一下，它不能设置为布尔值）。您的用例中不需要它。它会通过删除它来工作。

为避免expression has changed after it was checked错误，运行手动更改检测：

constructor(cdr: ChangeDetectorRef) {}

ngAfterViewInit() {
    this.childMessage = this.child.message;
    this.cdr.detectChanges();
  }

@viewChild return 在 Angular 中未定义

@viewChild return undefined in Angular

angular

viewchild