ViewChild 在调整大小事件期间未定义

Question

Let's see:

import {
  Component, OnInit, ViewEncapsulation, Input, ElementRef, ViewChild, AfterViewInit
} from '@angular/core';

@Component({
  selector: 'app-sidebar',
  template: `
  <aside #sidebar>
  Sidebar
  </aside>`,
})
export class AppSidebar implements OnInit, AfterViewInit {
  @ViewChild('sidebar')
  private sidebar: ElementRef;

  constructor() {
  }

  ngOnInit() {
    console.log('OnInit has:', this.sidebar)
    window.addEventListener('resize', this.resetSidebar);
  }

  ngAfterViewInit() {
    console.log('AfterViewInit has:', this.sidebar);
  }

  resetSidebar() {
    console.log(this.sidebar);
  }
}

this.sidebar is volatile here. It logs out correctly during the lifecycle hooks, but when you resize the window it becomes null. No camelCase problem , no conditional dom , so what's wrong with the code?

Answer 1

The problem is caused by how you register the event handlers.

With

 window.addEventListener('resize', this.resetSidebar);

this inside resetSidebar doesn't point to the AppSidebar class anymore.

Use .bind(this) to ensure this keeps working

 window.addEventListener('resize', this.resetSidebar.bind(this);

You can also use arrow functions https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions

 window.addEventListener('resize', (event) => this.resetSidebar(event);