[Angular] @ViewChildren and QueryLists (ngAfterViewInit)

When you use @ViewChildren, the value can only be accessable inside ngAfterViewInit lifecycle. This is somehow different from @ViewChild, which value can be accessed from ngAfterContentInit lifecycle.

import { Component, ChangeDetectorRef, Output, ViewChildren, AfterViewInit, EventEmitter, ContentChildren, QueryList, AfterContentInit } from ‘@angular/core‘;

import { AuthRememberComponent } from ‘./auth-remember.component‘;
import { AuthMessageComponent } from ‘./auth-message.component‘;

import { User } from ‘./auth-form.interface‘;

@Component({
  selector: ‘auth-form‘,
  template: `
    <div>
      <form (ngSubmit)="onSubmit(form.value)" #form="ngForm">
        <ng-content select="h3"></ng-content>
        <label>
          Email address
          <input type="email" name="email" ngModel>
        </label>
        <label>
          Password
          <input type="password" name="password" ngModel>
        </label>
        <ng-content select="auth-remember"></ng-content>
        <auth-message
          [style.display]="(showMessage ? ‘inherit‘ : ‘none‘)">
        </auth-message>
        <auth-message
          [style.display]="(showMessage ? ‘inherit‘ : ‘none‘)">
        </auth-message>
        <auth-message
          [style.display]="(showMessage ? ‘inherit‘ : ‘none‘)">
        </auth-message>
        <ng-content select="button"></ng-content>
      </form>
    </div>
  `
})
export class AuthFormComponent implements AfterContentInit, AfterViewInit {

  showMessage: boolean;

  @ViewChildren(AuthMessageComponent) message: QueryList<AuthMessageComponent>;

  @ContentChildren(AuthRememberComponent) remember: QueryList<AuthRememberComponent>;

  @Output() submitted: EventEmitter<User> = new EventEmitter<User>();

  constructor(private cd: ChangeDetectorRef) {}

  ngAfterViewInit() {
    console.log("this.message:", this.message); // QueryList {...}
    if (this.message) {
      this.message.forEach((message) => {
        message.days = 30;
      });
      this.cd.detectChanges();
    }
  }

  ngAfterContentInit() {
    console.log("this.message:", this.message); // undefined
    if (this.remember) {
      this.remember.forEach((item) => {
        item.checked.subscribe((checked: boolean) => this.showMessage = checked);
      });
    }
  }

  onSubmit(value: User) {
    this.submitted.emit(value);
  }

}

Here we try to modify the value inside ngAfterViewInit lifecycle. but in developement mode, there is change detection error! We cannot modify the ‘messages.day‘ after view init.

We can bypass this problem by using ‘ChangeDetectRef‘.

this.cd.detectChanges();

To tell Angular change detection everything is fine. And this error won‘t show up in production mode, only in development mode.