[Angular] @ContentChild and ngAfterContentInit

@ContentChild normally works with ngAfterContentInit lifecycle.

@ContentChild is used for looking into child component's props.

For example, we a app component:

      <auth-form
        (submitted)="loginUser($event)">
        <h3>Login</h3>
        <auth-remember
          (checked)="rememberUser($event)">
        </auth-remember>
        <button type="submit">
          Login
        </button>
      </auth-form>

Inside it define a 'auth-form' component, and for auth-form component it has 'h3', 'auth-remember' and 'button' component as children.

Auth-form:

    <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>
        <ng-content select="button"></ng-content>
      </form>
    </div>

Inside auth-form, it use <ng-content> (content projection) to show 'h3, button & auth-remember' component. So what we want to do here is "inside auth-form component, listen to auth-remember's checked event, using its value to toggle a message div".

Add a message div:

    <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>
        <div *ngIf="showMessage">You account will be kept for 30 days</div>
        <ng-content select="button"></ng-content>
      </form>
    </div>

Using @ContentChild to get the component object.

import { Component, Output, EventEmitter, AfterContentInit, ContentChild } from '@angular/core';

import { User } from './auth-form.interface';
import {AuthRememberComponent} from './auth-remember.component';

@Component({
  selector: 'auth-form',
  template: `
      ...
  `
})
export class AuthFormComponent implements AfterContentInit{
  showMessage: boolean;

  @ContentChild(AuthRememberComponent) remember: AuthRememberComponent;

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

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

  ngAfterContentInit(): void {
    if(this.remember) {
      this.remember.checked.subscribe((checked: boolean) => {
        this.showMessage = checked;
      })
    }
  }

}

If we check 'this.remember':

We can subscribe 'this.remember.checked' to get whether 'auth-remember' is checeked or not, and assign the value to 'showMessage' var.

@ContentChild is really powerful, it can get any props on the child component.

For exmaple, we can add an @Input value and a private prop on to the auth-remember component.

import { Component, Output, EventEmitter, Input } from '@angular/core';

@Component({
  selector: 'auth-remember',
  template: `
    <label>
      <input type="checkbox" (change)="onChecked($event.target.checked)">
      Keep me logged in
    </label>
  `
})
export class AuthRememberComponent {

  @Output() checked: EventEmitter<boolean> = new EventEmitter<boolean>();
  @Input() role: string;

  myName: string = "Auth-remember";

  onChecked(value: boolean) {
    this.checked.emit(value);
  }
}

And from the console log, we can see, we get everthing about the auth-remember component.