Angular 8: detect if a ng-content has content in it (or exists)

In Angular 8 you dont have to use the ngAfterViewInit life cycle hook. You can use the ngOnInit as long as you set the "static" value of the viewchild to true.

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

@Component({
  selector: 'app-test',
  templateUrl: './test.component.html',
  styleUrls: ['./test.component.scss']
})
export class TestComponent implements OnInit {
  @ViewChild('content', { read: ElementRef, static: true }) content: ElementRef;
  constructor() { }

  ngOnInit() {
    console.log(!!this.content.nativeElement.innerHTML);  // return true if there is a content
  }
}

Note that you must wrap the ng-content directive with html tag (such as div, span etc) and to set the templateRef on this outer tag.

<div #content>
  <ng-content></ng-content>
</div>

I putted it on stackblitz: https://stackblitz.com/edit/angular-8-communicating-between-components-mzneaa?file=app/app.component.html

You can get a reference to the ng-content (Template Variable) and then access that variable in your component to check the length on the content of that ng-content using ViewChild

Then you can use the ngAfterViewInit life cycle hook to check for ng-content length

Your code will be like this:

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

@Component({
  selector: 'app-promohero-message-unit',
  template: `
    <div>
      <h3 class="text-white">{{ title }}</h3>
      <p class="text-white">
        <ng-content select="[description]"></ng-content>
      </p>
      <p class="text-white" *ngIf="readMore">
        <ng-content #readMoreContent select="[readmoretext]"></ng-content>
      </p>
    </div>
    <p>
      <a class="text-white" (click)="showReadMore()" *ngIf="something"><u>Read more</u></a>
    </p>
  `
})
export class PromoheroMessageUnitComponent {
  @Input()
  title: string;
  @ViewChild('readMoreContent') readMoreContent: ElementRef;

  readMore = false;

  ngAfterViewInit() {
    if (this.readMoreContent.nativeElement.childNodes.length.value == 0){
      this.readMore = false
    }
  }

  showReadMore() {
    this.readMore = true;
  }
}