Nested Angular elements throwing ExpressionChangedAfterItHasBeenCheckedError

Question

I've done some research around this issue, now I'm not sure it's a bug or something wrong with my implementation.

I've got an element which uses a service to fetch some data, then it passes the data to a child element, but when I update the data, I've got that error.

parent element

import { Component, OnInit, ViewEncapsulation} from '@angular/core';
import { TestService } from '../services/test.service';

@Component({
    selector: 'dm-element-with-service-test',
    templateUrl: './element-with-service-test.component.html',
    encapsulation: ViewEncapsulation.Emulated
})
export class ElementWithServiceTestComponent implements OnInit {
currentCount: number;

constructor(private testService: TestService) { }

    ngOnInit() {
        this.testService.count.subscribe(count => this.currentCount = count); 
    }

    count() {
        this.testService.addCount();
    }

}

parent element view

<div>
    <button (click)="count()">count</button>
    <element-test [myNumber]="currentCount"></element-test>
</div>

child element

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

@Component({
    selector: 'dm-element-test',
    templateUrl: './element-test.component.html',
    encapsulation: ViewEncapsulation.Emulated
})
export class ElementTestComponent implements OnInit {
    @Input() myNumber: number;
    constructor() { }

    ngOnInit() {
    // do stuff...
    }
}

child element view

<p>
    current number in this component: {{myNumber}}
</p>

app module

import { BrowserModule } from '@angular/platform-browser';
import { NgModule, Injector, CUSTOM_ELEMENTS_SCHEMA } from '@angular/core';
import { AppComponent } from './app.component';
import { ElementTestComponent } from './element-test/element-test.component';
import { createCustomElement } from '@angular/elements';
import { ElementWithServiceTestComponent } from './element-with-service-test/element-with-service-test.component';


@NgModule({
    declarations: [
        AppComponent,
        ElementTestComponent,
        ElementWithServiceTestComponent
    ],
    imports: [
        BrowserModule
    ],
    providers: [],
    schemas: [CUSTOM_ELEMENTS_SCHEMA ],
    entryComponents: [ElementTestComponent, ElementWithServiceTestComponent]
})
export class AppModule {
    constructor(private injector: Injector) {
        const el = createCustomElement(ElementTestComponent, { injector });
        const el2 = createCustomElement(ElementWithServiceTestComponent, { injector });
        customElements.define('element-test', el);
        customElements.define('element-with-service-test', el2);
    }
    ngDoBootstrap() { }
}

Index

<div class="tests">
     <element-with-service-test></element-with-service-test>
</div>

When I load the app, everything's fine, but if I click the count button I've got that error.

Answer 1

That error will always happen when you change data passed from parents in child components. You should either

1) not do that

2) use changeDetectorRef.detectChanges() when you do modify data.

https://blog.angularindepth.com/everything-you-need-to-know-about-the-expressionchangedafterithasbeencheckederror-error-e3fd9ce7dbb4