ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked. Previous value: 'undefined'

问题

I know there are a lot of same questions already posted in stack-overflow and tried different solutions to avoid the run-time error but None of them are working for me.

Component and Html Code

export class TestComponent implements OnInit, AfterContentChecked {
    @Input() DataContext: any;
    @Input() Position: any;
    sampleViewModel: ISampleViewModel = { DataContext: : null, Position: null };
    constructor(private validationService: IValidationService, private modalService: NgbModal, private cdRef: ChangeDetectorRef) {
    }

    ngOnInit() {

    }
    ngAfterContentChecked() {

            debugger;
            this.sampleViewModel.DataContext = this.DataContext;
            this.sampleViewModel.Position = this.Position;

    }


<div class="container-fluid sample-wrapper text-center" [ngClass]="sampleViewModel.DataContext?.Style?.CustomCssClass +' samplewidget-'+ sampleViewModel.Position?.Columns + 'x' + sampleViewModel.Position?.Rows">
     //some other html here
</div>

Please Note : This Component is loaded dynamically using DynamicComponentLoader

After My trouble shooting I have identified couple of issues

First of all this child component is loaded dynamically by using DynamicComponentResolver and passing the input values like below

 ngAfterViewInit() {
    this.renderWidgetInsideWidgetContainer();

  }


  renderWidgetInsideWidgetContainer() {
    let component = this.storeFactory.getWidgetComponent(this.dataSource.ComponentName);
    let componentFactory = this._componentFactoryResolver.resolveComponentFactory(component);
    let viewContainerRef = this.widgetHost.viewContainerRef;
    viewContainerRef.clear();
    let componentRef = viewContainerRef.createComponent(componentFactory);
    debugger;
    (<IDataBind>componentRef.instance).WidgetDataContext = this.dataSource.DataContext;
    (<IDataBind>componentRef.instance).WidgetPosition = this.dataSource.Position;

  }

Even If I changed my child component html like below I am getting this same error.Just add a angular ngclass attribute

<div class="container-fluid ds-iconwidget-wrapper text-center" [ngClass]="Sample">

</div>

My databinding and everything are working fine.Do I need to do anything on parent component? I already tried all the lifecyle events in child component

回答1:

The ngAfterContentChecked lifecycle hook is triggered when bindings updates for the child components/directives have been already been finished. But you're updating the property that is used as a binding input for the ngClass directive. That is the problem. When Angular runs validation stage it detects that there's a pending update to the properties and throws the error.

To understand the error better, read these two articles:

Everything you need to know about the ExpressionChangedAfterItHasBeenCheckedError error
Everything you need to know about change detection in Angular

Think about why you need to change the property in the ngAfterViewInit lifecycle hook. Any other lifecycle that is triggered before ngAfterViewInit/Checked will work, for example ngOnInit or ngDoCheck or ngAfterContentChecked.

So to fix it move renderWidgetInsideWidgetContainer to the ngOnInit() lifecycle hook.

回答2:

you have to tell angular that you updated the content after ngAfterContentChecked you can import ChangeDetectorRef from @angular/core and call detectChanges

import {ChangeDetectorRef } from '@angular/core';

constructor( private cdref: ChangeDetectorRef ) {}


ngAfterContentChecked() {

this.sampleViewModel.DataContext = this.DataContext;
this.sampleViewModel.Position = this.Position;
this.cdref.detectChanges();

 }

回答3:

Two Solutions:

Make Sure if you have some binding variables then move that code to settimeout( { }, 0);
Move your related code to ngAfterViewInit method

回答4:

I had the same issue trying to do something the same as you and I fixed it with something similar to Richie Fredicson's answer.

When you run createComponent() it is created with undefined input variables. Then after that when you assign data to those input variables it changes things and causes that error in your child template (in my case it was because I was using the input in an ngIf, which changed once I assigned the input data).

The only way I could find to avoid it in this specific case is to force change detection after you assign the data, however I didn't do it in ngAfterContentChecked().

Your example code is a bit hard to follow but if my solution works for you it would be something like this (in the parent component):

export class ParentComponent implements AfterViewInit {
  // I'm assuming you have a WidgetDirective.
  @ViewChild(WidgetDirective) widgetHost: WidgetDirective;

  constructor(
    private componentFactoryResolver: ComponentFactoryResolver,
    private changeDetector: ChangeDetectorRef
  ) {}

  ngAfterViewInit() {
    renderWidgetInsideWidgetContainer();
  }

  renderWidgetInsideWidgetContainer() {
    let component = this.storeFactory.getWidgetComponent(this.dataSource.ComponentName);
    let componentFactory = this.componentFactoryResolver.resolveComponentFactory(component);
    let viewContainerRef = this.widgetHost.viewContainerRef;
    viewContainerRef.clear();
    let componentRef = viewContainerRef.createComponent(componentFactory);
    debugger;
    // This <IDataBind> type you are using here needs to be changed to be the component
    // type you used for the call to resolveComponentFactory() above (if it isn't already).
    // It tells it that this component instance if of that type and then it knows
    // that WidgetDataContext and WidgetPosition are @Inputs for it.
    (<IDataBind>componentRef.instance).WidgetDataContext = this.dataSource.DataContext;
    (<IDataBind>componentRef.instance).WidgetPosition = this.dataSource.Position;
    this.changeDetector.detectChanges();
  }
}

Mine is almost the same as that except I'm using @ViewChildren instead of @ViewChild as I have multiple host elements.

回答5:

If you are using <ng-content> with *ngIf you are bound to fall into this loop.

Only way out I found was to change *ngIf to display:none functionality

回答6:

setTimeout(() => { // your code here }, 0);

I wrapped my code in setTimeout and it worked

回答7:

 ngAfterViewInit() {
   setTimeout(() => {
     this.renderWidgetInsideWidgetContainer();
   }, 0);
  }

That's a good solution for resolve this problem.

来源：https://stackoverflow.com/questions/45467881/expressionchangedafterithasbeencheckederror-expression-has-changed-after-it-was

标签

angular

runtime-error