ExpressionChangedAfterItHasBeenCheckedError：表达式在检查后已更改。 以前的值：“未定义”

Question

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

Answer 1

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();

 }

Answer 2

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.

Answer 3

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

That is a good solution to resolve this problem.

Answer 4

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

Answer 5

I was having trouble with .

ERROR: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked. Previous value for 'mat-checkbox-checked': 'true'. Current value: 'false'.

The Problem here is that the updated value is not detected until the next change Detection Cycle runs.

The easiest solution is to add a Change Detection Strategy. Add these lines to your code:

import { ChangeDetectionStrategy } from "@angular/core";  // import

@Component({
  changeDetection: ChangeDetectionStrategy.OnPush,
  selector: "abc",
  templateUrl: "./abc.html",
  styleUrls: ["./abc.css"],
})

Answer 6

This shall solve the problem

  constructor(private changeDetector: ChangeDetectorRef) {}

  ngAfterViewChecked() {
    this.changeDetector.detectChanges();
  }

Answer 7

Two Solutions:

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

Answer 8

*NgIf会在这里产生问题，所以要么使用 display none CSS 或更简单的方法是使用[hidden]="!condition"

Answer 9

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

我用 setTimeout 包装了我的代码并且它起作用了

Answer 10

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.

Answer 11

This code solved my Problem.

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

constructor( private cdref: ChangeDetectorRef ) {}

ngAfterContentChecked() {
  this.cdref.detectChanges();
}

Answer 12

RxJS timer()

Like others suggested you can solve the issue by placing your code inside a setTimeout(..) function. It will essentially move the execution outside the event-loop stack, and hence into Angular's next change-detection cycle.

RxJS has its own timeout implementation called timer() - it accepts milliseconds as an argument and returns an observable. Since we only need the execution to happen after the event loop stack is clean (and Angular has finished all its rendering calculations), we can use timer() without any argument (it'll anyway default to 0):

ngAfterViewInit() {
    timer().subscribe(() => {
        // your code here
    })
}

JS event loop schema:

Answer 13

PS: My error details vary slightly, in the sense that mine detected a change in mat-focused value from true to false .

During my research, I noticed that the error was thrown because I tried to bind the SelectedIndex property of mat-tab-group to the index of the selected item in a mat-select field inside tab 1 of the tab group (code excerpt below).

HTML extract:

  <form [formGroup]="form" (ngSubmit)="onSubmit()">
    <mat-tab-group animationDuration="500ms" [selectedIndex]="titleId.value">
      <mat-tab label="Tab 1">
        <mat-radio-group formControlName="titleId">
          <mat-radio-button value="1">Mr</mat-radio-button>
          <mat-radio-button value="2">Mrs</mat-radio-button>
        </mat-radio-group>
      </mat-tab>
      <mat-tab label="tab 2">
        ...
      </mat-tab>
      <mat-tab label="tab 3">
        ...
      </mat-tab>
    </mat-tab-group>
  </form>

Although my code worked as expected (ie switching tabs based on a selected item from the dropdown list), the error was pretty frustrating to be ignored. Unfortunately, no suggested solution here worked for me. However, I noticed that moving mat-radio-group outside the mat-tab-group component, fixed the issue.

New implementation:

  <form [formGroup]="form" (ngSubmit)="onSubmit()">
    <mat-radio-group formControlName="titleId">
      <mat-radio-button value="1">Mr</mat-radio-button>
      <mat-radio-button value="2">Mrs</mat-radio-button>
    </mat-radio-group>
    <mat-tab-group animationDuration="500ms" [selectedIndex]="titleId.value">
      <mat-tab label="Tab 1">
        ...
      </mat-tab>
      <mat-tab label="tab 2">
        ...
      </mat-tab>
      <mat-tab label="tab 3">
        ...
      </mat-tab>
    </mat-tab-group>
  </form>

I expected the code below to fix the issue, but it didn't. Maybe I wasn't doing something right.

abc.component.ts code extract

  ngAfterViewInit() {
    setTimeout(() => {
      this.cdRef.detectChanges(); /*cdRef injected in constructor*/
    }, 0);
  }

I still believe there is a better way to fix this.

Answer 14

Try this, to call your code in ngOnInit()

someMethod() // emitted method call from output
{
    // Your code 
}

ngOnInit(){
  someMethod(); // call here your error will be gone
}