ngIf - Expression has changed after it was checked

问题

I have a simple scenario, but just can't get it working!

In my view I display some text in a box with limited height.

The text is being fetched from the server, so the view updates when the text comes in.

Now I have an 'expand' button that has a ngIf that should show the button if the text in the box is overflowing.

The problem is that because the text changes when it is fetched, the 'expand' button's condition turns to true after Angular's change detection has finished...

So I get this error: Expression has changed after it was checked. Previous value: 'false'. Current value: 'true'.

Obviously the button does not show...

see this Plunker (check the console to see the error...)

Any idea how to make this work?

回答1:

this error occur because you in dev mode:

In dev mode change detection adds an additional turn after every regular change detection run to check if the model has changed.

so, to force change detection run the next tick, we could do something like this:

export class App implements AfterViewChecked {

  show = false; // add one more property

  constructor(private cdRef : ChangeDetectorRef) { // add ChangeDetectorRef
    //...
  }
  //...
  ngAfterViewChecked() {
    let show = this.isShowExpand();
    if (show != this.show) { // check if it change, tell CD update view
      this.show = show;
      this.cdRef.detectChanges();
    }
  }

  isShowExpand()
  {
    //...
  }
}

Live Demo: https://plnkr.co/edit/UDMNhnGt3Slg8g5yeSNO?p=preview

回答2:

For some reason, @Tiep Phan's answer didn't work for me to force change detection, but using setTimeout (which also forces change detection) did.

I also only had to add it to the offending line, and it worked fine with the code I already had in ngOnInit instead of having to add ngAfterViewInit.

Example:

ngOnInit() {
    setTimeout(() => this.loadingService.loading = true);
    asyncFunctionCall().then(res => {
        this.loadingService.loading = false;
    })
}

More details here: https://github.com/angular/angular/issues/6005

回答3:

Causing change detector to run after ngAfterContentChecked solved the problem for me

example as below:

import { ChangeDetectorRef,AfterContentChecked} from '@angular/core'
export class example implements OnInit,AfterContentChecked{

     ngAfterContentChecked():void{
     this.changeDetector.detectChanges();

      }
     }

Although as i read some of the articles this issue gets solved in production mode without any required fix.

below is possible reason for such issue: it enforces a uni-directional data flow: when the data on our controller classes gets updated, change detection runs and updates the view.

But that updating of the view does not itself trigger further changes which on their turn trigger further updates to the view

https://blog.angular-university.io/how-does-angular-2-change-detection-really-work/

来源：https://stackoverflow.com/questions/43513421/ngif-expression-has-changed-after-it-was-checked

标签

angular

angular2-changedetection