Angular 2 / PrimeNG - Expression has changed after it was checked. Binding NgModel on last invalid form control - forms

Question

I'm having a problem where when the very last element in my form has a value bound to it the error "Expression has changed after it was checked." is thrown.
I will preface by saying this is based off of the Angular 2 website example here -
https://angular.io/docs/ts/latest/cookbook/dynamic-form.html#!#top
The way my app works is first I build a dynamic form with controls in my form component based off a model.
My form components html loops the questions in the model like so
<form *ngIf="showForm" [formGroup]="formGroup">
<!-- questions-->
<div *ngIf="questions.length > 0">
<div *ngFor="let question of questions">
<question [question]="question" [formGroup]="formGroup"></question>
</div>
</div>
<button pButton type="submit" label="Submit" icon="fa-check-circle-o" iconPos="left"
[disabled]="!formGroup.valid" (click)="submitFinalForm()"></button>
</form>
Below is the question component html that uses the data that was passed in from the form component to display certain types of questions via ngSwitch
<label [attr.for]="question.field">
{{ question.question }}
</label>
<div [ngSwitch]="question.type">
<!-- Radio / Checkbox -->
<radio-checkbox-question *ngSwitchCase="1" [formGroup]="formGroup" [question]="question"></radio-checkbox-question>
</div>
Finally here is the radio-checkbox-question component
<div *ngIf="showQuestion" [formGroup]="formGroup">
<!-- Radio -->
<div *ngIf="model.radiocheckbox == 'radio'">
<div *ngFor="let label of model.labels; let i = index;">
<p-radioButton name="{{model.field}}"
value="{{i}}"
label="{{label}}"
formControlName="{{model.field}}"
[(ngModel)]="questionAnswerRadio"></p-radioButton>
</div>
</div>
</div>
Here is the actual component TS
import { Component, Input, OnInit } from "#angular/core";
import { FormGroup } from "#angular/forms";
import { RadioCheckboxQuestion } from "../Questions/radio.checkbox.question.model";
#Component({
selector: "radio-checkbox-question",
templateUrl: "radio.checkbox.component.html"
})
export class RadioCheckboxComponent implements OnInit {
#Input() question: any;
#Input() formGroup: FormGroup;
model: RadioCheckboxQuestion = new RadioCheckboxQuestion();
showQuestion: boolean = false;
questionAnswerRadio: string = "";
ngOnInit(): void {
// question essential properties
if (this.question.hasOwnProperty("field") && this.question["field"] &&
this.question.hasOwnProperty("labels") && this.question["labels"]) {
this.model.field = this.question["field"];
this.model.labels = this.question["labels"];
// assume always radio for debugging
this.model.radiocheckbox = "radio";
// set existing answer
if (this.question.hasOwnProperty("QuestionAnswer") && this.question["QuestionAnswer"]) {
if (this.model.radiocheckbox == "radio") {
this.questionAnswerRadio = this.question["QuestionAnswer"];
}
}
this.showQuestion = true;
}
}
}
I've also seen many SO issues like the following
Angular 2 dynamic forms example with ngmodel results in "expression has changed after it was checked" which basically state that [(ngModel)] should not be used with dynamic forms, but the primeNG documentation says the components can work with model driven forms and the only way to set the answer (that I know of) is [(ngModel)]. I believe what might happen here is since I set the only question in the formGroup to a value that the formGroup becomes valid in between the change detection and causes the error
Error in ./FormComponent class FormComponent - inline template:17:48 caused by: Expression has changed after it was checked. Previous value: 'false'. Current value: 'true'.

Answer 1

From your template it looks like you are using both model drive (formControlName)
and template driven (ngModel).
<p-radioButton name="{{model.field}}"
value="{{i}}"
label="{{label}}"
formControlName="{{model.field}}"
[(ngModel)]="questionAnswerRadio"></p-
<radioButton>
Please select one way and try again.
I suggest you to remove the [(ngModel)]

Answer 2

The only way i've found to get the change detection to be happy with my multi-nested components and primeNG was to implement full change detection manually. What that basically means was in every component I had to add something like the following
import ChangeDetectorRef
constructor(private change: ChangeDetectorRef)
{}
ngOnInit() {
// code here that inits everything
this.change.markForCheck();
}
Anything less then this caused the change detection errors to pop-up in different and unique ways in the components that used primeNG.