Reactive Forms Not Working with ControlValueAccessor

user_3120232 0
Viewed 1015

I have built a control called TextBox, and for simplicity, I will only post the relevant parts of the control on here.

    @Component({
      selector: 'app-textbox',
      template:
      `
        <input [(ngModel)]="value" [disabled]="disabled" />
      `,
      styleUrls: ['./textbox.component.css']
    })
    export class TextboxComponent implements OnInit, ControlValueAccessor {
    
      constructor() { }
      writeValue(obj: any): void {
        this._value = obj;
      }
      registerOnChange(fn: any): void {
        this.onChange = fn;
      }
      registerOnTouched(fn: any): void {
        this.onTouch = fn;
      }
      setDisabledState?(isDisabled: boolean): void {
        this.disabled = isDisabled;
      }
    
      disabled = false;
    
      onChange:()=>{}
      onTouch:()=>{};
    
      private _value:string;
      public get value():string {
        return this._value
      } 
      public set value(value:string){
        this._value = value;
      }
    
      ngOnInit(): void {
      }

and my app.component.ts looks like:

    @Component({
      selector: 'app-root',
      template:
      `
        <form [formGroup]="form" novalidate>
          <div>
            <label >Name</label>
            <app-textbox formControlName="name"></app-textbox>
          </div>
        </form>
      `,
      styleUrls: ['./app.component.css']
    })
    export class AppComponent implements OnInit{
      /**
       *
       */
      constructor(private formBuilder:FormBuilder) {
      }
    
      form = this.formBuilder.group({
        name:['', Validators.required]
      })
    
      model:NameModel = {
        name:'test'
      }
    
      ngOnInit(): void {
        this.form.get('name').setValue(this.model.name);
      }
    }
    
    interface NameModel{
      name:string;
    }

When I run the application I would expect that the textbox would be populated with the value test.

Can someone please explain why it is not?

I will add when I do this.form.get('name')?.value I get the correct value.

angular-reactive-forms angular-forms angular typescript
2 Answers

You just have to move setting the value this.form.get('name').setValue(this.model.name); to ngAfterViewInit instead of ngOnInit, or you have to call after it:

this.form.updateValueAndValidity();

Btw, there are two notes here you should take care of them:

  1. You missed writing the value back to the form-control after it has been changed in your TextboxComponent, so you have to call the registered onChange method within the value setter like the following:
private _value: string;
public get value(): string {
  return this._value;
}
public set value(value: string) {
  this._value = value;
  this.onChange(value);
}
  1. It's better in your case to initialize the FormGroup within the ngOnInit, and set the default value of name directly, like the following:
form: FormGroup;

ngOnInit(): void {
  this.form = this.formBuilder.group({
    name: [this.model.name, Validators.required],
  });
}
user_151596030

In writeValue(obj), you need to assign received input obj to this._value

writeValue(obj: any): void {
  if (obj !== this._value) {
    this._value = obj;
  }
}

And don't forget to add providers for NG_VALUE_ACCESSOR and the multi-provider to extend existing providers. (Refer to References 1)

@Component({
  selector: 'app-textbox',
  template: `
    <input [(ngModel)]="value" [disabled]="disabled" />
  `,
  styleUrls: ['./textbox.component.css'],
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      useExisting: forwardRef(() => TextboxComponent),
      multi: true
    }
  ]
})
export class TextboxComponent implements OnInit, ControlValueAccessor {

  writeValue(obj: any): void {
    if (obj !== this.value) {
      this._value = obj;
    }
  }

  ...
}

Sample Solution on StackBlitz

References

Connect your custom control to ngModel with Control Value Accessor.

user_80176900
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