Best way to associate checkbox control with textbox control

Question

I've been trying many different methods have been failing all week, I need to be able to associate a checkbox control with an answer.

Pretty much, if the checkbox is checked then user must answer the question and it must have a validation of minlength 4.

The checkbox will contain a question and answer.

So if the user chooses that question he/she must provide an answer.

the questions are rendered from the server in an object such as;

 { 
                question_id: "1",
                selected: true,
                EN: "Question 1 - EN",
                FR: "Question 1 -FR",

            },
            { 
                question_id: "2",
                selected: false,
                EN: "Question 2 - EN",
                FR: "Question 2 -FR"
            }

I can post my code if required, it is however very long and complicated.

Answer 1

If you create a Reactive Form you can dynamically change the validation

component

this.questionForm = fb.group({
  questions: fb.array(this.questions.map(this.createQuestionControl(fb)))
});

createQuestionControl(fb: FormBuilder) {
  return (question, index) => {
    const checkbox = question.selected
    const answerbox = question.selected ? ['', [Validators.required, Validators.minLength(4)]] : ''
    return fb.group({question: checkbox, answer: answerbox, questionNumber: index + 1});
  }
}

changeValidator(selected, index) {
  const answerbox = this.questionForm.get('questions.' + index).get('answer')

  const validators = selected ? [Validators.required, Validators.minLength(4)] : null
  answerbox.setValidators(validators);
  answerbox.updateValueAndValidity();
}

The createQuestionControl() method will change each question into a control as below which the form builder can turn into a group with a question and an answer

{ question: true, answer: ['', [Validators.required, Validators.minLength(4)]], index: 4 }

The changeValidator() method will add or remove validators on the answer if the question is changed (note: do not forget updateValueAndValidity)

template

<form [formGroup]="questionForm" (ngSubmit)="submit(questionForm)">

  <div formArrayName="questions">
    <div *ngFor="let question of questionForm.get('questions').controls | orderBySelected; let i = index;" [formGroupName]="i">
      <!--{{questionForm.get('questions.' + i + '.questionNumber').value}}-->
      {{questions[questionForm.get('questions.' + i + '.questionNumber').value - 1]['EN']}}
      <input type="checkbox" formControlName="question" (ngModelChange)="changeValidator($event, i)"/>
      <input type="text" formControlName="answer" />
      <em *ngIf="questionForm.get('questions.' + i + '.answer').invalid">Minimum length 4</em>
    </div>
  </div>

  <button type="submit" [disabled]="questionForm.invalid">Submit</button>

</form>

Following a clarification in the comments:

Maximum of 5 can be checked at a given time

I have updated the array to have cross field validation of no more than 3 (easier to test you can change it to 5)

export function max3Selected(formArray) {

  let totalSelected = formArray.controls.reduce((selectedControls, control) => 
  {
    if (control.get('question').value) {
      selectedControls++
    }
    return selectedControls;
  }, 0)

  return totalSelected > 3 ? { moreThanThreeSelected: true } : null;
}

and you would change the fb.array to include the validator function

fb.array(this.questions.map(this.createQuestionControl(fb)), max3Selected)

Screenshot of result

Live plunker example