Form control valueChanges gives the previous value

问题

I have a form control with name 'question1' within the form object parentForm and I have subscribed to it in the following way.

Its a radio button with two options Yes and No, when I select No I get Yes and when I select Yes its a No.

this.parentForm.controls['question1'].valueChanges.subscribe(
  (selectedValue) => {
    // If option `No is selected`
    console.log(selectedValue);  // displays No. OK
    console.log(this.parentForm.value['question1']);  // displays Yes. Problem is here
  }
);

selectedValue variable has the correct value but if I do console.log(this.parentForm.value['question1'] it gives the previous value.

I tried to put a setTimeout() before retrieving value from this.parentForm.value['question1'], It just works fine.

setTimeout(() => {
  console.log(this.parentForm.value['question1']); // gives the correct value.
}, 500);

But my question is why parentForm is not updated when its control's value changes and that too I am retrieving its value only after value was changed.

Note: I don't want to observe for parentForm.valueChanges, not my requirement.

回答1:

valueChanges is an Observable so you can pipe pairwise to get the previous and next values in the subscription.

// No initial value. Will emit only after second character entered
this.form.get('fieldName')
  .valueChanges
  .pipe(pairwise())
  .subscribe(([prev, next]: [any, any]) => ... );

// Fill buffer with initial value, and it will emit immediately on value change
this.form.get('fieldName')
  .valueChanges
  .pipe(startWith(null), pairwise())
  .subscribe(([prev, next]: [any, any]) => ... );

Example of it working in StackBlitz: https://stackblitz.com/edit/angular-reactive-forms-vhtxua

回答2:

The valueChanges event is fired after the new value is updated to the FormControl value, and before the change is bubbled up to its parent and ancestors. Therefore, you will have to access the value of the FormControl itself (which has just been patched), not a field of the FormGroup value object (which is untouched during the event).

In light of that, use this.parentForm.get('question1').value instead:

this.parentForm.controls['question1'].valueChanges.subscribe(
    (selectedValue) => {
      console.log(selectedValue);
      console.log(this.parentForm.get('question1').value);     
    }
);

回答3:

There is one more option, but it might not be suitable in all cases: you can wait one tick before the selectedValue is updated in FormControl:

setTimeout( () => console.log( selectedValue ) );

More on sync/async forms topic: Reactive forms in Angular Guide docs.

回答4:

Try to do this

this.parentForm.controls['question1'].valueChanges.subscribe(
    (selectedValue) => {
      console.log(selectedValue);
      console.log(this.parentForm.value.question1);     
    }
);

if controls of FormBuilder was updated, you can immediately recover last values from FormBuilder object through the property value

来源：https://stackoverflow.com/questions/44898010/form-control-valuechanges-gives-the-previous-value

标签

angular

angular2-forms

angular2-formbuilder