How to identify which item in FormArray emitted valueChanges event?

问题

In Angular, is there a way to identify which FormGroup/FormControl in a dynamicFormArray emitted the valueChanges event?

My FormArray is dynamic. It starts out empty and users could add a FormGroup to the FormArray by clicking a button.

When valueChanges, I need to re-validate the control. Since I dont know which control emitted the event, I loop through the entire FormArray and validate all FormGroup/FormControl even though only one control changed - and this is every time when anything in the array changes. How can I avoid doing this?

        this.myFormArray
        .valueChanges
        .subscribe(data => this.onValueChanged(data));

    onValueChanged(data?: any): void {

    // the data I receive is an entire form array.
    // how can I tell which particular item emitted the event, 
    // so I don’t need to loop through entire array and run validation for all items.

    for (let control in this.myFormArray.controls) {
        // run validation on each control.
    }
}

回答1:

I resolved this issue by adding a formControl (named groupIndex) in the formGroup to track the index and subscribing to the valueChanges at the formGroup level instead of formArray level. On valueChanges event, I could then access the formControl that stored the current index.

this.myMainFormGroup = this.myFormBuilder.group({
    // other formControls
    myFormArray: this.myFormBuilder.array([this.addArrayItem()])
});

// this method will be called every time the add button is clicked
addArrayItem(): FormGroup {
  const itemToAdd = this.myFormBuilder.group({
    // dont forget add input control for groupIndex in html for this. It will give error otherwise.
    // i made the input control hidden and readonly
    groupIndex:"", 
    firstName:["", [validator1, validator2]]
    //other formControls

  });

  const myFormArray = <FormArray>this.myMainForm.get("myFormArray");

  //set groupIndex
  itemToAdd.get("groupIndex").patchValue(myFormArray.length -1);

  //subscribe to valueChanges
  itemToAdd.valueChanges
    .debounceTime(200)
    .subscribe(data => this.onValueChanged(data));

  myFormArray.push(itemToAdd);
}

onValueChanged(data?: any): void {
      const groupIndex = data["groupIndex"];

      const myChangedGroup = <FormArray>this.myMainForm.get("myFormArray").controls[groupIndex];

      // now I have hold of the group that changed without having to iterate through the entire array. 
      // run through the custom validator 
      this.generalValidator(myChangedGroup);
    }

回答2:

You can try something like this, but I am not sure that it will work

merge(...this.myFormArray.controls.map(control => control.valueChanges))
  .subscribe(this will be one of your form controls' value);

回答3:

Not in my PC to test, but maybe using the caller property of a function might guide you in the direction you want. Although this property is not recommended:

This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/caller

rough example:

this.myFormArray
    .valueChanges
    .subscribe(onValueChanged);

onValueChanged(data?: any): void {
     var whoYouGonnaCall = onValueChanged.caller.caller...caller;
     ...
}

Update

Another option is to store the last form value and use something like lodash for comparing the properties.

var lastFormValue = this.myFormArray.value; // maybe in an init function
this.myFormArray
    .valueChanges
    .subscribe(onValueChanged);

onValueChanged(data?: any): void {
     var diff = _.omitBy(data, function(v, k) {
         return lastFormValue[k] === v;
     });
     this.lastFormValue = this.myFormArray.value; // Update for future requests
     // diff will contain the properties if the form that changed.
     ...
}

回答4:

To build on epsilon's answer, which collects an array of valueChanges observables and merges them - you can also pipe the value changes thru a pipe, which adds necessary context to the changes stream via map.

merge(...this.formArray.controls.map((control: AbstractControl, index: number) =>
        control.valueChanges.pipe(map(value => ({ rowIndex: index, value })))))
      .subscribe(changes => {
        console.log(changes);
      });

Output:

{ 
  rowIndex: 0
  value: {
    <current value of the changed object>
  }
}

Note that the first call to map (on controls) is on an array. The second map (in the pipe) is an RxJs map. I really like this website to help get these operators straight and imagine what these streams of events look like: https://rxmarbles.com/#map

EDIT: Because I was watching a FormArray that could be modified by the user via the add/delete buttons, I added a changesUnsubscribe subject and reference that in the takeUntil. This allows me to discard the old set of watches and setup new ones when the list changes. So now I call watchForChanges() when items are added or removed from the list.

changesUnsubscribe = new Subject();
...
watchForChanges() {
  // cleanup any prior subscriptions before re-establishing new ones
  this.changesUnsubscribe.next();

  merge(...this.formArray.controls.map((control: AbstractControl, index: number) =>
            control.valueChanges.pipe(
                takeUntil(this.changesUnsubscribe),
                map(value => ({ rowIndex: index, control: control, data: value })))))
            .subscribe(changes => {
                this.onValueChanged(changes);
            });
}

来源：https://stackoverflow.com/questions/53654938/how-to-identify-which-item-in-formarray-emitted-valuechanges-event

标签

angular

rxjs

angular-reactive-forms