Reactive Forms in Angular: Listening for Changes

Reactive form instances like FormGroup and FormControl have a valueChages method that returns an observable that emits the latest values. You can therefore subscribe to valueChanges to update instance variables or perform operations.

Take a look at our intro to Reactive Forms if this is all new to you.

Here weโ€™ll create a very simple example that updates a template string every time a value changes in the form. First, letโ€™s initialize our reactive form with FormBuilder:

myForm: FormGroup;
formattedMessage: string;

constructor(private formBuilder: FormBuilder) {}

ngOnInit() {
  this.myForm = this.formBuilder.group({
    name: '',
    email: '',
    message: ''
  });

  this.onChanges();
}

Notice how we call an onChanges method in the ngOnInit lifecycle hook after having initialized our form. Hereโ€™s the content of our onChanges method:

onChanges(): void {
  this.myForm.valueChanges.subscribe(val => {
    this.formattedMessage =
    `Hello,

    My name is ${val.name} and my email is ${val.email}.

    I would like to tell you that ${val.message}.`;
  });
}

You can also listen for changes on specific form controls instead of the whole form group:

onChanges(): void {
  this.myForm.get('name').valueChanges.subscribe(val => {
    this.formattedMessage = `My name is ${val}.`;
  });
}

๐ŸŒŒ Since valueChanges returns an observable, thy sky is pretty much the limit in terms of what you can do with the values that are emitted.

  Tweet It

๐Ÿ•ต Search Results

๐Ÿ”Ž Searching...