Angular Reactive Forms valueChanges - UI changes only

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The Angular FormControl has a valueChanges Observable that states:

Emits an event every time the value of the control changes, in the UI or programmatically.

Is there a way to set the FormControl to ignore programmatic value changes? (Basically the equivalent of OneWayToSource binding in .NET)

Specifically, the issue I'm facing is that my valueChanges subscription, I'm updating a bunch the values bound to a bunch of other controls, which then causes valueChanges to fire for all of them as well, which is problematic as the actions they perform in their valueChanges handlers conflicts with the control the user actually touched.

You can skip emitting the valueChange event by passing the option { emitEvent: false } to the setValue call.

setValue(value: any, options: {
    onlySelf?: boolean;
    emitEvent?: boolean;
    emitModelToViewChange?: boolean;
    emitViewToModelChange?: boolean;
} = {}): void

Also you might want to take a look at other options.

If onlySelf is true, this change will only affect the validation of this FormControl and not its parent component. This defaults to false.

If emitEvent is true, this change will cause a valueChanges event on the FormControl to be emitted. This defaults to true (as it falls through to updateValueAndValidity).

If emitModelToViewChange is true, the view will be notified about the new value via an onChange event. This is the default behavior if emitModelToViewChange is not specified.

If emitViewToModelChange is true, an ngModelChange event will be fired to update the model. This is the default behavior if emitViewToModelChange is not specified.

Docs

Angular Reactive Forms valueChanges - UI changes only, By default, whenever a value of a FormControl changes, Angular runs We can tell Angular that we only want to run the validation function upon submit or blur . other hand we listen to the form's value changes and update the store. Whether it's entities arriving from the server or UI state data, Akita has  The ValueChanges is an event raised by the Angular forms whenever the value of the FormControl, FormGroup or FormArray changes. It returns an observable so that you can subscribe to it. The observable gets the latest value of the control. It allows us to track changes made to the value in real-time and respond to it.

The reactive form group have property yourFormName.pristine

You can use !yourFormName.pristine to detect only UI changes

 * A control is `pristine` if the user has not yet changed
 * the value in the UI.
 *
 * @returns True if the user has not yet changed the value in the UI; compare `dirty`.
 * Programmatic changes to a control's value do not mark it dirty.
 */
readonly pristine: boolean;

Angular Reactive Forms: Tips and Tricks, We will learn how to detect changes in forms and its controls. How To Listen Changes In Reactive Form Controls Using valueChanges In Angular Just change our onValueChanges() function , and get the form field  Reactive Forms in Angular: Listening for Changes. Reactive form instances like FormGroup and FormControl have a valueChanges method that returns an observable that emits the latest values. You can therefore subscribe to valueChanges to update instance variables or perform operations.

You can pass { emitEvent: false } as options for the below reactive form methods to prevent them from triggering the valueChanges event

this.form.patchValue(value, { emitEvent: false })
this.form.setValue(value, { emitEvent: false })
this.form.controls.email.updateValueAndValidity({ emitEvent: false })
this.form.disable({ emitEvent: false })

yes disable triggers the valueChanges event

PS: above this.form is a reactive form

Read this excellent post, it'll answer all your questions and even give some great insights on reactive forms:

https://netbasal.com/angular-reactive-forms-tips-and-tricks-bb0c85400b58

How To Listen Changes In Reactive Form Controls Using , valueChanges can be used for conditional validation in reactive form. when the value of control changes either using UI or programmatically. valueChanges: Observable<any>, A multicasting observable that emits an event every time the value of the control changes, in the UI or programmatically -- Angular Documentation In order to facilitate autosave, you can now easily subscribe to this observable, map the form value to something your server understands, and send off the data.

I could do it by putting this listener on the html file:

<ss-multiselect-dropdown ... (ngModelChange)="functionToTrigger($event)"></ss-multiselect-dropdown>

And on the .ts file:

functionToTrigger($event) {
    if (event === undefined) {
        // Put here are the instructions for UI changes
    } else {
        // Put here are the instructions for programmatic changes
}

I really don't know why it works this way. I came to this solution by trial and error.

Angular valueChanges and statusChanges, Listen Changes In Reactive Form Controls Using valueChanges In Angular These reactive form instances like FormGroup and FormControl have a valueChanges method Just change our onValueChanges() function, and get the form field firstname and How to create a registration form using Angular 8 and Kendo UI. Many of us love TypeScript as its Strongly-Typed. This gives the main power to the Angular Framework, But Angular Reactive Forms are not fully Strongly-Typed ☹️, Because of the substantial use of ‘any’ keyword, this creates a nasty code while developing the form in terms of mismatch typecasting, typo mistake, etc.

Listen Changes In Reactive Form Controls Using valueChanges In , Learn How to Create Your First Angular Reactive Form here Besides, that submit button would only be enabled when the form is valid. Form submit is Listening to changes; Put conditional validation In addition, if you haven't checked out Infragistics Ignite UI for Angular Components, be sure to do so! For example, if on the one hand we listen to the store’s changes and update the form accordingly, and on the other hand we listen to the form’s value changes and update the store. In order to avoid the above situation, we can tell Angular not to emit the valueChanges event whenever we update the form.

How to do Conditional Validation on valueChanges method in , But with Angular we also have another option of processing the form by Learning Objectives; Setting Up a Reactive Form; React to Changes in Our Form But in this example we just have a FormControl on its own, this is why we use the To react to changes on this form we need to subscribe to the valueChanges​  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).

Reactive Model Form • Angular, Forms Guide · Reactive Forms Guide A control is pristine if the user has not yet changed the value in the UI. valueChanges: Observable<any>, Read-Only. Angular is a platform for building mobile and desktop web applications. Join the community of millions of developers who build compelling user interfaces with Angular.

Comments
  • Thanks, this is helpful. However, in my particular case, I'm update the values bound to the other form controls, not the form controls themselves. So I don't have the setValue method available, and it seems Angular's change detection is picking up on the changes and firing the events.
  • Thanks, Tomasz!
  • Awsome, really helpful
  • pristine/dirty not reset after consequent patchValue's, are they?
  • The $event passed in the ngModelChange is just the value of the model. It has nothing to do with how the model was changed in the UI or not.