Reactive props

Last updated 2 months ago


This is a highly experimental technology and it may change, or be removed in the future. Follow the release notes to stay in tune.

Reactive prop - is a field's prop, which value is resolved automatically using the live subscriptions system. The latter allows to subscribe to props changes of another fields, and re-evaluate the reactive prop's resolver whenever the referenced props update.

This is a generic concept implemented in several features of React Advanced Form. Those are listed below in this section. Each feature may utilize this concept in a different way, although the interface strives to be unified.


Below, there is a list of behaviors applicable to reactive props, regardless of where they are used. For consistency's sake, Reactive field props interface is used in these examples.

Multiple field references

It is possible to reference multiple fields within a single reactive prop resolver function:

required={({ get }) => {
const foo = get(['firstName', 'valid'])
const bar = get(['lastName', 'required'])
return foo && !bar
}} />

This will create two observers, respectively, and each change of the referenced props will trigger re-evaluataion of that required resolver function.

Delegated subscription

Reactive prop resolver can also reference fields which haven't mounted yet at the moment when the resolver function is declared. In that case a direct subscription cannot be created. Instead, a delegated subscription is created.

The purpose of the delegated subscription is to listen for the field registration event and re-evaluate the resolver function after the mounting occurs.

Delegated subscription behaves the following way:

  1. Listens to the event when the referenced field becomes registered.

  2. Analyzes the resolver function once more, gathering the references props of the newly registered referenced field.

  3. Creates a subscription for the changes of the referenced props list.

  4. Removes the delegated subscription.

To illustrate this, consider the next scenario:

required={({ get }) => !!get(['fieldThree', 'value'])} />
value="doe" />

By the time fielTwo.props.required reactive prop is evaluated, the referenced fieldThree doesn't exist yet. Once the referenced field has been mounted, the resolver function (fieldTwo.props.required) is re-evaluated, and the value of required prop is updated according to the resolver logic (to true in the example above).

Reactive field props

To create a reactive field prop simply pass a function as its value, and use the exposed get method to reference props of another fields.

Note: At the moment reactive field props are supported only for the required prop.


<Input required={({ get }) => get(['path', 'to', 'field', 'prop'])} />


Whenever another field's prop is referenced using the get method, a live subscription for that referenced prop is created.

initialValue="foo" />
required={({ get }) => !!get(['fieldOne', 'value'])} />

Reactive props resolver is never assigned as a value of the reactive prop. Instead, it is copied and executed whenever the referenced prop updates, consequentially updating the prop, which is connected to the resolver.

Note: It is possible to reference any props from the Field's state (i.e. value, valid, validSync, and many more).

Reactive validation rules

The concept of reactive props is applicable to synchronous validation rules as well. Just as with the Reactive field props, whenever another field's prop is referenced within a synchronous validation rule declaration, the latter is re-evaluated each time the referenced props update.