Reactivity

Tanstack Form doesn't cause re-renders when interacting with the form. So you might find yourself trying to use a form or field state value without success.

If you would like to access reactive values, you will need to subscribe to them using one of two methods: useStore or the form.Subscribe component.

Some uses for these subscriptions are rendering up-to-date field values, determining what to render based on a condition, or using field values inside the logic of your component.

For situations where you want to "react" to triggers, check out the listener API.

The useStore hook is perfect when you need to access form values within the logic of your component. useStore takes two parameters. First, the form store. Second a selector to fine tune the piece of the form you wish to subscribe to.

const firstName = useStore(form.store, (state) => state.values.firstName)
const errors = useStore(form.store, (state) => state.errorMap)

const firstName = useStore(form.store, (state) => state.values.firstName)
const errors = useStore(form.store, (state) => state.errorMap)

You can access any piece of the form state in the selector.

Note, that useStore will cause a whole component re-render whenever the value subscribed to changes.

While it IS possible to omit the selector, resist the urge as omitting it would result in many unnecessary re-renders whenever any of the form state changes.

The form.Subscribe component is best suited when you need to react to something within the UI of your component. For example, showing or hiding ui based on the value of a form field.

<form.Subscribe>
  selector={(state) => state.firstName}
  children={(firstName) => (
    <form.Field>{(field) => <input name="lastName" value={field.state.lastName} onChange={field.handleChange}/></form.Field>
  )}
</form.Subscribe>

<form.Subscribe>
  selector={(state) => state.firstName}
  children={(firstName) => (
    <form.Field>{(field) => <input name="lastName" value={field.state.lastName} onChange={field.handleChange}/></form.Field>
  )}
</form.Subscribe>

The form.Subscribe component doesn't trigger component-level re-renders. Anytime the value subscribed to changes, only the form.Subscribe component re-renders.

The choice between whether to use useStore or form.Subscibe mainly boils down to that if it's rendered in the ui, reach for form.Subscribe for its optimizations perks, and if you need the reactivity within the logic, then useStore is the choice to make.