Programmatic Control

In some cases, you may want to control the state or context values of a machine programmatically via its props or based on certain conditions. This is typically know as "controlling" the components.

Zag provides a number of ways to control the state of a machine programmatically.

Setting initial context

All machines have an "internal" context value that is used to manage extended state (or data) within the machine. To extend the initial value of this context, we allow users pass it directly into the machine function.

This is used to set/override the initial context value of the machine. It can't be updated later on. Keep reading to see how to update it.

For example, if you want an accordion to start with a specific selected value. Here's how to acheive that:

const [state, send] = useMachine(
  // set initial context value
  accordion.machine({
    value: "item-1",
  }),
)

Using exposed methods

The connect method of the machines provide helpful methods (APIs) to change the machine state or update its context.

This approach is the recommended approach to programmatically update a machine.

Let's say we'd like to change the expanded accordion item in an accordion group. Here's how to do that:

function Accordion() {
  // 1. Bind the machine in your framework
  const [state, send] = useMachine(accordion.machine)

  // 2. Call the connect function
  const api = accordion.connect(state, send)

  // 3. Use exposed methods
  api.setValue("item-1")

  return (...)
}

Now, let's say you'd like to udpate the selected accordion based on props passed to the Accordion component, here is how to do that in React:

import useUpdateEffect from "use-update-effect"

function Accordion(props) {
  const [state, send] = useMachine(
    accordion.machine({
      value: props.value,
    }),
  )

  const api = accordion.connect(state, send)

  useUpdateEffect(() => {
    api.setValue(props.value)
  }, [props.value])

  return (...)
}

Transient updates

In some edge-cases, you may want to update the machine's context programmatically without sending an event to the machine or using the provided api.

You can pass the context value to the useMachine hook directly. We recommend memoizing the context to ensure it's stable between re-renders.

// ❌  Don't do this
const [state, send] = useMachine(accordion.machine, {
  context: {
    value: props.value,
  },
})

// ✅  Do this
const [state, send] = useMachine(accordion.machine, {
  context: useMemo(
    () => ({
      value: props.value,
    }),
    [props.value],
  ),
})

Controlled Usage in React

When using framework like React, you might want to fully control the value passed to a machine, and react to changes to that value (via a change handler).

There are some key things to note:

• Zag is "react" agnostic and doesn't follow the classic controlled "read-only" behavior in React. We assume uncontrolled by default and re-sync the DOM as needed.

• Zag works like native DOM, you can set and get an element's value, but you can't force it (except if you intercept the input native events and prevent it from happening).

To get this working in React, you can use the flushSync method from react-dom to ensure the changes are always in sync.

import { useState } from "react"
import { flushSync } from "react-dom"

const NumberInput = () => {
  const [value, setValue] = useState("123")

  const [state, send] = useMachine(machine({ id: "1" }), {
    context: {
      value,
      onChange(details) {
        flushSync(() => {
          setValue(details.value)
        })
      },
    },
  })
}

Edit this page on GitHub