What is a state machine?

A state machine is a tool for modeling stateful, reactive systems. It is useful for declaratively describing the behavior of an application or component.

To model any logic as using the state machine pattern, it must have:

A finite number of states.
A finite number of transitions between states.

Example of state machine

Consider a simple toggle or switch component that consists of two states, active and inactive. The initial state will be active

The supported transitions that can happen here:

In the active state, when we click the toggle, it should transition to the inactive state
In the inactive state, when we click the toggle, it should transition to the active state

Here's how we'll model the logic in code:

import { createMachine } from "@zag-js/core"

const machine = createMachine({
  // initial state
  initial: "active",
  // the finite states
  states: {
    active: {
      on: {
        CLICK: {
          // go to inactive
          target: "inactive",
        },
      },
    },
    inactive: {
      on: {
        CLICK: {
          // go to active
          target: "active",
        },
      },
    },
  },
})

The machine gives you access to these key information:

state: State is a representation of the machine at a specific point in time and contains the following properties:
- value: the current state value
- context: the current context or data stored in the machine
- event: the event object that triggered the transition of this current state
- matches(...): the function to check whether the machine is in a specific state. It's similar to state.value === <state>
send: A function to send events or signals to the machine.

Now that we've modelled the component logic, let's map that to DOM attributes and event handlers following the WAI-ARIA specification for the switch component.

We'll write a function called connect to do this.

function connect(state, send) {
  const active = state.matches("active")
  return {
    active,
    buttonProps: {
      type: "button",
      role: "switch",
      "aria-checked": active,
      onClick() {
        send("CLICK")
      },
    },
  }
}

Here's how to consume the toggle machine logic and connect in React.js.

import { useMachine } from "@zag-js/react"
import { machine, connect } from "./toggle"

function Toggle() {
  const [state, send] = useMachine(machine)
  const api = connect(state, send)

  return (
    <button
      {...api.buttonProps}
      style={{
        width: "40px",
        height: "24px",
        borderRadius: "999px",
        background: api.active ? "green" : "gray",
      }}
    >
      {api.active ? "ON" : "OFF"}
    </button>
  )
}

That's it! Now you've learned the fundamentals of a component state machine.

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