State machines with composables in Vue

Introduction

State machines help manage different states clearly. Using composables in Vue makes state machines reusable and easy to organize.

You want to control a button that can be loading, success, or error.

Managing form steps where each step is a different state.

Handling modal dialogs that open, close, or show errors.

Controlling a toggle switch with on and off states.

Syntax

Vue

import { ref } from 'vue';

export function useStateMachine() {
  const state = ref('initial');

  function transition(event) {
    switch (state.value) {
      case 'initial':
        if (event === 'start') state.value = 'running';
        break;
      case 'running':
        if (event === 'stop') state.value = 'stopped';
        break;
      case 'stopped':
        if (event === 'reset') state.value = 'initial';
        break;
    }
  }

  return { state, transition };
}

Use ref to keep track of the current state reactively.

The transition function changes states based on events.

Examples

Simple state machine with two states: 'idle' and 'active'.

Vue

import { ref } from 'vue';

const state = ref('idle');

function transition(event) {
  if (state.value === 'idle' && event === 'start') {
    state.value = 'active';
  }
}

A toggle machine switching between 'on' and 'off' states.

Vue

import { ref } from 'vue';

function useToggleMachine() {
  const state = ref('off');

  function toggle() {
    state.value = state.value === 'off' ? 'on' : 'off';
  }

  return { state, toggle };
}

Sample Program

This Vue component shows the current state and buttons to change it. Clicking buttons triggers state changes using the state machine logic.

Vue

<template>
  <div>
    <p>Current state: {{ state }}</p>
    <button @click="transition('start')">Start</button>
    <button @click="transition('stop')">Stop</button>
    <button @click="transition('reset')">Reset</button>
  </div>
</template>

<script setup>
import { ref } from 'vue';

const state = ref('initial');

function transition(event) {
  switch (state.value) {
    case 'initial':
      if (event === 'start') state.value = 'running';
      break;
    case 'running':
      if (event === 'stop') state.value = 'stopped';
      break;
    case 'stopped':
      if (event === 'reset') state.value = 'initial';
      break;
  }
}
</script>

Important Notes

State machines make your app predictable by clearly defining allowed state changes.

Composable functions keep state logic reusable and clean.

Use descriptive state names to make your code easy to understand.

Summary

State machines control app states clearly and predictably.

Vue composables help organize state machine logic for reuse.

Use ref and functions to manage and change states.

Practice

1. What is the main purpose of using a state machine in a Vue composable?

easy

A. To manage and control the different states of an application clearly and predictably

B. To style components dynamically based on user input

C. To fetch data from an API asynchronously

D. To create reusable UI components without state

Solution

1. Step 1: Understand state machine role

   A state machine helps track and control states in a clear way, avoiding unpredictable behavior.

2. Step 2: Relate to Vue composables

   Vue composables organize this logic so it can be reused across components easily.

3. Final Answer:

   To manage and control the different states of an application clearly and predictably -> Option A

4. Quick Check:

   State machine purpose = Manage states clearly [OK]

Hint: State machines organize app states clearly, not styling or fetching [OK]

Common Mistakes:

• Confusing state machines with styling or data fetching
• Thinking composables only create UI components
• Assuming state machines handle API calls

2. Which of the following is the correct way to declare a reactive state variable for a state machine in a Vue composable?

easy

A. const state = reactive('idle')

B. const state = ref('idle')

C. const state = computed('idle')

D. const state = watch('idle')

Solution

1. Step 1: Identify reactive state declaration

   In Vue composables, ref is used to create a reactive primitive value like a string.

2. Step 2: Check other options

   reactive is for objects, computed is for derived values, and watch is for side effects, so they are incorrect here.

3. Final Answer:

   const state = ref('idle') -> Option B

4. Quick Check:

   Use ref for reactive primitives [OK]

Hint: Use ref() for reactive simple values like strings [OK]

Common Mistakes:

• Using reactive() for primitive values
• Confusing computed() with state declaration
• Using watch() to declare state

3. Given this Vue composable code, what will be the value of state.value after calling next() twice?

import { ref } from 'vue'

export function useMachine() {
  const state = ref('idle')
  function next() {
    if (state.value === 'idle') state.value = 'loading'
    else if (state.value === 'loading') state.value = 'success'
  }
  return { state, next }
}

medium

A. 'idle'

B. 'loading'

C. 'success'

D. undefined

Solution

1. Step 1: Trace first next() call

   Initial state is 'idle'. Calling next() changes it to 'loading'.

2. Step 2: Trace second next() call

   Now state is 'loading'. Calling next() again changes it to 'success'.

3. Final Answer:

   'success' -> Option C

4. Quick Check:

   Two next() calls move state from idle -> loading -> success [OK]

Hint: Trace state changes step-by-step for each next() call [OK]

Common Mistakes:

• Assuming state resets to 'idle'
• Forgetting second next() changes state again
• Confusing state.value with state itself

4. Identify the error in this Vue composable state machine code:

import { ref } from 'vue'

export function useMachine() {
  const state = ref('idle')
  function next() {
    if (state === 'idle') state.value = 'loading'
    else if (state.value === 'loading') state.value = 'success'
  }
  return { state, next }
}

medium

A. next() function is not returned

B. Missing import of ref from 'vue'

C. Cannot assign to state.value, should assign to state

D. Incorrect comparison: should use state.value instead of state

Solution

1. Step 1: Check state comparisons

   state is a ref object, so to compare its value, use state.value, not state directly.

2. Step 2: Verify other parts

   Import is correct, assignment to state.value is correct, and next() is returned properly.

3. Final Answer:

   Incorrect comparison: should use state.value instead of state -> Option D

4. Quick Check:

   Compare ref values with state.value [OK]

Hint: Always compare ref values with .value, not the ref object [OK]

Common Mistakes:

• Comparing ref object instead of its value
• Forgetting to return functions from composable
• Misassigning to ref object instead of .value

5. You want to extend a Vue composable state machine to handle a 'failure' state after 'loading'. Which change correctly adds this new state transition when next() is called from 'loading'?

hard

A. Add an else if: else if (state.value === 'loading') state.value = 'failure'

B. Replace the existing loading transition with state.value = 'failure' only

C. Add a new function to set state to 'failure' instead of modifying next()

D. Change initial state to 'failure' to handle errors

Solution

1. Step 1: Understand current transitions

   Currently, next() changes 'idle' -> 'loading' -> 'success'. To handle 'failure' after 'loading' when next() is called, add a new else if condition for 'loading' to transition to 'failure'.

2. Step 2: Identify correct addition

   Adding an else if for state.value === 'loading' to set state.value = 'failure' extends the state machine correctly without removing existing transitions.

3. Final Answer:

   Add an else if: else if (state.value === 'loading') state.value = 'failure' -> Option A

4. Quick Check:

   Add new transition for loading -> failure without removing others [OK]

Hint: Add a new else if condition for the failure state [OK]

Common Mistakes:

• Adding a duplicate else if for 'loading' which conflicts or is unreachable
• Adding separate functions instead of modifying next()
• Changing initial state instead of the transition