parallel

README

Parallel States

This example models a computer input system where a keyboard and mouse operate independently inside a single active state. The keyboard tracks whether caps lock is on or off, while the mouse tracks whether a button is clicked or released. Because both regions live inside a Parallel state, they run simultaneously — a CLICK event only affects the mouse region and a CAPS_LOCK event only affects the keyboard region.

State Diagram

---
title: Input System
config:
    theme: default
    maxTextSize: 50000
    maxEdges: 500
    fontSize: 16
---
stateDiagram-v2
    state active {
        state active_fork <<fork>>
        state active_join <<join>>
        state keyboard {
            [*] --> caps_off
            state "caps_off" as caps_off
            state "caps_on" as caps_on
        }
        state mouse {
            [*] --> not_clicked
            state "clicked" as clicked
            state "not_clicked" as not_clicked
        }
    }
	active_fork --> keyboard
	keyboard --> active_join
	active_fork --> mouse
	mouse --> active_join
	[*] --> active
	caps_off --> caps_on: CAPS_LOCK
	caps_on --> caps_off: CAPS_LOCK
	clicked --> not_clicked: RELEASE
	not_clicked --> clicked: CLICK

SCXML

<?xml version="1.0" encoding="UTF-8"?>
<scxml xmlns="http://www.w3.org/2005/07/scxml" version="1.0" name="input_system" initial="active">
  <parallel id="active">
    <state id="keyboard" initial="caps_off">
      <state id="caps_off">
        <transition event="CAPS_LOCK" target="caps_on"></transition>
      </state>
      <state id="caps_on">
        <transition event="CAPS_LOCK" target="caps_off"></transition>
      </state>
    </state>
    <state id="mouse" initial="not_clicked">
      <state id="clicked">
        <transition event="RELEASE" target="not_clicked"></transition>
      </state>
      <state id="not_clicked">
        <transition event="CLICK" target="clicked"></transition>
      </state>
    </state>
  </parallel>
</scxml>

What Happens

When the actor starts, it enters the active parallel state, which activates both the keyboard and mouse regions at the same time. The keyboard begins in caps_off and the mouse begins in not_clicked.

Sending CLICK toggles the mouse region to clicked while the keyboard stays in caps_off — the event has no effect on a region that doesn't handle it. Sending CAPS_LOCK then toggles the keyboard to caps_on while the mouse stays in clicked. Neither region interferes with the other, and States() returns all active leaf states across both regions.

When To Use This

• Media players — playback, volume, and display mode each operate as independent regions, so pressing mute doesn't pause the video.
• Form validation — each field runs its own validation region independently, letting one field show an error while others remain valid.
• Monitoring dashboards — CPU, memory, and disk health are tracked as separate regions, each with its own normal/warning/critical states.

Output

--- Starting Parallel Actor ---
Active States: [active keyboard mouse not_clicked caps_off]

--- Sending 'CLICK' ---
Active States: [active keyboard mouse clicked caps_off]

--- Sending 'CAPS_LOCK' ---
Active States: [active mouse keyboard caps_on clicked]

--- Conclusion ---
Parallel states avoid the 'state explosion' problem by letting you
model independent behaviors without creating states for every combination.

Running

go run .