Transition Actions

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Each transition can carry an action block written in braces right after the event name: A -> B on event { ... }. The block executes synchronously after the transition is decided, but before .send returns — ideal for side effects that are cohesive with the state change (logs, hardware pulses, animation triggers).

Door with four transitions, each printing a message when fired.

02-actions.zolo

Playground

// Feature: transition actions — code that runs on transition

// Syntax: `A -> B on event { <body> }`. The block runs after the

// transition is decided but before `.send` returns.

// When to use: side effects tied to state changes — log lines,

// hardware pulses, animation triggers, audio events.


machine Door {
  state Closed, Open, Locked
  initial Closed
  Closed -> Open on open { print("door opened") }
  Open -> Closed on close { print("door closed") }
  Closed -> Locked on lock { print("door locked") }
  Locked -> Closed on unlock { print("door unlocked") }
}

let door = Door.new()
print(door.state)  // expected: Closed


door.send("open")  // expected: door opened

print(door.state)  // expected: Open


door.send("close")  // expected: door closed

print(door.state)  // expected: Closed


door.send("lock")  // expected: door locked

print(door.state)  // expected: Locked


door.send("unlock")  // expected: door unlocked

print(door.state)  // expected: Closed

Keep the blocks small. For heavier work, prefer spawning a coroutine with spawn inside the action instead of blocking send.