State machines are no good if you cannot monitor their state to do something about it. With State, you can register transition listeners quite easily. Just attach a callable to the transition you want to monitor:

<?php
$order->state->on('pay', [$order, 'addPayment']);
?>

This will trigger the addPayment() method on $order whenever the pay transition is performed. The listener should have the following signature:

<?php
function transitionListener($from, $to, $arg1, $arg2, $arg3 ...);
?>

It accepts a $from state, a $to state and all transition parameters you specified when calling the process() method or transition methods.

Sometimes you may want to monitor all transitions of a given state machine. This is especially useful for logging purposes (e.g. keeping a state transition log for orders of your e-commerce system). In this case, you can register a global listener by omitting the $transition parameter to the on() call:

<?php
$order->state->on([$order, 'logging']);
?>

The logging() method of $order will be called when any state transition happens within the machine. It accepts the $transition name, a $from state, a $to state, and an array of transition parameters.

<?php
function logging($transition, $from, $to, $parameters)
?>