Sometimes you may want to do some checking before a transition and allow or deny that transition conditionally. You can do exactly this with State using poilcies.

To create a policy for your machine, simply extend the OffbeatEngineer\State\Policy class and pass an instance of your policy class to the Machine constructor as the third argument.

<?php
use OffbeatEngineer\State\Policy;

class OrderPolicy extends Policy {
}

$order->state = new Machine("...", "...", $policy);
?>

Before each transition, the state machine will consult the policy by invoking a method named after the transition. It then proceeds only if that method call returns true. As an example:

<?php
class OrderPolicy extends Policy {

    public function pay($gateway){
        return 'stripe' == $gateway;
    }

}
...
// This transition will be denied,
// machine state stays unchanged.
$order->state->pay('paypal');

// This transition will be allowed,
// machine state changed to 'payed'.
$order->state->pay('stripe');
?>

Transitions not specified in the policy are allowed by default. If you prefer a safer policy scheme, override the $denial property on your policy class to true. That way, unspecified transitions are denied by default unless you explicitly allow them in your policy.

<?php
class OrderPolicy extends Policy {

    protected $denial = true;

    /* your other code */
}
?>

If no policy is specified upon machine instantiation, a policy allows all transitions will be used by default.