Signals and Slots

• Signals
• Slots

Signals

To define signals, a class must implement the Emitter interface. All non-static methods that return a SignalEvent instance are considered as signals.

// A class able to emit signals
class MyEmitter implements Emitter
{
    // A valid signal function
    public function signal(string $param1, int $param2): SignalEvent
    {
        return SignalEvent::auto();
    }
    
    // ... 
}

The goal of a SignalEvent instance is to encapsulate all input parameters of the signal in a single object. The static method SignalEvent::auto() uses reflection to facilitate parameter forwarding, thus preventing misordering of parameters.

A signal function SHOULD only return a SignalEvent instance and SHOULD NOT perform any other actions.

We recommend using the EmitterHelper trait to implement the Emitter interface. This trait provides the useful emit(SignalEvent $signalEvent): void method. The emit function is protected, by design, as it’s best to only emit signals from the class that defines them and its subclasses. You can find more details in the Advanced section.

class MyEmitter implements Emitter
{
    // ...
    public function processing(): void
    {
        // ...
        $this->emit(
            $this->signal('my string', 123)
        );
        // ...
    }
}

Calling a signal function is not sufficient to actually trigger the signal; the returned SignalEvent instance SHOULD be immediately emitted using the emit method.

• A class can define multiple signals.
• There are no restrictions on the type of parameters a signal can have.
• Although there are also no restrictions on the number of parameters, it is recommended to keep it low in order to be compatible with existing connection functions (see Connections).
• The visibility of a signal function only affects its capability to be called or connected, following the usual rules of visibility in PHP (see Connections Visibility).

Slots

Any non-static object method can be considered a slot. In most cases, it makes sense to call a slot as a regular method, without being triggered by a signal.

class MyReceiver
{
    // A valid slot function
    public function slot(string $param1, int $param2): void
    {
        // ...
    }
    
    // ...
}

• There are no restrictions on the type of parameters a slot can have.
• Although there are also no restrictions on the number of parameters, it is recommended to keep it low in order to be compatible with existing connection functions (see Connections).
• The visibility of a slot function only affects its capability to be called or connected, following the usual rules of visibility in PHP (see Connections Visibility).
• A slot SHOULD NOT return a value, as there is no way to retrieve it from the signal emitter (see Connections).