eventbus: add wildcard subscription type; getter to enumerate known types (#153)

event/bus.go

 package event
 
-import "io"
+import (
+	"io"
+	"reflect"
+)
 
 // SubscriptionOpt represents a subscriber option. Use the options exposed by the implementation of choice.
 type SubscriptionOpt = func(interface{}) error
@@ -11,6 +14,14 @@ type EmitterOpt = func(interface{}) error
 // CancelFunc closes a subscriber.
 type CancelFunc = func()
 
+// wildcardSubscriptionType is a virtual type to represent wildcard
+// subscriptions.
+type wildcardSubscriptionType interface{}
+
+// WildcardSubscription is the type to subscribe to to receive all events
+// emitted in the eventbus.
+var WildcardSubscription = new(wildcardSubscriptionType)
+
 // Emitter represents an actor that emits events onto the eventbus.
 type Emitter interface {
 	io.Closer
@@ -39,6 +50,11 @@ type Bus interface {
 	//
 	// Failing to drain the channel may cause publishers to block.
 	//
+	// If you want to subscribe to ALL events emitted in the bus, use
+	// `WildcardSubscription` as the `eventType`:
+	//
+	//  eventbus.Subscribe(WildcardSubscription)
+	//
 	// Simple example
 	//
 	//  sub, err := eventbus.Subscribe(new(EventType))
@@ -71,4 +87,11 @@ type Bus interface {
 	//  defer em.Close() // MUST call this after being done with the emitter
 	//  em.Emit(EventT{})
 	Emitter(eventType interface{}, opts ...EmitterOpt) (Emitter, error)
+
+	// GetAllEventTypes returns all the event types that this bus knows about
+	// (having emitters and subscribers). It omits the WildcardSubscription.
+	//
+	// The caller is guaranteed that this function will only return value types;
+	// no pointer types will be returned.
+	GetAllEventTypes() []reflect.Type
 }