Added TryFire

This allows handlers to return errors which will be returned to the caller.
Also replaced error returned from On with a panic.

bus.go

@@ -8,7 +8,7 @@ func New() *Bus {
 	return &Bus{topics: make(map[string]*topic)}
 }
 
-func (d *Bus) On(event string, receiver interface{}) error {
+func (d *Bus) On(event string, receiver interface{}) {
 	// TODO: make thread safe
 	t, hasTopic := d.topics[event]
 	if !hasTopic {
@@ -18,27 +18,43 @@ func (d *Bus) On(event string, receiver interface{}) error {
 
 	sub, err := newSubscriptionFromFunc(receiver)
 	if err != nil {
-		return err
+		panic(err)
 	}
 
 	t.addSubscriber(sub)
-	return nil
 }
 
 func (d *Bus) Fire(event string, args ...interface{}) {
+	_ = d.TryFire(event, args...)
+}
+
+func (d *Bus) TryFire(event string, args ...interface{}) error {
 	// TODO: make thead safe
 	topic, hasTopic := d.topics[event]
 	if !hasTopic {
-		return
+		return nil
 	}
 
 	preparedArgs := prepareArgs(args)
 
+	var errs []error
+
 	for sub := topic.head; sub != nil; sub = sub.next {
-		sub.handler.invoke(preparedArgs)
+		err := sub.handler.invoke(preparedArgs)
+		if err != nil {
+			errs = append(errs, err)
 		}
 	}
 
+	switch len(errs) {
+	case 0:
+		return nil
+	case 1:
+		return errs[0]
+	}
+	return HandlerError{topic: event, errs: errs}
+}
+
 type topic struct {
 	head *subscription
 	tail *subscription

errors.go

@@ -0,0 +1,16 @@
+package events
+
+import "fmt"
+
+type HandlerError struct {
+	topic string
+	errs  []error
+}
+
+func (h HandlerError) Error() string {
+	return fmt.Sprintf("caught %d errors from topic '%v': %v", len(h.errs), h.topic, h.errs)
+}
+
+func (h HandlerError) Unwrap() []error {
+	return h.errs
+}

handler.go

@@ -1,3 +1,4 @@
+//go:build !tinygo
 // +build !tinygo
 
 package events
@@ -21,7 +22,7 @@ func newReceiptHandler(receiver interface{}) (receiptHandler, error) {
 	return receiptHandler{receiverFunc: val, funcType: val.Type()}, nil
 }
 
-func (rh *receiptHandler) invoke(values preparedArgs) {
+func (rh *receiptHandler) invoke(values preparedArgs) error {
 	args := make([]reflect.Value, rh.funcType.NumIn())
 	for i := range args {
 		args[i] = reflect.Zero(rh.funcType.In(i))
@@ -32,7 +33,22 @@ func (rh *receiptHandler) invoke(values preparedArgs) {
 		}
 	}
 
-	rh.receiverFunc.Call(args)
+	rets := rh.receiverFunc.Call(args)
+	if len(rets) == 0 {
+		return nil
+	}
+
+	lastRet := rets[len(rets)-1]
+	switch {
+	case lastRet.IsNil():
+		return nil
+	case lastRet.CanInterface():
+		err, ok := lastRet.Interface().(error)
+		if ok {
+			return err
+		}
+	}
+	return nil
 }
 
 type preparedArgs []reflect.Value