Add ability to delete from fsm metadata by key

examples/data.go

@@ -16,6 +16,7 @@ func main() {
 		fsm.Events{
 			{Name: "produce", Src: []string{"idle"}, Dst: "idle"},
 			{Name: "consume", Src: []string{"idle"}, Dst: "idle"},
+			{Name: "remove", Src: []string{"idle"}, Dst: "idle"},
 		},
 		fsm.Callbacks{
 			"produce": func(_ context.Context, e *fsm.Event) {
@@ -27,7 +28,12 @@ func main() {
 				if ok {
 					fmt.Println("message = " + message.(string))
 				}
-
+			},
+			"remove": func(_ context.Context, e *fsm.Event) {
+				e.FSM.DeleteMetadata("message")
+				if _, ok := e.FSM.Metadata("message"); !ok {
+					fmt.Println("message removed")
+				}
 			},
 		},
 	)
@@ -48,4 +54,11 @@ func main() {
 
 	fmt.Println(fsm.Current())
 
+	err = fsm.Event(context.Background(), "remove")
+	if err != nil {
+		fmt.Println(err)
+	}
+
+	fmt.Println(fsm.Current())
+
 }

fsm.go

@@ -21,7 +21,6 @@
 //
 // Fysom for Python
 // https://github.com/oxplot/fysom (forked at https://github.com/mriehl/fysom)
-//
 package fsm
 
 import (
@@ -270,6 +269,13 @@ func (f *FSM) SetMetadata(key string, dataValue interface{}) {
 	f.metadata[key] = dataValue
 }
 
+// DeleteMetadata deletes the dataValue in metadata by key
+func (f *FSM) DeleteMetadata(key string) {
+	f.metadataMu.Lock()
+	delete(f.metadata, key)
+	f.metadataMu.Unlock()
+}
+
 // Event initiates a state transition with the named event.
 //
 // The call takes a variable number of arguments that will be passed to the