feat(fsm): support fallback transitions

support a transition that applies to any source state

fsm/fsm.go

@@ -78,11 +78,13 @@ func NewFSMHandler(world World, state StateType, stateKeyField StateKeyField, ev
 			if dst != nil {
 				d.stateHandlers[dst] = nil
 			}
-			if !reflect.TypeOf(src).AssignableTo(stateFieldType.Type) {
+			if src != nil && !reflect.TypeOf(src).AssignableTo(stateFieldType.Type) {
 				return nil, xerrors.Errorf("event `%s` source type is not assignable to: %s", name, stateFieldType.Type.Name())
 			}
 			d.transitions[eKey{name, src}] = dst
-			d.stateHandlers[src] = nil
+			if src != nil {
+				d.stateHandlers[src] = nil
+			}
 		}
 	}
 
@@ -131,6 +133,10 @@ func (d fsmHandler) Plan(events []statemachine.Event, user interface{}) (interfa
 	currentState := userValue.Elem().FieldByName(string(d.stateKeyField)).Interface()
 	e := events[0].User.(fsmEvent)
 	destination, ok := d.transitions[eKey{e.name, currentState}]
+	// check for fallback transition for any source state
+	if !ok {
+		destination, ok = d.transitions[eKey{e.name, nil}]
+	}
 	if !ok {
 		return d.completePlan(e, nil, 1, xerrors.Errorf("Invalid transition in queue, state `%+v`, event `%s`", currentState, e.name))
 	}

fsm/fsm_test.go

@@ -51,6 +51,11 @@ var events = fsm.Events{
 			uint64(2): nil,
 		},
 	},
+	"any": {
+		TransitionMap: fsm.TransitionMap{
+			nil: uint64(1),
+		},
+	},
 }
 
 var stateHandlers = fsm.StateHandlers{
@@ -305,3 +310,23 @@ func TestNoChangeHandler(t *testing.T) {
 	require.NoError(t, err)
 	<-tw.done
 }
+
+func TestAllStateEvent(t *testing.T) {
+	ds := datastore.NewMapDatastore()
+
+	tw := &testWorld{t: t, done: make(chan struct{}), proceed: make(chan struct{}), universalCalls: 0}
+	close(tw.proceed)
+	smm, err := fsm.New(ds, tw, statemachine.TestState{}, "A", events, stateHandlers)
+	require.NoError(t, err)
+
+	// any can run from any state and function like start
+	err = smm.Send(uint64(2), "any")
+	require.NoError(t, err)
+	<-tw.done
+
+	tw.done = make(chan struct{})
+	// here any can function like a restart handler
+	err = smm.Send(uint64(2), "any")
+	require.NoError(t, err)
+	<-tw.done
+}

fsm/types.go

@@ -47,6 +47,7 @@ type StateKeyField string
 // is designated the state key and must be comparable
 type StateKey interface{}
 
+// TransitionMap is a map from src state to destination state
 type TransitionMap map[StateKey]StateKey
 
 // EventDesc describes what happens when an event is triggered