add (un)subscribe function

src/index.ts

@@ -43,11 +43,12 @@ export interface Interpreter_T {
   context: any;
   eventQueue:Array<Event_T>;
   isTransitioning: boolean;
+  subscriptions: Record<string, SubscriptionCallbackFunction_T>
 }
 export function interpret(machine:Machine_T, options:{state?:string, context:any}) : Interpreter_T{
   let {state, context} = options;
   if(typeof state === 'undefined'){ state = machine.states[0].name; }
-  const interpreter = {machine, state, context, eventQueue:[], isTransitioning:false}
+  const interpreter = {machine, state, context, eventQueue:[], isTransitioning:false, subscriptions: {}}
   send(interpreter, ['entry', null] );
   return interpreter;
 }
@@ -80,6 +81,8 @@ export function send(interpreter : Interpreter_T, event:Event_T){
       processNextEvent(interpreter);
     }
     interpreter.isTransitioning = false;
+    // only run subscriptions here, once the machine's state has settled:
+    Object.values(interpreter.subscriptions).forEach((subscriptionCallbackFunction)=>{ subscriptionCallbackFunction(interpreter); });
   }
 }
 export const enqueue = send;
@@ -102,6 +105,7 @@ function processNextEvent(interpreter:Interpreter_T){
     });
     // processing of `goto` must be last:
     if(goto_ !== null){
+      send(interpreter, ['exit', null]);
       interpreter.state = goto_.targetStateName;
       send(interpreter, ['entry', null]);
     }
@@ -126,5 +130,22 @@ function categorizeReactions(reactions:Array<Reaction_T>) : {sideEffects:Array<S
   return {sideEffects, contextMutations, goto_};
 }
 
+export type SubscriptionCallbackFunction_T = (self:Interpreter_T)=>void;
+let subscriptionId : number = 0;
+export function subscribe(interpreter:Interpreter_T, callback:SubscriptionCallbackFunction_T){
+  subscriptionId++;
+  interpreter.subscriptions[subscriptionId.toString()] = callback;
+  return subscriptionId;
+}
+export function unsubscribe(interpreter:Interpreter_T, subscriptionId:string){
+  delete interpreter.subscriptions[subscriptionId.toString()];
+}
+
 export const Spawn = function(){};
-export const Unspawn = function(){};
+export const Unspawn = function(){};
+
+/*
+export function useMachine(machine, options){
+  return useMemo(()=>interpret(AppMachine, {context:{}}),[]);
+}
+*/