feat: Expose unmet guard conditions

Add a method to return list of non-permissible triggers from the current state, along with the descriptions of the un-met guard conditions.

Author: andrewmckaskill

src/Stateless/StateMachine.cs

@@ -147,6 +147,17 @@ public IEnumerable<TriggerDetails<TState, TTrigger>> GetDetailedPermittedTrigger
                 .Select(trigger => new TriggerDetails<TState, TTrigger>(trigger, _triggerConfiguration));
         }
 
+        /// <summary>
+        /// Gets any triggers for the current state that are not permissible, and the descriptions
+        /// of the unmet trigger conditions associated with them.
+        /// </summary>
+        /// <param name="args"></param>
+        /// <returns></returns>
+        public IEnumerable<Tuple<TTrigger, string[]>> GetTriggersWithUnmetConditions(params object[] args)
+        {
+            return CurrentRepresentation.GetTriggersWithUnmetGuardConditions(args);
+        }
+
         StateRepresentation CurrentRepresentation
         {
             get

src/Stateless/StateRepresentation.cs

@@ -330,6 +330,26 @@ public IEnumerable<TTrigger> GetPermittedTriggers(params object[] args)
                 return result;
             }
 
+            public IEnumerable<Tuple<TTrigger, string[]>> GetTriggersWithUnmetGuardConditions(params object[] args)
+            {
+                List<Tuple<TTrigger, string[]>> unmetConditions = new List<Tuple<TTrigger, string[]>>();
+
+                var behaviours = TriggerBehaviours.Values.SelectMany(x => x);
+                foreach (var triggerBehaviour in behaviours)
+                {
+                    if (triggerBehaviour.GuardConditionsMet(args))
+                        continue;
+                    
+                    var conditions = triggerBehaviour.UnmetGuardConditions(args);
+                    unmetConditions.Add(new Tuple<TTrigger, string[]>(triggerBehaviour.Trigger, conditions.ToArray()));
+                }
+                
+                if (Superstate != null)
+                    unmetConditions.AddRange(Superstate.GetTriggersWithUnmetGuardConditions(args));
+
+                return unmetConditions;
+            }
+
             internal void SetInitialTransition(TState state)
             {
                 InitialTransitionTarget = state;