changes that allow you to ask "machine.may_event?", and get back the boolean answer from any pending transition guards

lib/aasm/aasm.rb

@@ -49,6 +49,13 @@ module AASM
         sm.events[name] = AASM::SupportingClasses::Event.new(name, options, &block)
       end
 
+      # an addition over standard aasm so that, before firing an event, you can ask
+      # may_event? and get back a boolean that tells you whether the guard method
+      # on the transition will let this happen.
+      define_method("may_#{name.to_s}?") do |*args|
+        aasm_test_event(name)
+      end
+      
       define_method("#{name.to_s}!") do |*args|
         aasm_fire_event(name, true, *args)
       end
@@ -100,6 +107,12 @@ module AASM
     aasm_events_for_state(aasm_current_state)
   end
 
+  # filters the results of events_for_current_state so that only those that
+  # are really currently possible (given transition guards) are shown.
+  def aasm_permissible_events_for_current_state
+    aasm_events_for_current_state.select{ |e| self.send(("may_" + e.to_s + "?").to_sym) }
+  end
+  
   def aasm_events_for_state(state)
     events = self.class.aasm_events.values.select {|event| event.transitions_from_state?(state) }
     events.map {|event| event.name}

lib/aasm/event.rb

@@ -7,6 +7,24 @@ class AASM::SupportingClasses::Event
     update(options, &block)
   end
 
+  # a neutered version of fire - it doesn't actually fir the event, it just
+  # executes the transition guards to determine if a transition is even
+  # an option given current conditions.
+  def may_fire?(obj, to_state=nil)
+    transitions = @transitions.select { |t| t.from == obj.aasm_current_state }
+    return false if transitions.size == 0
+    
+    result = false
+    transitions.each do |transition|
+      next if to_state and !Array(transition.to).include?(to_state)
+      if transition.perform(obj)
+        result = true
+        break
+      end
+    end
+    result
+  end
+  
   def fire(obj, to_state=nil, *args)
     transitions = @transitions.select { |t| t.from == obj.aasm_current_state }
     raise AASM::InvalidTransition, "Event '#{name}' cannot transition from '#{obj.aasm_current_state}'" if transitions.size == 0