Workflow: before_transition implementation

module Workflow
  class Specification
    attr_accessor :before_transition_proc
  private
    def before_transition(&proc)
      @before_transition_proc = proc
    end
  end
  module WorkflowInstanceMethods
    def process_event!(name, *args)
      event = current_state.events[name.to_sym]
      raise NoTransitionAllowed.new(
        "There is no event #{name.to_sym} defined for the #{current_state} state") \
        if event.nil?
      @halted_because = nil
      @halted = false
      
      check_transition(event)
      
      run_before_transition(current_state, spec.states[event.transitions_to], name, *args)
      return false if @halted
        
      return_value = run_action(event.action, *args) || run_action_callback(event.name, *args)
      return false if @halted
        
      run_on_transition(current_state, spec.states[event.transitions_to], name, *args)
      transition_value = transition(current_state, spec.states[event.transitions_to], name, *args)
      return_value.nil? ? transition_value : return_value
    end
  private
    def run_on_transition(from, to, event, *args)
      instance_exec(from.name, to.name, event, *args, &spec.on_transition_proc) if spec.on_transition_proc
    end  
    def run_before_transition(from, to, event, *args)
      instance_exec(from.name, to.name, event, *args, &spec.before_transition_proc) if spec.before_transition_proc
    end  
  end
end