module StateMachine
  # Provides a set of helper methods for evaluating methods within the context
  # of an object.
  module EvalHelpers
    # Evaluates one of several different types of methods within the context
    # of the given object.  Methods can be one of the following types:
    # * Symbol
    # * Method / Proc
    # * String
    # 
    # == Examples
    # 
    # Below are examples of the various ways that a method can be evaluated
    # on an object:
    # 
    #   class Person
    #     def initialize(name)
    #       @name = name
    #     end
    #     
    #     def name
    #       @name
    #     end
    #   end
    #   
    #   class PersonCallback
    #     def self.run(person)
    #       person.name
    #     end
    #   end
    # 
    #   person = Person.new('John Smith')
    #   
    #   evaluate_method(person, :name)                            # => "John Smith"
    #   evaluate_method(person, PersonCallback.method(:run))      # => "John Smith"
    #   evaluate_method(person, Proc.new {|person| person.name})  # => "John Smith"
    #   evaluate_method(person, lambda {|person| person.name})    # => "John Smith"
    #   evaluate_method(person, '@name')                          # => "John Smith"
    # 
    # == Additional arguments
    # 
    # Additional arguments can be passed to the methods being evaluated.  If
    # the method defines additional arguments other than the object context,
    # then all arguments are required.
    # 
    # For example,
    # 
    #   person = Person.new('John Smith')
    #   
    #   evaluate_method(person, lambda {|person| person.name}, 21)                              # => "John Smith"
    #   evaluate_method(person, lambda {|person, age| "#{person.name} is #{age}"}, 21)          # => "John Smith is 21"
    #   evaluate_method(person, lambda {|person, age| "#{person.name} is #{age}"}, 21, 'male')  # => ArgumentError: wrong number of arguments (3 for 2)
    def evaluate_method(object, method, *args, &block)
      case method
        when Symbol
          klass = (class << object; self; end)
          args = [] if (klass.method_defined?(method) || klass.private_method_defined?(method)) && object.method(method).arity == 0
          object.send(method, *args, &block)
        when Proc, Method
          args.unshift(object)
          arity = method.arity
          
          # Procs don't support blocks in < Ruby 1.9, so it's tacked on as an
          # argument for consistency across versions of Ruby
          if block_given? && Proc === method && arity != 0
            if [1, 2].include?(arity)
              # Force the block to be either the only argument or the 2nd one
              # after the object (may mean additional arguments get discarded)
              args = args[0, arity - 1] + [block]
            else
              # Tack the block to the end of the args
              args << block
            end
          else
            # These method types are only called with 0, 1, or n arguments
            args = args[0, arity] if [0, 1].include?(arity)
          end
          
          method.is_a?(Proc) ? method.call(*args) : method.call(*args, &block)
        when String
          eval(method, object.instance_eval {binding}, &block)
        else
          raise ArgumentError, 'Methods must be a symbol denoting the method to call, a block to be invoked, or a string to be evaluated'
      end
    end
  end
end
