and other languages…

 Also called introspection  Program can examine/modify its own state set variables call methods add new methods define new classes

What class am I? o.class Who’s my parent? o.superclass Am I an X? (synonyms) o.is_a? o.kind_of? Can I do this? o.respond_to? What are my variables? o.local_variables (private) o.global_variables o.instance_variables What else can I do? o.methods

 As to why both is_a? and kind_of? exist: I suppose it's part of Ruby's design philosophy. Python would say there should only be one way to do something; Ruby often has synonymous methods so you can use the one that sounds better. It's a matter of preference. It may partly be due to Japanese influence: I'm told that they will use a different word for the same number depending on the sentence in order to make it sound nicer. Matz may have carried that idea into his language design.

class Cat def initialize(name, = = age end def purr puts "purrrrrr" end def show_local x = 5 local_variables.each do |var| puts var end def show_methods //false = only my method, not inherited Cat.instance_methods(false).each do |method| puts method.to_s end cat = Cat.new("Fluffy", 6) puts cat.class puts "Cat? #{cat.is_a? Cat}" puts "String? #{cat.is_a? String}" puts "Kind of cat? #{cat.kind_of? Cat}" puts "Kind of object? #{cat.kind_of? Object}" puts "Instance variables" cat.instance_variables.each do |var| puts var end puts "Local variables" cat.show_local puts "Methods" cat.show_methods look up: instance_variable_set and instance_variable_get

 Think about the unit tests in Ruby. You write a method with the name test_something. It gets run automatically.  How does that happen??

 Think about a game program. The user has a game piece that’s become really powerful. They now roll the die and are able to “spawn” a new object of that type.  How can we create a new instance of that class? this_obj.class.new(params)

 Anyone heard of Rails? ActiveRecord?  Object-relational mappers… associate the rows of a database table with objects, and the columns of a database table with instance variables.  Lot of cool “magic” in frameworks! Including the ability to dynamically generate “views” to accept data. How do they do that??

 8/what-is-reflection-and-why-is-it-useful

class Message_framework def message_hello puts "hello" end def message_goodbye puts "goodbye" end def show_all_messages o = self puts "The message names" o.methods.each do |method| if (method.to_s[0..6] == "message") puts method.to_s end def run_all_messages # now execute the methods puts "\nThe messages" self.methods.each do |method| if (method.to_s[0..6] == "message") self.send method end messages = Message_framework.new messages.show_all_messages messages.run_all_messages How does this compare to a unit test framework?

 Write a program that uses reflection to prompt for user input (like a form generator, but no real GUI)  Example: I have a Cat class with two attributes, name and age  Program output: What data will we accept? How do I know the class name? Google – how to set

 Put your class in one file (e.g., my Cat is in cat.rb) The only methods you need are initialize (so that you set up the instance variables) and to_s (for convenient display)  Create a class named Input_framework Initialize an object of type Input_framework with an object of your type (e.g., I would pass in a Cat object) Get started by printing the instance variable names (e.g., I Using reflection, prompt the user to enter the data. Using reflection, store the data entered in the object  To verify, print the contents of the object (uses the to_s method of your class)

 What other languages have reflection?  Is it used for different purposes depending on the language?