RubyBeans, a short example of Ruby metaprogramming

As I explained in my recent post about metaprogramming, implementing a JavaBean is a task that would greatly benefit from metaprogramming. Here, I illustrate how this would be done in Ruby. Note that the resulting code has little interest for Ruby as JavaBeans are a Java idiom. I hope however that it demonstrates my purpose. Let’s dive straight into the code:

class RubyBean

  def initialize
    @listeners = []
  end

  def register_listener(l)
    @listeners.push(l) unless @listeners.include?(l)
  end

  def unregister_listener(l)
    @listeners.delete(l)
  end

These methods implement the management of the property change listeners on the bean. It is a subset of what java.beans.PropertyChangeSupport provides. Now comes the most interesting part:

  def RubyBean.property(*properties)
    properties.each { |property|
      class_eval(%Q[
        def #{property}
          @#{property}
        end

        def #{property}=(value)
          old_value = @#{property}
          return if (value == old_value)
          @listeners.each { |listener|
            listener.property_changed(:#{property},
                                     old_value,
                                     value)
          }
          @#{property} = value
        end
      ])
    }
  end
end

I define property as a class method that takes a variable number of arguments. For each of its arguments, property dynamically creates and adds to the RubyBean class, a getter and a setter to the bean. This follows the Ruby conventions for accessors: the getter is simply the property name, the setter is the property name followed by the equal sign. The setter also notifies the registered property change listeners. Note that in Ruby, you would use the attr_accessor method to create these accessors.

Now, writing a bean is as simple as:

class TestBean < RubyBean
  property :name, :firstname
end

Simple, no? Finally, here is how this code could be used:

class LoggingPropertyChangeListener
  def property_changed(property, old_value, new_value)
    print property, " changed from ",
      old_value, " to ",
      new_value, "\n"
  end
end

test = TestBean.new
listener = LoggingPropertyChangeListener.new
test.register_listener(listener)
test.name = "Parker"
test.firstname = "Charlie"
test.firstname = "Maceo"
test.unregister_listener(listener)

and the output it generates:

name changed from nil to Parker
firstname changed from nil to Charlie
firstname changed from Charlie to Maceo

Do not hesitate to comment if you have any query, suggestion, critic or simply if you want to say Hi!

• Update 06/03/2006: see RubyBeans revisited for a better implementation.
• Update 01/05/2007: see RubyBeans revisited (again) – Closures in Ruby for a more elegant solution.

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