inspector_gadget.rb

class InspectorGadget < BasicObject
  def initialize(target, notifier)
    @target = target
    @notifier = notifier
  end

  def method_missing(name, *args, &block)
    result = @target.public_send(name, *args, &block)
    @notifier.invocation(name, args, block, result)
    result
  rescue ::Exception => exception
    @notifier.invocation(name, args, block, exception)
    Kernel.raise
  end

  def respond_to_missing?(method_name, include_private = false)
    @target.respond_to_missing?(method_name, include_private) || super
  end
end

print_notifier.rb

class PrintNotifier
  def initialize(output)
    @output = output
  end

  def invocation(name, args, block, result)
    inspected_args = args.map(&:inspect).join(', ')
    if block
      inspected_args << "&#{block.inspect}"
    end
    @output.puts "#{name}(#{inspected_args})"
    @output.puts "=> #{result.inspect}"
  end
end