NotifyWhen, a Ruby module you can extend any object with to get on-the-fly tracing of any later method calls on it

notify_when.rb

# So you want to find out what is messing with your object somewhere deep in your stack...
# Behold, your on-the-fly tracer...

module NotifyWhen
  def notify_when(*meths, &callback)
    class << self; self; end.instance_eval do
      meths.each do |meth|
        alias_method "unnotified_#{meth}".to_sym, meth
        define_method(meth) do |*args, &block|
          callback.yield(*([meth, args, self][0..callback.arity]))
          send("unnotified_#{meth}", *args, &block)
        end
      end
    end
  end
end


h = {} # can be any object
h.extend NotifyWhen
h.notify_when(:[], :[]=){|m, args| puts "#{m} was called with args #{args}"}

h[5] = 'five'
h[5]
h[6]

# []= was called with args [5, "five"]
# [] was called with args [5]
# [] was called with args [6]