Question

Is there a quick way to track the methods that are being called on an object? Often, when I’m working with a gem at a level just below their public interface, I run into errors that are hard to track down. Ultimately, I end up tracking the object through the source code and keeping everything in my head.

But it would be nice to be able to call something like a #log_method_calls on an object so that, say, all methods called on it get printed to stdout or something. Is there any way to accomplish this?

Answer 1

There are several methods to do it, depending on the situation.

If it’ possible to create a new object instead of the observed, you can easily write an observer class using method_missing.

class LogProxy  
  def initialize obj
    @obj = obj
  end

  def method_missing(name, *args)
    puts "#{name} => #{args.to_s}"  
    @desk.send(name, *args)
  end
end

If it’s not possible, you still may use alias_method. It’s a bit more tricky, but using Module.instance_methods you can chain every method of anything.

Something like:

module Logger

  def self.included(mod)
    mod.instance_methods.each do |m|
      next if m =~ /with_logging/
      next if m =~ /without_logging/

      mod.class_eval do

        define_method "#{m}_with_logging" do |*args|
          puts "#{m} called #{args.to_s}"
          self.send_without_logging "#{m}_without_logging", *args
        end

        alias_method "#{m}_without_logging", m
        alias_method m, "#{m}_with_logging"
      end

    end
  end

end

TargetClass.send(:include, Logger)