Trace method entrances and exists. Requires ActiveSupport.

tracer.rb

module Tracer
  def trace(method)
    method, ending = split_method_name(method)
    define_method "#{method}_with_trace#{ending}" do |*arguments|
      puts "called #{self.class}##{method}#{ending}(#{arguments.map(&:inspect).join(', ')})"
       returning(send("#{method}_without_trace#{ending}", *arguments)) do |v|
         puts "#{self.class}##{method}#{ending} => #{v.inspect}"
       end
     end
     alias_method_chain "#{method}#{ending}", :trace
   end
   private
   def split_method_name(method)
     if ['?', '!'].include?(method.to_s.last)
       [method.to_s[0..-2], method.to_s.last]
     else
       [method, nil]
     end
   end
end

usage_example.rb

class Foo
  extend Tracer
  def foo(*args)
    'foo'
  end
  trace :foo
end

>> Foo.new.foo
called Foo#foo()
Foo#foo => "foo"
=> "foo"
>> Foo.new.foo(1)
called Foo#foo(1)
Foo#foo => "foo"
=> "foo"