CSC/ECE 517 Fall 2007/wiki1b 2 22
Introduction - method_missing
The method_missing is a method that called whenever someone tries to call a method in your object that doesn't exist. When you call a method, Ruby looks for a method to invoke with the same name as the method you call. First it looks in the current self object’s own instance methods. Then it looks in the list of instance methods that all objects of that class share, and then in each of the included modules of that class. Then it looks in that class’s superclass, and then in the superclass’s included modules, all the way up until it reaches the class Object. If it still can’t find a method, the very last place it looks is in the Kernel module, included in the class Object. When a method call is not exist, Ruby use method_missing as the last resort. [1]
Examples
1. Object composition
class SimpleCallLogger
def initialize(o)
@obj = o
end
def method_missing(methodname, *args)
puts "called: #{methodname}(#{args})"
a = @obj.send(methodname, *args)
puts "\t-> returned: #{a}"
return a
end
end
This object "intercepts" all method calls made to it, prints out a message and forwards on the method call to an internal object using the 'send' method, without knowing anything about the object passed to it. It can be used to debug some code without littering it with print statements. [2]
2. Factorial
Let's create a Computer class that contains a factorial method (you know the famous n! thing).
class Computer
def factorial n
raise ArgumentError if n < 0
f = 1
n.downto(1) do |i|
f = f * i
end
f
end
end
computer = Computer.new
puts computer.factorial(4)
We would like to use some notation close to the usual notation:
computer = Computer.new puts computer._4!
Obviously, we cannot create methods for every integer, to do it we use the method_missing.
def method_missing(meth, *args) meth.to_s =~ /_([0-9]*)!/ return super if ! $1 factorial($1.to_i) end
If we use the special notation (_<digits>!) the method_missing implementation extracts the number, from the method name, and calls the factorial method to get the result. Each time and for any method the same processing happens. [3]
3. Enumerator
class Enumerable::Enumerator
def method_missing(sym,*args,&blk)
each{ |x| x.send(sym,*args,&blk) }
end
end
The benefit is a number of nice conveniences, eg.
[1,2,3].map + 3 => [4,5,6] [1,2,3].select > 2 => [3] [1,2,3].reject > 1 => [1] [1,2,3].find > 1 => 2
4. Roman numbers
For example, Roman [10] returns "X" and Roman.CLIX returns 159(through method_missing).
module Roman
Sorted = [
["M", 1000], ["CM", 900], ["D", 500], ["CD", 400],
["C", 100], ["XC", 90], ["L", 50], ["XL", 40],
["X", 10], ["IX", 9], ["V", 5], ["IV", 4], ["I", 1] ]
Values = Hash[*Sorted.flatten]
Values.default = 0 # Needed to cope with bad input and boundary conditions.
# Convert an integer to a Roman numeral.
def self.[](dec)
raise ArgumentError, dec.to_s if dec.class != Fixnum or dec < 1
Sorted.map { |str, num| str * (x, dec = dec.divmod num)[0] }.join
end
# Convert a Roman numeral to an integer.
def self.method_missing(id)
id = id.id2name
dec = (0...id.length).inject(0) { |sum, ix|
(n = Values[id[ix, 1]]) < Values[id[ix+1, 1]] ? sum - n : sum + n
}
raise ArgumentError, "#{id}? #{dec}=#{self[dec]}" if id != self[dec]
dec
end
end
5. Ruby Torrent
RubyTorrent uses method_missing to make all the bencoded fields in the .torrent file easy to access. (bencoding being the BitTorrent encoding scheme.) This is basically identical to the Javascript attributes thing above, but with some type-checking. For example:
class MetaInfoInfoFile
def initialize(dict=nil)
@s = TypedStruct.new do |s|
s.field :length => Integer, :md5sum => String, :sha1 => String,
:path => String
s.array :path
s.required :length, :path
end
end
def method_missing(meth, *args)
@s.send(meth, *args)
end
end
6. Ruby moment of zen
For example, let’s say you have a User model, and that user has a ton of profile data. You want to abstract that data, so you make a Profile class.
class User < ActiveRecord::Base has_one :profile end
class Profile < ActiveRecord::Base belongs_to :user end
However, you don’t want to call user.profile.street_address every time you need to access an attribute in a user’s profile. You also don’t want to define a bunch of reader methods like this:
def street_address profile.street_address end
Here’s all you have to do:
class User < ActiveRecord::Base
...
# Attempts to pass any missing methods to
# the associated +profile+.
def method_missing(meth, *args, &block)
if profile.respond_to?(meth)
profile.send(meth)
else
super
end
end
end
So, we now have this:
@user.street_address == @user.profile.street_address
7. Generic Handler
class NoBar
def method_missing(methodname, *args)
define_method(:bar) if "bar" == methodname.to_s
define_method(:nobar) if "nobar" == methodname.to_s
end
end
This is an example of using method_missing as a generic handler to handle when a calling method is not exist. You can use missing_method to dynamically create a method at a runtime.
Advantages
- Allows to catch problem at runtime.
- Allows to define a generic method_missing and handle any undefined method. This is a big advantage over Java. In Java, when you call an undefined method, the program will not compile.
- The use of method_missing falls under the general technique of meta-programming. You can employ meta-programming in missing_function to write a another function to handle the call.
References
- 10 things you should know about method_missing.http://www.thirdbit.net/articles/2007/08/01/10-things-you-should-know-about-method_missing/
- More Ruby: method_missing http://blog.mauricecodik.com/2005/12/more-ruby-methodmissing.html
- Hey Ruby, how much for the method_missing? http://www.alef1.org/ruby/method_missing/index.html
Further reading
- Evaluation Options in Ruby. http://www.infoq.com/articles/eval-options-in-ruby
- IF YOU BUILD IT .. will they come. http://www.ddj.com/blog/architectblog/archives/2007/08/dependency_inje.html
External links
- http://www.thirdbit.net/articles/2007/08/01/10-things-you-should-know-about-method_missing/
- http://blog.mauricecodik.com/2005/12/more-ruby-methodmissing.html
- http://www.alef1.org/ruby/method_missing/index.html
- http://www.ruby-forum.com/topic/112986
- http://redhanded.hobix.com/inspect/theBestOfMethod_missing.html
- http://cleanair.highgroove.com/articles/2006/07/04/ruby-moment-of-zen-method_missing