CSC/ECE 517 Fall 2007/wiki1b 2 22: Difference between revisions
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== 5. Ruby moment of zen == | == 5. Ruby moment of zen == | ||
Below is the model class for a User. If the user has a ton of profile data and we want to abstract that data, we will make a Profile class. | |||
However, we don’t want to call user.profile.street_address every time we need to access an attribute in a user’s profile. We also don’t want to define a bunch of reader methods. [http://cleanair.highgroove.com/articles/2006/07/04/ruby-moment-of-zen-method_missing] | |||
class User < ActiveRecord::Base | class User < ActiveRecord::Base | ||
has_one :profile | has_one :profile | ||
end | end | ||
class Profile < ActiveRecord::Base | class Profile < ActiveRecord::Base | ||
belongs_to :user | belongs_to :user | ||
end | end | ||
class User < ActiveRecord::Base | class User < ActiveRecord::Base | ||
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end | end | ||
So, we now have this: | So, we can now have this: | ||
@user.street_address == @user.profile.street_address | @user.street_address == @user.profile.street_address | ||
== 6. Generic Handler == | == 6. Generic Handler == |
Latest revision as of 18:58, 10 October 2007
Introduction - method_missing
The method - method_missing - is invoked by Ruby, when an object sent a message that it cannot handle. symbol is the symbol for the method called, and args are any arguments that were passed to it. By default, the interpreter raises an error when this method is called. However, it is possible to override the method to provide more dynamic behavior.
obj.method_missing(symbol [, *args] ) => result
When a method is called, Ruby looks for a same name method. It searches the method in the following sequence - self object’s methods, instance methods of all objects of this class, included modules in this class, superclass, superclass’s included modules, all the way till class Object. If it still can’t find a method, it looks in the Kernel module, included in the class Object. When a method does not exist, Ruby invokes method_missing. [1]
Examples
1. Object composition
The below implementation of method_missing can be used to debug some code without littering it with print statements. 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. [2]
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
2. Factorial
Below is the - Computer class which contains a factorial method. (n!). If we want to 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. [3]
class Computer def factorial n raise ArgumentError if n < 0 f = 1 n.downto(1) do |i| f = f * i end f end def method_missing(meth, *args) meth.to_s =~ /_([0-9]*)!/ return super if ! $1 factorial($1.to_i) end end
So instead of using traditional method call, we can use some thing like _<digits>!
computer = Computer.new puts computer.factorial(4) /*Traditional*/ puts computer._4! /*method_missing will return correct result.*/
This way, we won't have to create methods for every integer, by using method_missing.
3. Enumerator
We can enhance the functionalities of existing classes with the use of method_missing. Below is the implementation of class Enumerable. It shows how simply and in a user friendly manner, we can add an integer to all the elements of map, select, reject and find specifying criteria. [4]
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. 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.). It also helps ensuring type-checking.
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
5. Ruby moment of zen
Below is the model class for a User. If the user has a ton of profile data and we want to abstract that data, we will make a Profile class.
However, we don’t want to call user.profile.street_address every time we need to access an attribute in a user’s profile. We also don’t want to define a bunch of reader methods. [5]
class User < ActiveRecord::Base has_one :profile end class Profile < ActiveRecord::Base belongs_to :user end
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 can now have this:
@user.street_address == @user.profile.street_address
6. 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