CSC/ECE 517 Summer 2008/wiki1 5 bk

Understanding Hooks

The hooking mechanism in Ruby allows certain functional events inside a running program to be identified while providing an action that will occur upon detection of the specified event. In this way, hooks serve as a detector of actions within the code, when event x is encountered, peform y. Examples of where such a technique would prove useful include:

• Recording when or how many times a method is called.
• Recording when an object is created.
• Debugging problematic code.

Example of a Hook

module Kernel

 alias_method :original_system, :system
 def system(*args)
   puts "Your program executed the \"#{args.join(', ')}\" command." 
   puts "It was executed at: " + `date`
   puts "The command output the following: "
   original_system(*args)
 end

end system("ls -al")

Your program executed the "ls -al" command. It was executed at: Mon Jun 2 21:19:27 EDT 2008 The command output the following: total 20 drwxr-xr-x 2 brking brking 4096 2008-06-02 21:19 . drwxr-xr-x 18 brking brking 4096 2008-06-02 21:19 .. -rw-r--r-- 1 brking brking 134 2008-06-01 22:01 fib.rb -rw-r--r-- 1 brking brking 235 2008-06-02 20:29 hooking.rb -rw-r--r-- 1 brking brking 287 2008-06-02 21:19 moreHooking.rb