Defly is a debugging and tracing tool for Ruby that allows users to trace method calls, instance variables, and provides better error messages to inspect the point of error. It works by allowing users to trace specific methods and variables on a class and see the flow of method calls and variable changes. The document demonstrates using Defly to trace method calls and variable changes on a Warrior class and inspect where an error occurs in a Rocket class.

1. Defly
Debugging and Tracing Tool for Ruby
http://github.com/eggegg/defly

2. Andrew Liu
• Collage Student
• Intern @ Cardinalblue
• eggegg @ github
• Ruby, iOS, Android
• andrewliu33@gmail.com

3. gem install defly

4. Defly is for...
• Trace method calls
• Trace instance variables
• Better error messages
• Inspect the point of error

5. class Warrior
attr_accessor :hp, :mp
def sleep
self.hp += 10
self.mp += 2
end
end

6. class Warrior
attr_accessor :hp, :mp
def sleep
puts "BEFORE: #{@hp}, #{@mp}"
self.hp += 10
puts "AFTER ADDING HP: #{@hp}, #{@mp}"
self.mp += 2
puts "AFTER ADDING MP: #{@hp}, #{@mp}"
end
end

7. class Warrior
attr_accessor :hp, :mp
def sleep
self.hp += 10
self.mp += 2
end
end
require 'defly'
Warrior.debug!
Warrior.new.trace([:hp, :hp=, :mp, :mp=, :sleep],
[:@hp, :@mp]) do |warrior|
warrior.hp = 10
warrior.mp = 20
warrior.sleep
end

8. Tracing hp, hp=, mp, mp=, sleep on Warrior instance
Tracing @hp, @mp on Warrior instance
<<<<< Warrior#hp=(10) # (irb):14:in `block in irb_binding'
@hp = 10 # undefined
@mp = nil # undefined
>>>>> 10
<<<<< Warrior#mp=(20) # (irb):15:in `block in irb_binding'
@mp = 20 # undefined
>>>>> 20
<<<<< Warrior#sleep() # (irb):16:in `block in irb_binding'
<<<<< Warrior#hp() # (irb):7:in `sleep'
>>>>> 10
<<<<< Warrior#hp=(20) # (irb):7:in `sleep'
@hp = 20 # 10 -> 20
>>>>> 20
<<<<< Warrior#mp() # (irb):8:in `sleep'
>>>>> 20
<<<<< Warrior#mp=(22) # (irb):8:in `sleep'
@mp = 22 # 20 -> 22
>>>>> 22
>>>>> 22

9. NoMethodError
bug.rb
debugging = nil
debugging.is_annoying
irb
irb(main):001:0> require 'bug'
NoMethodError: undefined method `is_annoying' for nil:NilClass
from /Users/eggegg/bug.rb:2:in `<top (required)>'
...
Where is the bug???

10. irb(main):003:0> require 'defly'
=> true
irb(main):004:0> require 'bug'
NoMethodError: undefined method `is_annoying' for nil:NilClass
bug.rb:2> debugging.<<is_annoying>>
from /Users/andrewliu/bug.rb:2:in `<top (required)>'

11. Inspecting Errors
class Rocket
def launch!
@reason = "Bugs invasion"
raise "Engine Fail"
end
end
Rocket.debug!
rocket = Rocket.new
rocket.watch_error "Engine Fail"
rocket.launch!

12. Ruby shell (Rib by godfat)
to inspect errors!
>>>>> Error received:
"Engine Fail"
>>>>>
#<Rocket:0(0)>> @reason
=> "Bugs invasion"
#<Rocket:0(0)>>

13. Thanks!
Any Question?