Method Swizzling Method Decorator Refinement

1. Magic in Ruby
David Lin
davll.xc@gmail.com
<beer>
Oct. 18, 2013
</beer>

2. Suppose You Know
1. Ruby Programming Language
- class, module, method
- block
- mixin (include, extend)
2. Metaprogramming
- class_eval
- define_method
3. Monkey Patching

3. Outline
1. Smalltalk-style method calling
2. Method swizzling
3. Decorator
4. Refinement

4. Ruby
is
Smalltalk-style!
Objective-C, too.

5. 丟訊息
給物件
看看物件有什麼反應

6. obj.hello(“world”)

7. obj.hello(“world”)
這是 message

8. obj.hello(“world”)
這是 receiver

9. obj.hello(“world”)

10. obj.send(:hello,
“world”)
Yes, it sends a message!

11. obj 接收了message
內容為 hello(“world”)

12. 現在 obj 就要

13. 暴走
同步率400%

14. obj 找一個對應的
method body
根據 message name :hello

15. 然後...
引爆 invoke
呼叫這個 method body

16. DEFINE
A METHOD
事實上是定義一個 method body
然後連到對應的 message name

17. def hello(s)
define a method named ‘hello’
define a method for the name ‘hello’
a method body
:hello

18. method body
&
message name
沒有綁死在一起
而是可以改變的

19. HACK IT!
Let’s change behaviour

20. Method
Swizzling
把 method body “抽換”掉

21. Example.rb
class MyObj
def hello(s)
“Hello #{s}”
end
end
method body
A
:hello

22. Hack1.rb (part 1)
class MyObj
alias_method :hello_original,
:hello
end
:hello_original
method body
A
:hello

23. Hack1.rb (part 2)
class MyObj
def hello_with_bracket(s)
“{” + hello_original(s) + ”}”
end
end
method body
B
:hello_with_bracket
send message
:hello_original

24. Hack1.rb (part 3)
class MyObj
alias_method :hello,
:hello_with_bracket
end
method body
B
method body
A
:hello_with_bracket
:hello

25. Hack1.rb (final)
:hello
method body
B
:hello_with_bracket
send message
method body
A
:hello_original

26. 可不可以
更簡潔些？
太多沒必要的 message names

27. Hack2.rb (to expect)
:hello
method body
B
call directly
method body
A
:hello_with_bracket
send message
:hello_original

28. Hack2.rb (to expect)
:hello
method body
B
call directly
method body
A

29. Hack2.rb (yes, that’s all)
class MyObj
m = instance_method(:hello)
define_method(:hello) do |s|
“{” + m.bind(self).call(s) + “}”
end
end

30. Hack2.rb
class MyObj
m = instance_method(:hello)
define_method(:hello) do |s|
“{” + m.bind(self).call(s) + “}”
end
end
m (a local var)
method body
A
:hello

31. Hack2.rb
class MyObj
m = instance_method(:hello)
define_method(:hello) do |s|
“{” + m.bind(self).call(s) + “}”
end
end
method body
B
method body
A
:hello

32. Hack2.rb (Ah, ha!)
class MyObj
m = instance_method(:hello)
define_method(:hello) do |s|
“{” + m.bind(self).call(s) + “}”
end
end
method body
B
call directly
method body
A
:hello

33. Decorator
Method Wrapping
(it’s more complicant than Python, though)

34. Example (Unsafe!)
def send_money(from, to, amount)
from.balance -= amount
to.balance += amount
from.save!
to.save!
end

35. Example (Safer)
def send_money(from, to, amount)
ActiveRecord::Base.transcation do
from.balance -= amount
to.balance += amount
from.save!
to.save!
end
end

36. Example in Ruby
def send_money(from, to, amount)
ActiveRecord::Base.transcation do
# do operations
end
end

37. Example in Python
def send_money(from, to, amount):
try:
db.start_transcation()
# do operations
db.commit_transcation()
except:
db.rollback_transcation()
raise

38. Decorating in Python
Form 1:
@transcational
def send_money(from, to, amount):
# do operations
Form 2:
def send_money(from, to, amount):
# do operations
send_money = transcational(send_money)

39. Decorator in Python
def transcational(func):
def func2(*args):
try:
db.start_transcation()
func(*args) # call decoratee
db.commit_transcation()
except:
db.rollback_transcation()
raise
return func2

40. Decorating in Ruby
class Bank
extend Transcational # include decorator
def send_money(from, to, amount)
# do operations
end
transcational :send_money # decorate!
end

41. Decorator in Ruby
module Transcational
def transcational(mthd_name)
mthd = instance_method(mthd_name)
define_method(mthd_name) do |*args, &blk|
ActiveRecord::Base.transcation {
# call decoratee
mthd.bind(self).call(*args, &blk)
}
end
end
end

42. 這只是
Method
Swizzling
since Ruby has methods but functions

43. Refinement
For Ruby 2.0+

44. 當你要...
Monkey Patching
Modify existing classes of other libs

45. Instead of monkey patching
class KlassOrModule
# define instance methods...
end
module MyLibrary
# using modified KlassOrModule...
end

46. 住手！
You’re gonna RUIN EVERYTHING!

47. Use refinement
module MyLibrary
refine KlassOrModule do
# define instance methods...
end
# using modified KlassOrModule...
end

48. Example
module MyLibrary
refine String do # modify String
def hello
“Hello #{self}”
end
end
end

49. Example (cont.)
# Outside MyLibrary
puts “Kitty”.hello
# => NoMethodError: hello

50. Example (cont.)
# Outside MyLibrary
using MyLibrary
puts “Kitty”.hello
# => Hello Kitty

51. Inside Refinement
Actually, refinement creates an annoymous
module to mixin a class.
module MyLib
refine String do
puts self.is_a?(Module) # => true
puts self
# => #<refinement:String@MyLibrary>
end
end

52. Inside Refinement
module MyLib
refine String do
module
<refinement:String@MyLibrary>
end
end

53. It is ...
Automatic Mixining
自動化MIXIN

54. When using MyLibrary ...
every new instance of String is extended!
using MyLibrary
puts “x”.hello # “x” is extended
puts String.new(“x”).hello

55. using is lexical in scope
just like Java’s import

56. using is lexical
1. Refinements are activated only at top-level
- Not inside class, module, and method scope
2. Scope of the effect is the whole source code

57. using is lexical
Please DO NOT…
class Foo
using MyLib
end
module Moo
using MyLib
end
def bar
using MyLib
end

58. using is lexical
# refinement of MyLib is deactived
# “World”.hello # => NoMethodError
using MyLib # activate!
# refinement of MyLib is activated
def hello_world
“World”.hello
end
# END OF FILE
Scope of
MyLib
Refinement

59. Refinement in Ruby
1. Actually, it is based on Mixining
2. Avoid global scope corruption
3. Prefer lexical scope to runtime scope
=> easy to use, just like import in Java

60. Better Refinement
than
Monkey Patching
please, get rid of monkey patching.

61. 魔法のRuby
まほうのRuby

62. 本簡報
用高橋流製作
在此向たかはしさん致敬

63. <FIN>