CSC/ECE 517 Spring 2014/ch1 1w1l m: Difference between revisions

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<code>
<code>
 
  class Report
  class Report
   attr_reader :title, :text
   attr_reader :title, :text
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   end
   end
  end
  end
 
  HTML_FORMATTER = lambda do |context|
  HTML_FORMATTER = lambda do |context|
   puts('<html>')
   puts('<html>')

Revision as of 02:10, 24 February 2014

Design Patterns Involving Closures

Background

Explanation of Closures

Very simply, a closure is a function that can use a variable that was valid within the scope that the closure was defined, but need not be in-scope where the closure is called. A quick example is very illustrative. 

def closure_builder(message="Default"):
    def closure():
        # Message is in-scope here
        print message
    return closure

# Build two functions
default_closure = closure_builder()
custom_closure = closure_builder("Custom")
del closure_builder

# Call the closures you built
default_closure()  # Amazingly, prints "Default"
custom_closure()  # Amazingly, prints "Custom"

Examples

Decorators

Decorators are an interesting and powerful language feature that can be implemented elegantly with closures. 

#!/usr/bin/env python

def decorate(func):
    def decorated_func():
        print "About to call func"
        func()
        print "Back from calling func"
    return decorated_func

@decorate
def func_to_decorate():
    print "In func_to_decorate"

func_to_decorate()


[~517/wiki]$ chmod ug+x decorator.py
[~517/wiki]$ ./decorator.py
About to call func
In func_to_decorate
Back from calling func


Command Pattern

The command Pattern can be implemented using proc objects, which are the closure of an object

In the command pattern, commands for objects to be implemented are queued and can be executed at any time. Arguments are considered passed when the method is called. 

count = 0

commands = []
(1..10).each do |i|
  commands << proc { count +=i }
end

puts "Count is initially #{count}"
commands.each { |cmd| cmd.call }
puts "Performed all commands. count is #{count}"

Strategy Pattern

class Report
 attr_reader :title, :text
 attr_accessor :formatter
 def initialize(&formatter)
  @title = 'Monthly Report'
  @text = [ 'Things are going', 'really, really well.' ]
  @formatter = formatter
 end
 def output_report
  @formatter.call( self )
 end
end

HTML_FORMATTER = lambda do |context|
 puts('<html>')
 puts(' <head>')
 puts(" <title>#{context.title}</title>")
 puts(' </head>')
 puts(' <body>')
 context.text.each do |line|

puts("

#{line}

" ) 

 end
 puts(' </body>')
 puts(' </html>')
end

See Also

References

<references />

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