CSC/ECE 517 Spring 2014/ch1 1w1l m
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 />