The Decorator Pattern in Ruby

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“Each pattern describes a problem that occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice.”

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Context Forces Solution

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Context: An object’s behaviour needs to be extended. The new behaviour only applies to some instances. Forces Solution

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Context: An object’s behaviour needs to be extended. The new behaviour only applies to some instances. Forces: Addition of feature(s) should be invisible to client code. Base class should not know about additional behaviour. There are multiple independent extensions. Solution

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Solution: Wrap the object in a decorator. Implement or override feature-speciﬁc methods. Pass other messages to the wrapped object. Context: An object’s behaviour needs to be extended. The new behaviour only applies to some instances. Forces: Addition of feature(s) should be invisible to client code. Base class should not know about additional behaviour. There are multiple independent extensions.

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UML ahead

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class Coffee def ingredients [:beans, :water] end ! def price BigDecimal("2.00") end end

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class LargeCoffee < Coffee def price super * BigDecimal("1.2") end end

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class WhiteCoffee < Coffee def ingredients super + [:milk] end end

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class LargeWhiteCoffee < Coffee def price super * BigDecimal("1.2") end ! def ingredients super + [:milk] end end

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drink = LargeWhiteCoffee.new ! drink.ingredients # => [:beans, :water, :milk] ! "%.2f" % drink.price # => "2.40"

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class LargeDrink def initialize drink @drink = drink end ! def ingredients @drink.ingredients end ! def price @drink.price * BigDecimal("1.2") end end

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class MilkDrink def initialize drink @drink = drink end ! def ingredients @drink.ingredients + [:milk] end ! def price @drink.price end end

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drink = MilkDrink.new( LargeDrink.new(Coffee.new) ) ! drink.ingredients # => [:beans, :water, :milk] ! "%.2f" % drink.price # => "2.40"

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drink = Coffee.new |> LargeDrink.new |> MilkDrink.new Meanwhile, in a parallel universe…

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class MilkDrink def initialize drink @drink = drink end ! def ingredients @drink.ingredients + [:milk] end ! def price @drink.price end end

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class MilkDrink def initialize drink @drink = drink end ! def ingredients @drink.ingredients + [:milk] end ! def price @drink.price end end

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class MilkDrink def initialize drink @drink = drink end ! def ingredients @drink.ingredients + [:milk] end ! def method_missing *args, &block @drink.send(*args, &block) end end

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drink = MilkDrink.new( LargeDrink.new(Coffee.new) ) ! drink.ingredients # => [:beans, :water, :milk] ! "%.2f" % drink.price # => "2.40"

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class Decorator def initialize obj @obj = obj end ! attr_reader :obj ! def method_missing *args, &block obj.send(*args, &block) end end

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class LargeDrink < Decorator def price obj.price * BigDecimal("1.2") end end ! class MilkDrink < Decorator def ingredients obj.ingredients + [:milk] end end

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drink = MilkDrink.new( LargeDrink.new(Coffee.new) ) ! drink.ingredients # => [:beans, :water, :milk] ! "%.2f" % drink.price # => "2.40"

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require "delegate" ! class LargeDrink < SimpleDelegator def price super * BigDecimal("1.2") end end ! class MilkDrink < SimpleDelegator def ingredients super + [:milk] end end

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drink = MilkDrink.new( LargeDrink.new(Coffee.new) ) ! drink.ingredients # => [:beans, :water, :milk] ! "%.2f" % drink.price # => "2.40"

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module LargeDrink def price super * BigDecimal("1.2") end end ! module MilkDrink def ingredients super + [:milk] end end

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drink = Coffee.new drink.extend LargeDrink drink.extend MilkDrink ! drink.ingredients # => [:beans, :water, :milk] ! "%.2f" % drink.price # => "2.40"

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In the wild

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FileInputStream f = new FileInputStream("foo.txt"); ! BufferedInputStream buf = new BufferedInputStream(f);

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Not to be confused with…

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An adapter changes  a class’s interface. A decorator changes  its behaviour.

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A composite wraps  multiple objects. A decorator  only wraps one.

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A proxy acts as a temporary stand-in. A decorator augments another object.

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A façade presents a  reduced interface. A decorator preserves the object’s interface.

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https://www.flickr.com/photos/tinou/453593446 https://www.flickr.com/photos/15016964@N02/5917784319 https://www.flickr.com/photos/snowpeak/8516781169 https://en.wikipedia.org/wiki/File:Christopher_Alexander.jpg https://www.flickr.com/photos/txmx-2/6145817075 https://www.flickr.com/photos/johannahobbs/473289801 https://www.flickr.com/photos/sabeth718/14816607620

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