Design Patterns in Ruby [Digest 3] Strategy-优快云博客

本文介绍了一种设计模式——策略模式，该模式使用组合而非继承来实现算法的动态切换。通过实例展示了如何利用策略模式简化报告生成过程，并讨论了上下文与策略间的数据共享问题。

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The Template method is build around inheritance, the inheritance has the nature born relationship.

So the Strategy give a delegate solution.

So we use the strategy implementation:

class Formatter
	def output_report( title, text )
	raise 'Abstract method called'
	end
end

class HTMLFormatter < Formatter
	def output_report( title, text )
		puts('<html>')
		puts(' <head>')
		puts(" <title>#{title}</title>")
		puts(' </head>')
		puts(' <body>')
		text.each do |line|
			puts(" <p>#{line}</p>" )
		end
		puts(' </body>')
		puts('</html>')
	end
end

class PlainTextFormatter < Formatter
	def output_report(title, text)
		puts("***** #{title} *****")
		text.each do |line|
		puts(line)
		end
	end
end

So the Report is much simpler:

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.output_report( @title, @text )
	end
end

Usage is as below:

report = Report.new(HTMLFormatter.new)
report.output_report

The key idea of Strategy is to define a family of strategy objects those are used by the context class,

its use composition and delegation rather than inheritance, it is easy to switch strategies at runtime

there is a problem about sharing data between context and strategies,

the writer gives a example as below:

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.output_report(self)
	end
end

class Formatter
	def output_report(context)
		raise 'Abstract method called'
	end
end

class HTMLFormatter < Formatter
	def output_report(context)
		puts('<html>')
		puts(' <head>')
		puts(" <title>#{context.title}</title>")
		puts(' </head>')
		puts(' <body>')
		context.text.each do |line|
			puts(" <p>#{line}</p>")
		end
		puts(' </body>')
		puts('</html>')
	end
end

This technique simplifies the data flow but increases the coupling.

Then Ruby Duck Typing philosophy shows that we don't need the Formatter class any more.