We need to vary part of an algorithm— something we previously solved using the Template Method pattern— although we want to avoid its drawbacks, introduced by the fact that it's built around inheritance.
To avoid problems introduced by inheritance we should use delegation. Instead of creating subclasses (like in the Template Method pattern), we tear out the varying part of the code and isolate it in its own class and create one of them for each variation. The key idea of the Strategy pattern is to define a family of objects (strategies), which all do (almost) the same thing and support the same interface. Then, the user of the strategy (context) can treat the strategies as interchangeable parts.
Following the example of the Template Method pattern, we can refactor the code so that every format is a class (strategy), instead of a subclass. That way, the Report class would become much simpler, it would play the context object role and would be provided with the strategy.
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
endThe implementation of every format would have its own class, which allows us to achieve a better separation of concerns.
class HTMLFormatter
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
class PlainTextFormatter
def output_report(context)
puts("***** #{context.title} *****")
context.text.each do |line|
puts(line)
end
end
endTo use it, we just provide a formatted object (strategy) to the report (context)
report = Report.new(HTMLFormatter.new)
report.output_report
report.formatter = PlainTextFormatter.new
report.output_report