Design Patterns in Ruby [Digest 2] - Template
When we have a complex algorithm including several steps to build something, which would be vary in the middle.
Now you need Template method.
It comes up with an example:
class Report
def initialize
@title = 'Monthly Report'
@text = [ 'Things are going', 'really, really well.' ]
end
def output_report
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
The usage of the class is as below:
report = Report.new report.output_report
When they need something more? It will support plain text or RTF or PostScript.
The code will mess up with many if ... elsif ... else conditions
So, let's do the seperate thing:
Separate the Things That Stay the Same
class Report
def initialize
@title = 'Monthly Report'
@text = ['Things are going', 'really, really well.']
end
def output_report
output_start
output_head
output_body_start
output_body
output_body_end
output_end
end
def output_body
@text.each do |line|
output_line(line)
end
end
def output_start
raise 'Called abstract method: output_start'
end
def output_head
raise 'Called abstract method: output_head'
end
def output_body_start
raise 'Called abstract method: output_body_start'
end
def output_line(line)
raise 'Called abstract method: output_line'
end
def output_body_end
raise 'Called abstract method: output_body_end'
end
def output_end
raise 'Called abstract method: output_end'
end
end
class HTMLReport < Report
def output_start
puts('<html>')
end
def output_head
puts(' <head>')
puts(" <title>#{@title}</title>")
puts(' </head>')
end
def output_body_start
puts('<body>')
end
def output_line(line)
puts(" <p>#{line}</p>")
end
def output_body_end
puts('</body>')
end
def output_end
puts('</html>')
end
end
class PlainTextReport < Report
def output_start
end
def output_head
puts("**** #{@title} ****")
puts
end
def output_body_start
end
def output_line(line)
puts(line)
end
def output_body_end
end
def output_end
end
end
Then the usage is really straight:
report = HTMLReport.new report.output_report report = PlainTextReport.new report.output_report
Ruby doesn't support abstract methods and abstract classes, then we can use the methods which raise exception meet our demand.
"In the Template Method pattern, the abstract base class controls the higher-level processing through the template method; the subclasses simply fill in the details."
Then it goes to hook methods. but really, I don't agree with the writer's opinion about the example
Report with the default method implementations. Maybe the default implementations will brought in some wrong format if the coder obmit to implement some methods, such as missing the output_start or output_end or something context related.
So maybe we should implement the must be right thing for the common abstract class.
Lazziness must be secondary in front of correctness
Then we go to duck typing issues:
If it looks like a duck and quacks like a duck, then it is a duck~
The statically typed languages are working like aristocracies, they always ask about your genealogy.
The dynamically typed language are working like meritocracies, they only concern about what you have rather than where do you get the methods from.
The writer wrote it at the end of the part~
Dynamically typed languages rarely ask about an object’s ancestry; instead, they simply say, “I don’t care who you are related to, Mac. All I want to know is what you can do.”