Design Patterns in Ruby [Digest 2] - Template

最新推荐文章于 2023-10-17 19:39:34 发布

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本文介绍了一种软件设计模式——模板方法模式，并通过一个报告输出的例子详细解释了如何将复杂算法的不同部分抽象出来，使得子类可以复用相同的流程，同时允许在特定步骤中进行定制化实现。

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.”