Design Patterns in Ruby [Digest 2]

jlaky · 发表于 2013-2-7 21:14:15

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>') endend

The usage of the class is as below:
report = Report.newreport.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'  endend

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>')  endend

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  endend
Then the usage is really straight:

report = HTMLReport.newreport.output_reportreport = PlainTextReport.newreport.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.”

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Design Patterns in Ruby [Digest 2]

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