Python Design Patterns 简明教程

Chain of Responsibility

责任链模式用于在软件中实现松耦合，其中客户端的指定请求通过其中的对象链传递。它有助于构建对象链。请求从一端进入，从一个对象移动到另一个对象。

The chain of responsibility pattern is used to achieve loose coupling in software where a specified request from the client is passed through a chain of objects included in it. It helps in building a chain of objects. The request enters from one end and moves from one object to another.

此模式允许对象发送命令，而不了解哪个对象将处理请求。

This pattern allows an object to send a command without knowing which object will handle the request.

How to implement the chain of responsibility pattern?

现在我们将看到如何实现责任链模式。

We will now see how to implement the chain of responsibility pattern.

class ReportFormat(object):
   PDF = 0
   TEXT = 1
class Report(object):
   def __init__(self, format_):
      self.title = 'Monthly report'
      self.text = ['Things are going', 'really, really well.']
      self.format_ = format_

class Handler(object):
   def __init__(self):
      self.nextHandler = None

   def handle(self, request):
      self.nextHandler.handle(request)

class PDFHandler(Handler):

   def handle(self, request):
      if request.format_ == ReportFormat.PDF:
         self.output_report(request.title, request.text)
      else:
         super(PDFHandler, self).handle(request)

   def output_report(self, title, text):
      print '<html>'
      print ' <head>'
      print ' <title>%s</title>' % title
      print ' </head>'
      print ' <body>'
      for line in text:
         print ' <p>%s' % line
      print ' </body>'
      print '</html>'

class TextHandler(Handler):

   def handle(self, request):
      if request.format_ == ReportFormat.TEXT:
         self.output_report(request.title, request.text)
      else:
         super(TextHandler, self).handle(request)

   def output_report(self, title, text):
      print 5*'*' + title + 5*'*'
      for line in text:
         print line

class ErrorHandler(Handler):
   def handle(self, request):
      print "Invalid request"

if __name__ == '__main__':
   report = Report(ReportFormat.TEXT)
   pdf_handler = PDFHandler()
   text_handler = TextHandler()

   pdf_handler.nextHandler = text_handler
   text_handler.nextHandler = ErrorHandler()
	pdf_handler.handle(report)

Output

上述程序生成以下输出 −

The above program generates the following output −

Explanation

上面的代码为每月任务创建了一个报告，其中它通过每个函数发送命令。它需要两个处理程序 - 用于 PDF 和文本。一旦所需对象执行每个函数，它便会打印输。

The above code creates a report for monthly tasks where it sends commands through each function. It takes two handlers – for PDF and for text. It prints the output once the required object executes each function.