Python实现IOC控制反转

思路:

用一个字典存储beanName和资源
初始化时先将beanName和资源注册到字典中
然后用一个Dscriptor类根据beanName动态请求资源,从而实现控制反转

# -*- coding:utf-8 -*-
import os
class BeanFactory: 
    """
    Python版控制反转
    context: 存储bean的名字和对应的类或者值的字典
    allowRepalce: 是否允许替换已经注入的bean
    """

    def __init__(self,allowReplace=False):
        """构造函数
        allowReplace:是否允许替换已经注入的bean
        """
        self.context = {}
        self.allowReplace = allowReplace
    def setBean(self,beanName,resource,*args,**kwargs):
        if not self.allowReplace:
            assert not beanName in self.context,"该BeanFactory不允许重复注入%r,请修改beanName" % beanName
        def call():
            """定义一个函数闭包,如果注入的resource是可调用类型,就将*args和**kwargs传入并调用该函数,然后将返回值返回
            如果是一个不可调用对象,就直接返回
            """
            if callable(resource):
                return resource(*args,**kwargs)
            else:
                return resource
        #将call闭包与beanName建立映射
        self.context[beanName]=call
    def __getitem__(self,beanName):
        """重载__getitem__方法,使得BeanFactory支持使用[]获取beanName对应的注册的资源
        """
        try:
            # 从context字典中取出beanName对应的资源
            resource = self.context[beanName]
        except KeyError:
            raise KeyError("%r 未注册" % beanName)
        # 返回闭包函数调用后的结果
        return resource()

AppFactory = BeanFactory()

def NoAssertion(obj): return True

def IsInstanceOf(*classes):
   def test(obj): return isinstance(obj, classes)
   return test

def HasAttributes(*attributes):
   def test(obj):
      for each in attributes:
         if not hasattr(obj, each): return False
      return True
   return test

def HasMethods(*methods):
   def test(obj):
      for each in methods:
         try:
            attr = getattr(obj, each)
         except AttributeError:
            return False
         if not callable(attr): return False
      return True
   return test


#
#
#Descriptor就是一类实现了__get__(), __set__(), __delete__()方法的对象
#若一个类的成员是descriptor,在访问它的值时是通过__get__()函数访问的
#用这个特性实现在访问一个类的成员时自动到BeanFactory中请求对应的资源

class RequiredResource(object):
   def __init__(self, beanName, assertion=NoAssertion):
      self.beanName =  beanName
      self.assertion = assertion
   def __get__(self, obj, T):#每次访问descriptor时都会调用__get__方法
      return self.result # <-- .操作符会自动调用__getattr__
   def __getattr__(self, name):
      assert name == 'result', "Unexpected attribute request other then 'result'"
      self.result = self.Request()
      return self.result
   def Request(self):
      obj = AppFactory[self.beanName]
      assert self.assertion(obj), 
             "The value %r of %r does not match the specified criteria" 
             % (obj, self.feature)
      return obj




class Component(object):
   "Symbolic base class for components"
class Bar(Component):
   # HasMethods是一个闭包函数,传入RequiredResource后用于检查'Console'
   # 对应的注册的那个feature是否有'WriteLine'方法
   # IsinstanceOf(str) 是一个闭包,传入RequiredResource后会被调用,用于
   # 检查注册的'AppTitle'对应资源是否是一个字符串
   # IsinstanceOf(str) 是一个闭包,传入RequiredResource后会被调用,用于
   # 检查'CurrentUser'对应的资源是否是一个字符串

   # RequiredFeatuire是desciptor,每次访问descriptor（即实现了__get__的类），都会先经过__get__函数。

   con   = RequiredResource('Console', HasMethods('WriteLine'))
   title = RequiredResource('AppTitle', IsInstanceOf(str))
   user  = RequiredResource('CurrentUser', IsInstanceOf(str))
   flist = RequiredResource('show_dir',IsInstanceOf(list))
   def __init__(self):
      self.X = 0
   def PrintYourself(self):
      self.con.WriteLine('-- Bar instance --')
      # title 由 RequiredResource('AppTitle',IsInstanceOf(str))生成
      #'AppTitle'对应的值在__main__代码块注册了一个值
      self.con.WriteLine('Title: %s' % self.title)
      self.con.WriteLine('User: %s' % self.user)
      self.con.WriteLine('X: %d' % self.X)
      for f in self.flist:
           self.con.WriteLine(f)

class BetterConsole(Component):
   def __init__(self, prefix=''):
      self.prefix = prefix
   def WriteLine(self, s):
      lines = s.split('
')
      for line in lines:
         if line:
            print(self.prefix, line)
         else:
            print


def GetCurrentUser():
   return os.getenv('USERNAME') or 'Some User' # USERNAME is platform-specific
def ShowDir():
    return os.listdir()

if __name__ == '__main__':
   print('
*** IoC Demo ***')
   #Provide(feature,provider,*args,**kwargs) feature是要生成的对象的父类类型 provider是要注入的子类或者值
   AppFactory.setBean('AppTitle', 'Inversion of Control ...

... The Python Way')
   AppFactory.setBean('CurrentUser', GetCurrentUser)
   AppFactory.setBean('Console', BetterConsole, prefix='-->') # <-- transient lifestyle
   AppFactory.setBean('show_dir',ShowDir)

   bar = Bar()
   bar.PrintYourself()