代理设计模式包括一个新对象，称为"代理"代替现有对象，称为"真实主题".创建真实主题的代理对象必须位于同一个界面上，使客户端不应该知道代理用于代替真实对象.客户端向代理生成的请求将通过真实主题传递.
代理模式的UML表示如下 :
现在让我们看看如何实现代理模式.
class Image:
def __init__( self, filename ):
self._filename = filename
def load_image_from_disk( self ):
print("loading " + self._filename )
def display_image( self ):
print("display " + self._filename)
class Proxy:
def __init__( self, subject ):
self._subject = subject
self._proxystate = None
class ProxyImage( Proxy ):
def display_image( self ):
if self._proxystate == None:
self._subject.load_image_from_disk()
self._proxystate = 1
print("display " + self._subject._filename )
proxy_image1 = ProxyImage ( Image("HiRes_10Mb_Photo1") )
proxy_image2 = ProxyImage ( Image("HiRes_10Mb_Photo2") )
proxy_image1.display_image() # loading necessary
proxy_image1.display_image() # loading unnecessary
proxy_image2.display_image() # loading necessary
proxy_image2.display_image() # loading unnecessary
proxy_image1.display_image() # loading unnecessary上面的程序生成以下输出 :
代理模式设计有助于复制我们创建的图像. display_image()函数有助于检查是否在命令提示符下打印了值.
