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-- flyaflya [<<DateTime(2005-08-04T09:15:17Z)>>]
<<TableOfContents>>
== 模式名称 ==
=== FlyweightPattern ===
运用共享技术有效地支持大量细粒度的对象。

=== 代码 ===
{{{
#!python
#实现过程类似于singleton模式

import weakref
#weekref产生的value不能保持对象存在。当对象不包括weakref在内的引用计数达到0时，对象将被删除。

class Instrument(object):
    _InstrumentPool = weakref.WeakValueDictionary()

    def __new__(cls, name):
        '''Instrument(name)
        Create a new instrument object, or return an existing one'''
        obj = Instrument._InstrumentPool.get(name, None)

        if not obj:
            print "new",name
            obj = object.__new__(cls)
            Instrument._InstrumentPool[name] = obj

        return obj

    def __init__(self, name):
        '''Complete object construction'''
        self.name = name
        print "New instrument @%04x, %s" % (id(self), name)

        # ... connect instrument to datasource ...
}}}
=== 　测试例子　 ===
{{{
#!python
import unittest

class InstrumentTests(unittest.TestCase):
    def testInstrument(self):
        ibm1 = Instrument("IBM")
        ms = Instrument("MS")
        ibm2 = Instrument("IBM")
        self.assertEquals(id(ibm1), id(ibm2))
        self.assertNotEquals(id(ibm1), id(ms))

        self.assertEquals(2, len(Instrument._InstrumentPool),
            "Total instruments allocated")

        # This bit assumes CPython memory allocation:
        del(ibm1)
        del(ibm2)
        # 每一次调用Instrument，因其函数__new__中的Instrument._InstrumentPool[name]是weakref，只有obj = object.__new__(cls)引用一次。所以del(ibm1)和del(imb2)后引用计数达到0，对象被清理。
        self.assertEquals(1, len(Instrument._InstrumentPool),
            "Total instruments allocated")


if __name__=='__main__':
    unittest.main()
}}}

=== 特殊说明 ===