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文章目录

23种设计模式及Python实现 -- 行为型(二)

PySuper
2019-10-22 / 0 评论 / 0 点赞 / 15 阅读 / 11817 字
温馨提示:
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中介者

用一个中介对象来封装一系列的对象交互。中介者使各对象不需要显式地相互引用,从而使其耦合松散,而且可以独立地改变它们之间的交互

  • 一组对象以定义良好但是复杂的方式进行通信,产生的相互依赖关系结构混乱且难以理解
  • 一个对象引用其他很多对象并且直接与这些对象通信,导致难以复用该对象
import time

class TC:
    def __init__(self):
        self._tm = tm
        self._bProblem = 0

    def setup(self):
        print("Setting up the Test")
        time.sleep(1)
        self._tm.prepareReporting()

    def execute(self):
        if not self._bProblem:
            print("Executing the test")
            time.sleep(1)
        else:
            print("Problem in setup. Test not executed.")

    def tearDown(self):
        if not self._bProblem:
            print("Tearing down")
            time.sleep(1)
            self._tm.publishReport()
        else:
            print("Test not executed. No tear down required.")

    def setTM(self, TM):
        self._tm = tm

    def setProblem(self, value):
        self._bProblem = value

class Reporter:
    def __init__(self):
        self._tm = None

    def prepare(self):
        print("Reporter Class is preparing to report the results")
        time.sleep(1)
 
    def report(self):
        print("Reporting the results of Test")
        time.sleep(1)
 
    def setTM(self, TM):
        self._tm = tm
 
class DB:
    def __init__(self):
        self._tm = None
 
    def insert(self):
        print("Inserting the execution begin status in the Database")
        time.sleep(1)
        import random
        if random.randrange(1, 4) == 3:
            return -1
 
    def update(self):
        print("Updating the test results in Database")
        time.sleep(1)
 
    def setTM(self, TM):
        self._tm = tm
 
class TestManager:
    def __init__(self):
        self._reporter = None
        self._db = None
        self._tc = None

    def prepareReporting(self):
        rvalue = self._db.insert()
        if rvalue == -1:
            self._tc.setProblem(1)
            self._reporter.prepare()

    def setReporter(self, reporter):
        self._reporter = reporter

    def setDB(self, db):
        self._db = db

    def publishReport(self):
        self._db.update()
        rvalue = self._reporter.report()

    def setTC(self, tc):
        self._tc = tc


if __name__ == '__main__':
    reporter = Reporter()
    db = DB()
    tm = TestManager()
    tm.setReporter(reporter)
    tm.setDB(db)
    reporter.setTM(tm)
    db.setTM(tm)
    while (True):
        tc = TC()
        tc.setTM(tm)
        tm.setTC(tc)
        tc.setup()
        tc.execute()
        tc.tearDown()

备忘录

在不破坏封装性的前提下,捕获一个对象的内部状态,并在该对象之外保存这个状态。这样以后就可将该对象恢复到原先保存的状态

  • 必须保存一个对象在某一个时刻的(部分)状态, 这样以后需要时它才能恢复到先前的状态
  • 如果一个用接口来让其它对象直接得到这些状态,将会暴露对象的实现细节并破坏对象的封装性
import copy
 
def Memento(obj, deep=False):
    state = (copy.copy, copy.deepcopy)[bool(deep)](obj.__dict__)
 
    def Restore():
        obj.__dict__.clear()
        obj.__dict__.update(state)
    return Restore
 
class Transaction:
    deep = False
 
    def __init__(self, *targets):
        self.targets = targets
        self.Commit()
 
    def Commit(self):
        self.states = [Memento(target, self.deep) for target in self.targets]
 
    def Rollback(self):
        for st in self.states:
            st()
 
class transactional(object):
    def __init__(self, method):
        self.method = method
 
    def __get__(self, obj, T):
        def transaction(*args, **kwargs):
            state = Memento(obj)
            try:
                return self.method(obj, *args, **kwargs)
            except:
                state()
                raise
        return transaction
 
class NumObj(object):
    def __init__(self, value):
        self.value = value
 
    def __repr__(self):
        return '<%s: %r>' % (self.__class__.__name__, self.value)
 
    def Increment(self):
        self.value += 1
 
    @transactional
    def DoStuff(self):
        self.value = '1111'  # <- invalid value
        self.Increment()     # <- will fail and rollback

if __name__ == '__main__':
    n = NumObj(-1)
    print(n)
    t = Transaction(n)
    try:
        for i in range(3):
            n.Increment()
            print(n)
        t.Commit()
        print('-- commited')
        for i in range(3):
            n.Increment()
            print(n)
        n.value += 'x'  # will fail
        print(n)
    except:
        t.Rollback()
        print('-- rolled back')
    print(n)
    print('-- now doing stuff ...')
    try:
        n.DoStuff()
    except:
        print('-> doing stuff failed!')
        import traceback
        traceback.print_exc(0)
        pass
    print(n)

观察者

定义对象间的一种一对多的依赖关系,当一个对象的状态发生改变时, 所有依赖于它的对象都得到通知并被自动更新

  • 当一个抽象模型有两个方面, 其中一个方面依赖于另一方面。将这二者封装在独立的对象中以使它们可以各自独立地改变和复用
  • 当对一个对象的改变需要同时改变其它对象, 而不知道具体有多少对象有待改变
  • 当一个对象必须通知其它对象,而它又不能假定其它对象是谁。换言之, 你不希望这些对象是紧密耦合的
class Subject(object):
    def __init__(self):
        self._observers = []
 
    def attach(self, observer):
        if not observer in self._observers:
            self._observers.append(observer)
 
    def detach(self, observer):
        try:
            self._observers.remove(observer)
        except ValueError:
            pass
 
    def notify(self, modifier=None):
        for observer in self._observers:
            if modifier != observer:
                observer.update(self)

class Data(Subject):
    def __init__(self, name=''):
        Subject.__init__(self)
        self.name = name
        self._data = 0
 
    @property
    def data(self):
        return self._data
 
    @data.setter
    def data(self, value):
        self._data = value
        self.notify()
 
class HexViewer:
    def update(self, subject):
        print('HexViewer: Subject %s has data 0x%x' %
              (subject.name, subject.data))
 
class DecimalViewer:
    def update(self, subject):
        print('DecimalViewer: Subject %s has data %d' %
              (subject.name, subject.data))

def main():
    data1 = Data('Data 1')
    data2 = Data('Data 2')
    view1 = DecimalViewer()
    view2 = HexViewer()
    data1.attach(view1)
    data1.attach(view2)
    data2.attach(view2)
    data2.attach(view1)
 
    print("Setting Data 1 = 10")
    data1.data = 10
    print("Setting Data 2 = 15")
    data2.data = 15
    print("Setting Data 1 = 3")
    data1.data = 3
    print("Setting Data 2 = 5")
    data2.data = 5
    print("Detach HexViewer from data1 and data2.")
    data1.detach(view2)
    data2.detach(view2)
    print("Setting Data 1 = 10")
    data1.data = 10
    print("Setting Data 2 = 15")
    data2.data = 15
 
if __name__ == '__main__':
    main()

状态

允许一个对象在其内部状态改变时改变它的行为。对象看起来似乎修改了它的类

  • 一个对象的行为取决于它的状态, 并且它必须在运行时刻根据状态改变它的行为
  • 一个操作中含有庞大的多分支的条件语句,且这些分支依赖于该对象的状态
class State(object):
    def scan(self):
        self.pos += 1
        if self.pos == len(self.stations):
            self.pos = 0
        print("Scanning... Station is", self.stations[self.pos], self.name)
 
 
class AmState(State):
    def __init__(self, radio):
        self.radio = radio
        self.stations = ["1250", "1380", "1510"]
        self.pos = 0
        self.name = "AM"
 
    def toggle_amfm(self):
        print("Switching to FM")
        self.radio.state = self.radio.fmstate
 
class FmState(State):
    def __init__(self, radio):
        self.radio = radio
        self.stations = ["81.3", "89.1", "103.9"]
        self.pos = 0
        self.name = "FM"
 
    def toggle_amfm(self):
        print("Switching to AM")
        self.radio.state = self.radio.amstate
 
class Radio(object):
    def __init__(self):
        self.amstate = AmState(self)
        self.fmstate = FmState(self)
        self.state = self.amstate
 
    def toggle_amfm(self):
        self.state.toggle_amfm()
 
    def scan(self):
        self.state.scan()

if __name__ == '__main__':
    radio = Radio()
    actions = [radio.scan] * 2 + [radio.toggle_amfm] + [radio.scan] * 2
    actions = actions * 2
 
    for action in actions:
        action()

策略

定义一系列的算法,把它们一个个封装起来, 并且使它们可相互替换。本模式使得算法可独立于使用它的客户而变化

  • 许多相关的类仅仅是行为有异。“策略”提供了一种用多个行为中的一个行为来配置一个类的方法
  • 需要使用一个算法的不同变体。例如,你可能会定义一些反映不同的空间/时间权衡的算法。当这些变体实现为一个算法的类层次时[H087] ,可以使用策略模式
  • 算法使用客户不应该知道的数据。可使用策略模式以避免暴露复杂的、与算法相关的数据结构
  • 一个类定义了多种行为, 并且这些行为在这个类的操作中以多个条件语句的形式出现
import types
 
 
class StrategyExample:
    def __init__(self, func=None):
        self.name = 'Strategy Example 0'        
        if func is not None:
            self.execute = types.MethodType(func, self)     
 
    def execute(self):        
        print(self.name)  
 
def execute_replacement1(self):
    print(self.name + ' from execute 1')  
 
def execute_replacement2(self):
    print(self.name + ' from execute 2') 
 
if __name__ == '__main__':
    strat0 = StrategyExample()    
 
    strat1 = StrategyExample(execute_replacement1)
    strat1.name = 'Strategy Example 1'    
 
    strat2 = StrategyExample(execute_replacement2)
    strat2.name = 'Strategy Example 2'
 
    strat0.execute()
    strat1.execute()    
    strat2.execute()

访问者

定义一个操作中的算法的骨架,而将一些步骤延迟到子类中。TemplateMethod 使得子类可以不改变一个算法的结构即可重定义该算法的某些特定步骤

  • 一次性实现一个算法的不变的部分,并将可变的行为留给子类来实现
  • 各子类中公共的行为应被提取出来并集中到一个公共父类中以避免代码重复
  • 控制子类扩展
class Node(object):
    pass
 
class A(Node):
    pass
 
class B(Node):
    pass
 
class C(A, B):
    pass
 
class Visitor(object):
    def visit(self, node, *args, **kwargs):
        meth = None
        for cls in node.__class__.__mro__:
            meth_name = 'visit_'+cls.__name__
            meth = getattr(self, meth_name, None)
            if meth:
                break
 
        if not meth:
            meth = self.generic_visit
        return meth(node, *args, **kwargs)
 
    def generic_visit(self, node, *args, **kwargs):
        print('generic_visit '+node.__class__.__name__)
 
    def visit_B(self, node, *args, **kwargs):
        print('visit_B '+node.__class__.__name__)
 
a = A()
b = B()
c = C()
visitor = Visitor()
visitor.visit(a)
visitor.visit(b)
visitor.visit(c)
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