Question

我试图了解对责任链模式的理解，作为练习，我编写了一些代码。这段代码

class SomeObject:
    def __init__(self):
        self.integer_field = 0
        self.float_field = 0.0
        self.string_field = 'hi'


class EventGet:
    def __init__(self, data_type):
        self.data_type = data_type

class EventSet:
    def __init__(self, value):
        self.value = value


class NullHandler:
    def __init__(self, successor=None):
        self.__successor = successor

    def handle(self, obj, event):
        if self.__successor:
            self.__successor.handle(obj, event)


class IntHandler(NullHandler):
    def handle(self, obj, event):
        if isinstance(event, EventGet) and event.data_type is int:
            return obj.integer_field
        elif isinstance(event, EventSet) and isinstance(event.value, int):
            obj.integer_field = event.value
        else:
            super().handle(obj, event)


class FloatHandler(NullHandler):
    def handle(self, obj, event):
        if isinstance(event, EventGet) and event.data_type is float:
            return obj.float_field
        elif isinstance(event, EventSet) and isinstance(event.value, float):
            obj.float_field = event.value
        else:
            super().handle(obj, event)


class StrHandler(NullHandler):
    def handle(self, obj, event):
        if isinstance(event, EventGet) and event.data_type is str:
            return obj.string_field
        elif isinstance(event, EventSet) and isinstance(event.value, str):
            obj.string_field = event.value
        else:
            super().handle(obj, event)

但是它不符合我的预期。通过以下方式被调用

obj = SomeObject()
chain = IntHandler(FloatHandler(StrHandler(NullHandler())))

print(chain.handle(obj, EventGet(int)))
print(chain.handle(obj, EventGet(float)))
print(chain.handle(obj, EventGet(str)))

chain.handle(obj, EventSet(1))
print(obj.integer_field)    

chain.handle(obj, EventSet(1.1))
print(obj.float_field)

chain.handle(obj, EventSet('str'))
print(obj.string_field)

它会产生令人困惑的输出：

0 # That's ok
None # Why not 0.0?
None # Why not hi?
1 # That's ok
1.1 # That's ok
str # That's ok

我不知道为什么调用chain.handle(obj, EventGet(float))和chain.handle(obj, EventGet(str))会产生None。毕竟，在相应位置（即return和return obj.float_field）有一个return obj.string_field语句。

能否请您向我解释此代码中的错误以及如何使其按预期工作？提前非常感谢！

责任链模式的菜鸟尝试

0 个答案: