如何触发价值变化的功能? [英] How to trigger function on value change?

问题描述

我意识到这个问题与事件处理有关，并且我已经阅读了有关 Python 事件处理程序和调度程序的信息，所以要么它没有回答我的问题，要么我完全错过了信息.

I realise this question has to do with event-handling and i've read about Python event-handler a dispatchers, so either it did not answer my question or i completely missed out the information.

我希望在值 v 发生变化时触发对象 A 的方法 m():

I want method m() of object A to be triggered whenever value v is changing:

例如(假设金钱使人快乐):

For instance (assuming money makes happy):

global_wealth = 0

class Person()
    def __init__(self):
        self.wealth = 0
        global global_wealth
        # here is where attribute should be
        # bound to changes in 'global_wealth'
        self.happiness = bind_to(global_wealth, how_happy)

    def how_happy(self, global_wealth):
        return self.wealth / global_wealth

因此，每当 global_wealth 值更改时，Person 类的所有实例都应相应更改其 happiness 值.

So whenever the global_wealth value is changed, all instances of the class Person should change their happiness value accordingly.

NB:我不得不编辑这个问题，因为第一个版本似乎表明我需要 getter 和 setter 方法.很抱歉造成混乱.

NB: I had to edit the question since the first version seemed to suggest i needed getter and setter methods. Sorry for the confusion.

推荐答案

你需要使用 观察者模式.在以下代码中，一个人订阅接收来自全球财富实体的更新.当全球财富发生变化时，该实体会提醒其所有订阅者(观察者)发生变化.Person 然后更新自己.

You need to use the Observer Pattern. In the following code, a person subscribes to receive updates from the global wealth entity. When there is a change to global wealth, this entity then alerts all its subscribers (observers) that a change happened. Person then updates itself.

我在这个例子中使用了属性，但它们不是必需的.一个小警告:属性仅适用于新样式类，因此类声明后的 (object) 是强制性的.

I make use of properties in this example, but they are not necessary. A small warning: properties work only on new style classes, so the (object) after the class declarations are mandatory for this to work.

class GlobalWealth(object):
    def __init__(self):
        self._global_wealth = 10.0
        self._observers = []

    @property
    def global_wealth(self):
        return self._global_wealth

    @global_wealth.setter
    def global_wealth(self, value):
        self._global_wealth = value
        for callback in self._observers:
            print('announcing change')
            callback(self._global_wealth)

    def bind_to(self, callback):
        print('bound')
        self._observers.append(callback)


class Person(object):
    def __init__(self, data):
        self.wealth = 1.0
        self.data = data
        self.data.bind_to(self.update_how_happy)
        self.happiness = self.wealth / self.data.global_wealth

    def update_how_happy(self, global_wealth):
        self.happiness = self.wealth / global_wealth


if __name__ == '__main__':
    data = GlobalWealth()
    p = Person(data)
    print(p.happiness)
    data.global_wealth = 1.0
    print(p.happiness)

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