使用装饰器在Python3中实现观察者模式

要注册回调，您需要有一个实际的对象.在您的代码中，应该如何@Observable.register_observer查找应该在哪个实例上注册?

To register the callback, you need to have an actual object. In your code, how is @Observable.register_observer supposed to find which instance is should register on?

请删除Observable这是一种Java风格的东西，在python中很麻烦.

Please drop that Observable thing that's a javaism, cumbersome in python.

看看这个.

#!/usr/bin/env python

class SomeData(object):
    def __init__(self, value):
        self.callbacks = []
        self.foo = value

    def register(self, callback):
        self.callbacks.append(callback)
        return callback

    def notify(self, *args, **kwargs):
        for callback in self.callbacks:
            callback(self, *args, **kwargs)

class SomeGUI(object):
    def redraw(self, obj, key, newvalue):
        print('redrawing %s with value %s' % (self, newvalue))


if __name__ == '__main__':
    my_data = SomeData(42)

    # Register some function using decorator syntax
    @my_data.register
    def print_it(obj, key, value):
        print('Key %s changed to %s' % (key, value))

    # Register the SomeGUI element
    my_gui = SomeGUI()
    my_data.register(my_gui.redraw)

    # Try changing it. Note my_data is dumb for now, notify manually.
    my_data.foo = 10
    my_data.notify("foo", 10)

我有意删除了自动通知，以说明注册本身.

I intentionally removed automatic notifications to illustrate registration by itself.

让我们重新添加.但是使用该Observable类没有意义.只需定义一个事件类，我们就可以减轻它的负担.

Let's add it back. But there is no point using that Observable class. Let's make it lighter, simply defining an event class.

#!/usr/bin/env python3


class Event(object):
    def __init__(self):
        self.callbacks = []

    def notify(self, *args, **kwargs):
        for callback in self.callbacks:
            callback(*args, **kwargs)

    def register(self, callback):
        self.callbacks.append(callback)
        return callback

class SomeData(object):
    def __init__(self, foo):
        self.changed = Event()
        self._foo = foo

    @property
    def foo(self):
        return self._foo

    @foo.setter
    def foo(self, value):
        self._foo = value
        self.changed.notify(self, 'foo', value)

class SomeGUI(object):
    def redraw(self, obj, key, newvalue):
        print('redrawing %s with value %s' % (self, newvalue))


if __name__ == '__main__':
    my_data = SomeData(42)

    # Register some function using decorator syntax
    @my_data.changed.register
    def print_it(obj, key, value):
        print('Key %s changed to %s' % (key, value))

    # Register the SomeGUI element
    my_gui = SomeGUI()
    my_data.changed.register(my_gui.redraw)

    # Try changing it.
    my_data.foo = 10

您现在可能已经注意到，装饰器语法在以下情况下很有用:

As you probably noted now, the decorator syntax is useful in those circumstances:

• 您有一个注册表.单例或类本身的类都是一阶对象，大多数是单例.
• 您可以动态定义函数并随时注册.

现在，如果您有许多为什么不将它们排除在外，那么您拥有的那些手动获取器/设置器也很麻烦?

Now, those manual getters/setters you have are cumbersome as well, if you have many why not factor them out?

#!/usr/bin/env python3


class Event(object):
    def __init__(self):
        self.callbacks = []

    def notify(self, *args, **kwargs):
        for callback in self.callbacks:
            callback(*args, **kwargs)

    def register(self, callback):
        self.callbacks.append(callback)
        return callback

    @classmethod
    def watched_property(cls, event_name, key):
        actual_key = '_%s' % key

        def getter(obj):
            return getattr(obj, actual_key)

        def setter(obj, value):
            event = getattr(obj, event_name)
            setattr(obj, actual_key, value)
            event.notify(obj, key, value)

        return property(fget=getter, fset=setter)


class SomeData(object):
    foo = Event.watched_property('changed', 'foo')

    def __init__(self, foo):
        self.changed = Event()
        self.foo = foo



class SomeGUI(object):
    def redraw(self, obj, key, newvalue):
        print('redrawing %s with value %s' % (self, newvalue))


if __name__ == '__main__':
    my_data = SomeData(42)

    # Register some function using decorator syntax
    @my_data.changed.register
    def print_it(obj, key, value):
        print('Key %s changed to %s' % (key, value))

    # Register the SomeGUI element
    my_gui = SomeGUI()
    my_data.changed.register(my_gui.redraw)

    # Try changing it.
    my_data.foo = 10

作为参考，这三个程序都输出完全相同的东西:

For reference, all three programs output the exact same thing:

$ python3 test.py
Key foo changed to 10
redrawing <__main__.SomeGUI object at 0x7f9a90d55fd0> with value 10