ObjectProperty 类的使用

问题描述

我刚开始学习 kivy，我对 ObjectProperty 类的用法以及它如何将 None 作为参数感到非常困惑.有人可以解释一下吗?我在 kivy 教程中找到的:

I Just started to learn kivy and I am very confused on the usage of the ObjectProperty class, and how it takes None as an argument. Could somebody explain it please? I found it in the kivy tutorial:

class PongGame(Widget):
    ball = ObjectProperty(None)

    def update(self, dt):
        self.ball.move()

        # bounce off top and bottom
        if (self.ball.y < 0) or (self.ball.top > self.height):
            self.ball.velocity_y *= -1

        # bounce off left and right
        if (self.ball.x < 0) or (self.ball.right > self.width):
            self.ball.velocity_x *= -1

推荐答案

Kivy Property 是一个类似于 Python 自己的 property 的便利类，但也提供类型检查，验证和事件.ObjectProperty 是 Property 类的一个特殊子类，因此它具有与它相同的初始化参数:

The Kivy Property is a convenience class similar to Python's own property but that also provides type checking, validation and events. ObjectProperty is a specialised sub-class of the Property class, so it has the same initialisation parameters as it:

默认情况下，属性始终采用默认值[.] 默认值value 必须是与 Property 类型一致的值.例如，您不能将列表设置为 StringProperty，因为 StringProperty将检查默认值.

By default, a Property always takes a default value[.] The default value must be a value that agrees with the Property type. For example, you can’t set a list to a StringProperty, because the StringProperty will check the default value.

None 是一种特殊情况:您可以将 Property 的默认值设置为无，但您不能在之后将 None 设置为属性.如果你真的想要这样做，您必须使用 allownone=True[.]

None is a special case: you can set the default value of a Property to None, but you can’t set None to a property afterward. If you really want to do that, you must declare the Property with allownone=True[.]

(来自 Kivy 属性文档)

(from the Kivy Property documentation)

在您的代码中，PongGame 有一个 ball 属性，该属性最初设置为 None，稍后将被分配一个球对象.这是在 kv 文件中定义的:

In your code, PongGame has a ball property that is initially set to None and will later be assigned a ball object. This is defined in the kv file:

<PongGame>:
    ball: pong_ball

    PongBall:
        id: pong_ball
        center: self.parent.center

因为没有对象被传递给初始化器，所以 任何 对象都可以分配给该属性.您可以通过使用虚拟值对其进行初始化来将其限制为仅容纳球对象:

Because no object was passed to the initialiser, any object can be assigned to that property. You could restrict it to only hold ball objects by initialising it with a dummy value:

ball = ObjectProperty(PongBall())

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