問題描述

我剛開始學(xué)習(xí) kivy，我對(duì) ObjectProperty 類的用法以及它如何將 None 作為參數(shù)感到非常困惑.有人可以解釋一下嗎?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

推薦答案

Kivy Property 是一個(gè)類似于 Python 自己的 property 的便利類，但也提供類型檢查，驗(yàn)證和事件.ObjectProperty 是 Property 類的一個(gè)特殊子類，因此它具有與它相同的初始化參數(shù):

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:

默認(rèn)情況下，屬性始終采用默認(rèn)值[.] 默認(rèn)值value 必須是與 Property 類型一致的值.例如，您不能將列表設(shè)置為 StringProperty，因?yàn)?StringProperty將檢查默認(rèn)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 是一種特殊情況:您可以將 Property 的默認(rèn)值設(shè)置為無，但您不能在之后將 None 設(shè)置為屬性.如果你真的想要這樣做，您必須使用 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[.]

(來自 Kivy 屬性文檔)

(from the Kivy Property documentation)

在您的代碼中，PongGame 有一個(gè) ball 屬性，該屬性最初設(shè)置為 None，稍后將被分配一個(gè)球?qū)ο?這是在 kv 文件中定義的:

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?yàn)闆]有對(duì)象被傳遞給初始化器，所以 任何 對(duì)象都可以分配給該屬性.您可以通過使用虛擬值對(duì)其進(jìn)行初始化來將其限制為僅容納球?qū)ο?

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())

這篇關(guān)于ObjectProperty 類的使用的文章就介紹到這了，希望我們推薦的答案對(duì)大家有所幫助，也希望大家多多支持html5模板網(wǎng)！