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pythonkivy

Centering elements in kivy: using root vs self.parent

I'm reading the kivy documentations. In it, there's an example on how to make your own pong game.

In general, the tutorial is very clear. However, at some point there was something subtle I wasn't sure about.

They make a ball in the .kv file:

<PongBall>:
    size: 50,50
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size

Then, they add an instance of a PongBall to a previously defined PongGame class:

<PongGame>:
    canvas:
        Rectangle:
            pos:self.center_x - 5 , 0
            size : 10, self.height
    Label:
        font_size : 70
        center_x : root.width / 4
        top : root.top - 50
        text : "0"
    Label:
        font_size : 70
        center_x : root.width / 4 * 3
        top : root.top - 50
        text: "0"
    PongBall:
        center: self.parent.center

In the PongGame labels they added, the set the location using root, which references the PongGame For the PongBall they set the location using self.parent.center, where self.parent appears to reference the PongGame. I am assuming there is a reason they used:

<PongBall>:
    PongBall:
        center: self.parent.center

instead of 

<PongBall>:
    PongBall:
        center: root.center

Why did they not use root.center as they did for the labels? Is there a reason for picking one over the other? Is there a difference?

Solution

  • They do this, because the ball should be relative to the position of the game widget. The labels on the other hand are "dead". They just have to "exist" at the point they got in the beginning of the game, but the ball has to "live"