##language:zh
#pragma section-numbers on
'''
[[StartProgramming| （首页）开始编程之旅]]
翻译自Lee Harr的[[http://staff.easthighschool.net/lee/computers/book/Start_Programming.html|Start Programming]]

本文是使用[[http://www.nongnu.org/pygsear/|pygsear]]+[[http://pygame.org|pygame]]作为开发环境，以初级用户角度来分步分阶段学习[[http://www.python.org|PYTHON]]基本概念，并以小游戏开发项目为具体案例，介绍的十分详细。编写风格清新朴实，没有象一般教科书那样枯燥，极其适合初级用户来激发兴趣时使用。
'''
<<TableOfContents>>
== StartProgramming-4-2 Ball ==

We have experience with a ball from the `WhoaBall` class.

'''我们已经从 `WhoaBall` 获得了一个球的经验。'''

We can draw on that experience, but I am not going to inherit from `WhoaBall` for the Pong ball. It is just too different.

'''我们可以从那个经验开始画起，但是我不打算从 `WhoaBall` 继承到 Pong ball ，这有太多的差异。'''

{{{#!python
from pygsear.Drawable import Square

class Ball(Square):
    def __init__(self):
        Square.__init__(self, size=15)
        self.center()
        self.path.set_velocity(vx=150, vy=100)

    def walls(self):
        vx, vy = self.path.get_velocity()
        if not self.onscreen(top=-5, bottom=-5, jail=1)
            self.path.set_velocity(vy=-vy)
        if not self.onscreen(right=-5, jail=1)
            self.path.set_velocity(vx=-vx)

    def move(self):
        self.walls()
        Square.move(self)
}}}

Now we can add the new Ball in to our Pong game and see what happens.

'''现在我们可以把新 Ball 添加到我们的 Pong 游戏中并且看看发生了什么。'''

{{{#!python
class Pong(Game):
    def initialize(self):
        paddle = Paddle()
        self.sprites.add(paddle)
        ball = Ball()
        self.sprites.add(ball)
}}}

Here is our progress so far. We have the paddle and the ball:

'''这是我们目前的进程。我们有了 paddle 和 ball：'''

{{{#!python
# Pong-0.2.py
from pygsear.Drawable import Rectangle, Square
from pygsear.Game import Game
from pygsear.Event import KEYDOWN_Event, KEYUP_Event

from pygame.locals import K_UP, K_DOWN


class Paddle(Rectangle):
    def __init__(self):
        Rectangle.__init__(self, width=15, height=50)
        self.center(x=10)

        self.up_pressed = 0
        self.down_pressed = 0

    def up(self, ev):
        self.up_pressed = 1

    def noup(self, ev):
        self.up_pressed = 0

    def down(self, ev):
        self.down_pressed = 1

    def nodown(self, ev):
        self.down_pressed = 0 

    def setVel(self):
        if self.up_pressed and not self.down_pressed:
            self.path.set_velocity(vy=-100)
        elif self.down_pressed and not self.up_pressed:
            self.path.set_velocity(vy=100)
        else:
            self.path.set_velocity(vy=0)

    def move(self):
        self.setVel()
        Rectangle.move(self) 
        self.onscreen(top=-5, bottom=-5, jail=1)


class Ball(Square):
    def __init__(self):
        Square.__init__(self, side=15)
        self.center()
        self.path.set_velocity(vx=150, vy=100)

    def walls(self):
        vx, vy = self.path.get_velocity()
        if not self.onscreen(top=-5, bottom=-5, jail=1):
            self.path.set_velocity(vy=-vy)
        if not self.onscreen(right=-5, jail=1):
            self.path.set_velocity(vx=-vx)

    def move(self):
        self.walls()
        Square.move(self) 


class Pong(Game):
    def initialize(self):
        paddle = Paddle()
        self.sprites.add(paddle)
        ball = Ball()
        self.sprites.add(ball)    

        self.events.add(KEYDOWN_Event(key=K_UP, callback=paddle.up))
        self.events.add(KEYUP_Event(key=K_UP, callback=paddle.noup))

        self.events.add(KEYDOWN_Event(key=K_DOWN, callback=paddle.down))
        self.events.add(KEYUP_Event(key=K_DOWN, callback=paddle.nodown))


if __name__ == '__main__':
    game = Pong()
    game.mainloop()
}}}

You can run the new game with:

'''你可以运行你的新游戏用：'''

{{{
python Pong-0.2.py
}}}

The only problem is that the ball passes right through the paddle!

'''唯一的问题是 ball 正常穿越了 paddle !'''

To fix this we need to introduce collisions. 

'''要修复这个问题我们需要介绍碰撞。'''