为什么我的球矢量在重新初始化后保留其组件

Question

我正在用Pygame编写一个Pong游戏，这是到目前为止我的Ball类的代码：

class Ball(pygame.sprite.Sprite):
    def __init__(self, game, vector=Vec2D.Vec2D()):
        super(Ball, self).__init__()
        self.image = pygame.Surface((BALL_RADIUS*2, BALL_RADIUS*2))
        self.rect = self.image.get_rect()
        self.__draw_ball()
        screen = pygame.display.get_surface()
        self.area = screen.get_rect().inflate(-GAP*2, 0)
        self.vector = vector
        self.game = game
        self.reinit()
    def __draw_ball(self):
        self.image.fill(BLACK)
        self.image.set_colorkey(BLACK, RLEACCEL)
        pygame.draw.circle(self.image, WHITE, (self.rect.centerx, self.rect.centery), BALL_RADIUS)
    def reinit(self):
        self.rect.centerx = self.area.centerx
        self.rect.centery = self.area.centery
        self.vector = Vec2D.Vec2D.from_magn_and_angle(BALL_SPEED, 0)
    def update(self):
        self.rect = self.calcnewpos()
        self.handle_collision()
    def calcnewpos(self):
        (dx, dy) = self.vector.get_xy()
        return self.rect.move(dx, dy)
    def handle_collision(self):
        (dx, dy) = self.vector.get_xy()
        if not self.area.contains(self.rect):
            if self.__hit_topbottom():
                dy = -dy
            elif self.__hit_leftright():
                self.game.increase_score()
                self.reinit()
        else:
            for paddle in self.hit_paddle(dx):
                if dx < 0: self.rect.left = GAP + PADDLE_WIDTH
                elif dx > 0: self.rect.right = SCREEN_WIDTH - (GAP + PADDLE_WIDTH)
                dx = -dx
                dy = paddle.hitpos / 4
                paddle.collided = True
        self.vector = Vec2D.Vec2D(dx, dy)
    def _hit_topbottom(self):
        return self.rect.top < 0 or self.rect.bottom > SCREEN_HEIGHT
    def _hit_leftright(self):
        return self.rect.left < self.area.left or self.rect.right > self.area.right
    def hit_paddle(self, dx):
        if dx < 0: paddle = self.game.paddles['left']
        elif dx > 0: paddle = self.game.paddles['right']
        if self.rect.colliderect(paddle.rect): return [paddle]
        else: return []  

好吧，在玩家（或AI）得分后，球调用其reinit方法，将球放在屏幕中间并重置矢量：

def reinit(self):
    self.rect.centerx = self.area.centerx
    self.rect.centery = self.area.centery
    self.vector = Vec2D.Vec2D.from_magn_and_angle(BALL_SPEED, 0)

但是，不知何故，球仍然保持着在reinit被调用之前拥有的矢量。因此，当球以 （-5.0 -2.0） 这样的向量穿过左侧时，它会在 reinit 调用中快速变化，然后变回 （-5.0， -2.0）。有人可以告诉我为什么会这样吗？

Answer 1

问题出在您的handle_collision方法中。

短篇小说：
该函数做的第一件事是将 dx 和 dy 设置为当前向量。然后它调用reinit().然后，它会将它们设置回函数末尾的第一个。

要修复：
改变

def handle_collision(self):
    (dx, dy) = self.vector.get_xy() # <-- It first sets (dx, dy) to the old vector
    if not self.area.contains(self.rect):
        if self.__hit_topbottom():
            ...
        elif self.__hit_leftright():
            ...  # <-- here is where the reinit gets called, which changes the vector to new values.
    else:
        for paddle in self.hit_paddle(dx):
            ...
    self.vector = Vec2D.Vec2D(dx, dy)  # <-- Then the vector gets changed again, to the OLD vector saved above in (dx, dy)

自

def handle_collision(self):
    (dx, dy) = self.vector.get_xy()
    if not self.area.contains(self.rect):
        if self.__hit_topbottom():
            ...
        elif self.__hit_leftright():
            ...
            return  # <-- With the return here, the parsing never gets to change it back
    else:
        for paddle in self.hit_paddle(dx):
            ...
    self.vector = Vec2D.Vec2D(dx, dy)

这将防止矢量被设置回原来的状态，同时在需要时仍然设置它。