使用 Pyxel 入门 2D 游戏（第 8 部分）：生成小行星

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1. 创建小行星精灵

首先，我们为小行星准备一个精灵类。在 sprite.py 中添加一个新的 AsteroidSprite 类。构造函数中使用随机值来选择要显示的小行星图像。

# sprite.py
# add this class

class AsteroidSprite(BaseSprite):

    def __init__(self, x, y):
        """Constructor"""
        super().__init__(x, y)
        self.index = random.randint(2, 7)  # Asteroid image index

    def draw(self):
        """Draw"""
        pyxel.blt(
            self.x, self.y, 0,
            self.w * self.index, 0,
            self.w, self.h, 0
        )  # Asteroid

通过这种设置，每次生成小行星时，它们都会拥有不同的外观。

---

2. 准备常量和变量

接下来，在 main.py 中添加与小行星相关的常量。将这些数值归类为常量后，后期调整游戏难度会更加方便。

# main.py
# add constants

ASTEROID_INTERVAL = 20   # Spawn interval (frames)
ASTEROID_LIMIT    = 30   # Maximum number of asteroids

ASTEROID_SPD_MIN = 1.0   # Minimum speed
ASTEROID_SPD_MAX = 2.0   # Maximum speed
ASTEROID_DEG_MIN = 30    # Minimum angle
ASTEROID_DEG_MAX = 150   # Maximum angle

在 Game 类的构造函数中添加用于管理小行星的变量。

# main.py
# add to Game.__init__()

# Asteroids
self.asteroid_time = 0   # Spawn interval counter
self.asteroids = []      # List of asteroids

3. 小行星生成逻辑

向 Game 类添加一个 check_interval() 方法。该方法会在固定间隔（每 ASTEROID_INTERVAL 帧）生成小行星。小行星的总数量受 ASTEROID_LIMIT 限制。生成位置随机选择在屏幕顶部边缘，速度和角度也在预定义的范围内随机选取。

# main.py
# add to Game class

def check_interval(self):
    # Asteroid spawn interval
    self.asteroid_time += 1
    if self.asteroid_time = ASTEROID_LIMIT:
        return

    # Add a new asteroid
    x   = random.random() * W
    y   = 0
    spd = random.uniform(ASTEROID_SPD_MIN, ASTEROID_SPD_MAX)
    deg = random.uniform(ASTEROID_DEG_MIN, ASTEROID_DEG_MAX)

    asteroid = sprite.AsteroidSprite(x, y)
    asteroid.move(spd, deg)
    self.asteroids.append(asteroid)

---

4. 更新与绘制小行星

更新

在 Game 类的 update() 方法中，处理小行星的生成和更新。

# main.py
# add to Game.update()

self.check_interval()  # Spawn asteroids

# Update asteroids
for asteroid in self.asteroids:
    asteroid.update()
    self.overlap_spr(asteroid)

绘制

在 draw() 方法中，绘制所有小行星。

# main.py
# add to Game.draw()

# Draw asteroids
for asteroid in self.asteroids:
    asteroid.draw()

现在，多个小行星将会在屏幕上流动。

完整代码

以下是实现本章节所有功能的完整代码。

sprite.py

import pyxel
import math
import random

class BaseSprite:

    def __init__(self, x, y, w=8, h=8):
        """Constructor"""
        self.x = x
        self.y = y
        self.w = w
        self.h = h
        self.vx = 0
        self.vy = 0

    def update(self):
        """Update"""
        self.x += self.vx
        self.y += self.vy

    def draw(self):
        """Draw (implemented in subclasses)"""
        pass

    def move(self, spd, deg):
        """Move"""
        rad = deg * math.pi / 180
        self.vx = spd * math.cos(rad)  # x‑axis velocity
        self.vy = spd * math.sin(rad)  # y‑axis velocity

    def flip_x(self):
        """Flip movement in the x direction"""
        self.vx *= -1

class ShipSprite(BaseSprite):

    def __init__(self, x, y):
        """Constructor"""
        super().__init__(x, y)

    def draw(self):
        """Draw"""
        pyxel.blt(
            self.x, self.y, 0,
            0, 0,
            self.w, self.h, 0
        )  # Ship

class AsteroidSprite(BaseSprite):

    def __init__(self, x, y):
        """Constructor"""
        super().__init__(x, y)
        self.index = random.randint(2, 7)  # Asteroid image index

    def draw(self):
        """Draw"""
        pyxel.blt(
            self.x, self.y, 0,
            self.w * self.index, 0,
            self.w, self.h, 0
        )  # Asteroid

main.py

import pyxel
import math
import random
import sprite

W, H = 160, 120
SHIP_SPD = 1.4

ASTEROID_INTERVAL = 20   # Spawn interval
ASTEROID_LIMIT    = 30   # Maximum number of asteroids

ASTEROID_SPD_MIN = 1.0   # Minimum speed
ASTEROID_SPD_MAX = 2.0   # Maximum speed
ASTEROID_DEG_MIN = 30    # Minimum angle
ASTEROID_DEG_MAX = 150   # Maximum angle

class Game:
    def __init__(self):
        pyxel.init(W, H, caption="Asteroid Shooter")
        self.ship = sprite.ShipSprite(W // 2, H - 20)

        # Asteroids
        self.asteroid_time = 0
        self.asteroids = []

    def check_interval(self):
        # Asteroid spawn interval
        self.asteroid_time += 1
        if self.asteroid_time = ASTEROID_LIMIT:
            return

        # Add a new asteroid
        x   = random.random() * W
        y   = 0
        spd = random.uniform(ASTEROID_SPD_MIN, ASTEROID_SPD_MAX)
        deg = random.uniform(ASTEROID_DEG_MIN, ASTEROID_DEG_MAX)

        asteroid = sprite.AsteroidSprite(x, y)
        asteroid.move(spd, deg)
        self.asteroids.append(asteroid)

    def overlap_spr(self, asteroid):
        # Placeholder for collision detection logic
        pass

    def update(self):
        # Ship movement (example)
        if pyxel.btn(pyxel.KEY_LEFT):
            self.ship.x -= SHIP_SPD
        if pyxel.btn(pyxel.KEY_RIGHT):
            self.ship.x += SHIP_SPD

        self.check_interval()  # Spawn asteroids

        # Update asteroids
        for asteroid in self.asteroids:
            asteroid.update()
            self.overlap_spr(asteroid)

    def draw(self):
        pyxel.cls(0)

        # Draw ship
        self.ship.draw()

        # Draw asteroids
        for asteroid in self.asteroids:
            asteroid.draw()

Game()
pyxel.run(Game().update, Game().draw)

---

最大角度

# Game
class Game:
    def __init__(self):
        """Constructor"""

        # Initialize score
        self.score = 0

        # Initialize player
        self.ship = sprite.ShipSprite(W / 2, H - 40)
        deg = 0 if random.random() < 0.5 else 180
        self.ship.move(SHIP_SPD, deg)

        # Asteroids
        self.asteroid_time = 0   # Spawn interval counter
        self.asteroids = []      # Asteroid list

        # Start Pyxel
        pyxel.init(W, H, title="Hello, Pyxel!!")
        pyxel.load("shooter.pyxres")
        pyxel.run(self.update, self.draw)

    def update(self):
        """Update"""

        # Update player
        self.ship.update()
        self.control_ship()
        self.overlap_spr(self.ship)

        self.check_interval()  # Spawn asteroids

        # Update asteroids
        for asteroid in self.asteroids:
            asteroid.update()
            self.overlap_spr(asteroid)

    def draw(self):
        """Draw"""
        pyxel.cls(0)

        # Draw score
        pyxel.text(
            10, 10,
            "SCORE:{:04}".format(self.score), 12
        )

        # Draw player
        self.ship.draw()

        # Draw asteroids
        for asteroid in self.asteroids:
            asteroid.draw()

    def control_ship(self):
        """Action"""
        if pyxel.btnp(pyxel.KEY_SPACE):
            self.ship.flip_x()  # Reverse movement

    def overlap_spr(self, spr):
        """Move to the opposite side when leaving the screen"""
        if spr.x < -spr.w:
            spr.x = W
            return
        if W < spr.x:
            spr.x = -spr.w
            return
        if spr.y < -spr.h:
            spr.y = H
            return
        if H < spr.y:
            spr.y = -spr.h
            return

    def check_interval(self):
        # Asteroid spawn interval
        self.asteroid_time += 1
        if self.asteroid_time < ASTEROID_INTERVAL:
            return
        self.asteroid_time = 0

        # Do not exceed the maximum number of asteroids
        if ASTEROID_LIMIT < len(self.asteroids):
            return

        # Add a new asteroid
        x = random.random() * W
        y = 0
        spd = random.uniform(ASTEROID_SPD_MIN, ASTEROID_SPD_MAX)
        deg = random.uniform(ASTEROID_DEG_MIN, ASTEROID_DEG_MAX)

        asteroid = sprite.AsteroidSprite(x, y)
        asteroid.move(spd, deg)
        self.asteroids.append(asteroid)

def main():
    """Main"""
    Game()

if __name__ == "__main__":
    main()

进入全屏模式

退出全屏模式

运行游戏时，它的效果如下：

下次…

感谢阅读！

在下一章中，我们将介绍 “Let’s Fire Bullets.”

敬请期待！