巡逻的史莱姆

本课属于 Coddy Pygame 学习旅程的“游戏世界”单元。第 24 课，共 36 课。

两个 blob 向右走，接着走到空中。一个巡逻敌人来回走动：碰到墙或走到地面的边缘时就会转向。Blob 类会保存它的 direction，1 表示向右，-1 表示向左。

每走一步后，检查 blob 前脚前方 1 像素处、与脚同高的位置。如果没有墙支撑那个位置，下一步就会走到空中：

if self.direction > 0:
    ahead = self.rect.right
else:
    ahead = self.rect.left - 1
hit_wall = any(self.rect.colliderect(wall) for wall in walls)
ground_ahead = any(wall.collidepoint(ahead, self.rect.bottom) for wall in walls)
if hit_wall or not ground_ahead:
    self.rect.x -= self.direction
    self.direction = -self.direction

只要至少有一面墙通过测试，any(...) 就会得到 True。转向时会撤销这一步，并反转方向。blob 的 Rect 与它的身体大小相符，为 30 × 26，小于 48 像素的图像。绘制图像时，它的底边中点会与 Rect 的底边中点对齐。

挑战

困难

让 blob 在遇到墙壁和平台边缘时转身。

在 Blob.update 中，移动一步后，如果向右移动，就将 ahead 设为 self.rect.right，否则设为 self.rect.left - 1。
检查它是否与墙壁重叠，以及是否有墙壁包含 (ahead, self.rect.bottom)。
如果撞到墙壁，或者前方没有地面，就撤销这一步，并反转 self.direction。

按下“开始”：每个 blob 都会在自己的平台上来回移动。然后点击“检查”。

亲手试试

import pygame

TILE = 36
SPEED = 4
GRAVITY = 1
JUMP = -15
MAX_FALL = 12

LEVEL = [
    "................",
    "................",
    "................",
    "................",
    "..........####..",
    "................",
    "...####.........",
    "................",
    "#.P............#",
    "################",
]

pygame.init()
screen = pygame.display.set_mode((576, 360))
pygame.display.set_caption("Patrol")
clock = pygame.time.Clock()

background = pygame.image.load("assets/backgrounds/clouds.png").convert_alpha()
block_img = pygame.image.load("assets/images/block.png").convert_alpha()
player_img = pygame.image.load("assets/images/player.png").convert_alpha()
blob_img = pygame.image.load("assets/images/blob.png").convert_alpha()


class Blob(pygame.sprite.Sprite):
    def __init__(self, midbottom):
        super().__init__()
        self.image = blob_img
        self.rect = pygame.Rect(0, 0, 30, 26)
        self.rect.midbottom = midbottom
        self.direction = 1

    def update(self):
        self.rect.x += self.direction

walls = []
for row, line in enumerate(LEVEL):
    for col, char in enumerate(line):
        if char == "#":
            walls.append(pygame.Rect(col * TILE, row * TILE, TILE, TILE))
        elif char == "P":
            start = (col * TILE + TILE // 2, row * TILE + TILE)

blobs = pygame.sprite.Group(Blob((432, 144)), Blob((432, 324)))
player = player_img.get_rect(midbottom=start)
vel_y = 0
on_ground = False

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.KEYDOWN and event.key in (pygame.K_SPACE, pygame.K_UP):
            if on_ground:
                vel_y = JUMP
        elif event.type == pygame.KEYUP and event.key in (pygame.K_SPACE, pygame.K_UP):
            if vel_y < -4:
                vel_y = -4

    keys = pygame.key.get_pressed()
    dx = 0
    if keys[pygame.K_LEFT]:
        dx -= SPEED
    if keys[pygame.K_RIGHT]:
        dx += SPEED

    player.x += dx
    for wall in walls:
        if player.colliderect(wall):
            if dx > 0:
                player.right = wall.left
            elif dx < 0:
                player.left = wall.right

    vel_y = min(vel_y + GRAVITY, MAX_FALL)
    player.y += vel_y
    on_ground = False
    for wall in walls:
        if player.colliderect(wall):
            if vel_y > 0:
                player.bottom = wall.top
                on_ground = True
            elif vel_y < 0:
                player.top = wall.bottom
            vel_y = 0

    blobs.update()

    for y in range(0, 360, 72):
        for x in range(0, 576, 72):
            screen.blit(background, (x, y))
    for wall in walls:
        screen.blit(block_img, wall)
    for blob in blobs:
        screen.blit(blob.image, blob.image.get_rect(midbottom=blob.rect.midbottom))
    screen.blit(player_img, player)
    pygame.display.flip()
    clock.tick(60)

pygame.quit()