fix issue with block placement

main.py

@@ -1,6 +1,7 @@
 import pygame
 
 from module import WIDTH, HEIGHT, BLOCK_SIZE, PLAYER_SPEED, GRAVITY, COLORS
+from module.block import AirBlock, GrassBlock, StoneBlock, SandBlock, WaterBlock
 from module.player import Player
 from module.utils import generate_terrain
 
@@ -23,20 +24,22 @@ while running:
             mx, my = pygame.mouse.get_pos()
             bx = mx // BLOCK_SIZE
             by = my // BLOCK_SIZE
+
+            block = world[bx][by]
             if event.button == 1:  # 左键放置方块
-                if world[bx][by] == "air":
-                    world[bx][by] = player.selected_block
+                if block.id == "air":
+                    world[bx][by] = player.selected_block(bx, by)
             elif event.button == 3:  # 右键拆除方块
-                world[bx][by] = "air"
+                world[bx][by] = AirBlock(bx, by)
         elif event.type == pygame.KEYDOWN:
             if event.key == pygame.K_1:
-                player.selected_block = "grass"
+                player.selected_block = GrassBlock
             elif event.key == pygame.K_2:
-                player.selected_block = "stone"
+                player.selected_block = StoneBlock
             elif event.key == pygame.K_3:
-                player.selected_block = "sand"
+                player.selected_block = SandBlock
             elif event.key == pygame.K_4:
-                player.selected_block = "water"
+                player.selected_block = WaterBlock
 
     # 玩家移动
     keys = pygame.key.get_pressed()
@@ -73,7 +76,7 @@ while running:
 
     # 显示提示文字
     font = pygame.font.SysFont('KaiTi', 24)
-    text = font.render(f"Selected: {player.selected_block} (1-4切换方块)", True, (255, 255, 255))
+    text = font.render(f"Selected: {player.selected_block.id} (1-4切换方块)", True, (255, 255, 255))
     screen.blit(text, (10, 10))
 
     pygame.display.flip()

module/block.py

@@ -52,3 +52,13 @@ class AirBlock(Block):
 
     def __init__(self, x: int, y: int):
         super().__init__(x, y)
+
+
+class WaterBlock(Block):
+    id = 'water'
+    color = COLORS.get(id)
+    flowable = True
+    collision = False
+
+    def __init__(self, x: int, y: int):
+        super().__init__(x, y)

module/player.py

@@ -1,4 +1,5 @@
 from module import WIDTH, HEIGHT, JUMP_FORCE
+from module.block import AirBlock
 
 
 # 玩家类
@@ -8,7 +9,7 @@ class Player:
         self.y = HEIGHT // 2
         self.velocity = 0
         self.on_ground = False
-        self.selected_block = "grass"
+        self.selected_block = AirBlock
 
     def move(self, dx, dy):
         new_x = self.x + dx