开发人员希望在 Python 中创建一个 "鱼缸"，其中有一些鱼类，这些鱼类可以通过按一个按钮产生，并按 dx、dy 方向移动，其中 dx 和 dy 都是为每条鱼产生的 -3 到 3 之间的随机值。一旦它们接近鱼缸的边缘，它们就会在相反的方向弹回（类似于 DVD 屏幕保护程序）。
当前代码中有两个问题：
canvas.coords()方法并没有更新所有条目，无法正确更新鱼的位置。- 第一条鱼不能正常移动。
详细代码如下：
import time
import random
from Tkinter import *
tank = Tk()
tank.title("Fish Tank")
tankwidth = 700 # (the background image is 700 by 525)
tankheight = 525
x = tankwidth/2
y = tankheight/2
fishwidth = 78 # (the fish image is 78 by 92)
fishheight = 92
fishx = fishwidth/2
fishy = fishheight/2
dx = 0
dy = 0
canvas = Canvas(tank,width=tankwidth,height=tankheight)
canvas.grid(row=0, column=0, columnspan=3)
bg = PhotoImage(file = "tank.gif")
left = PhotoImage(file = "fishleft.gif")
right = PhotoImage(file = "fishright.gif")
background = canvas.create_image(x,y,image=bg)
rightfish = canvas.create_image(-1234,-1234, image=right)
leftfish = canvas.create_image(-1234,-1234, image=left)
def newfish():
x = random.randrange(fishx+5, tankwidth-(fishx+5)) # +5 here so even the biggest dx or dy
y = random.randrange(fishy+5, tankheight-(fishy+5)) # won't get stuck between the border
dx = random.randrange(-3,4)
dy = random.randrange(-3,4)
leftfish = canvas.create_image(x,y, image=left)
rightfish = canvas.create_image(-1234,-1234, image=right)
updatefish(leftfish,rightfish,x,y,dx,dy)
def updatefish(left,right,x,y,dx,dy):
x += dx
y += dy
if dx < 0:
whichfish = left
canvas.coords(right,-1234,-1234)
if dx > 0:
whichfish = right
canvas.coords(left,-1234,-1234)
if x < fishx or x > tankwidth-fishx:
dx = -dx
if y < fishy or y > tankheight-fishy:
dy = -dy
print x, y, dx, dy
if dx == 0 and dy == 0:
return
canvas.coords(whichfish, x,y)
canvas.after(100, updatefish, leftfish,rightfish,x,y,dx,dy)
newfish()
new = Button(tank, text="Add Another Fish", command=newfish)
new.grid(row=1,column=1,sticky="NS")
tank.mainloop()
2、解决方案
为了解决上述问题，需要对以下方法进行修改：
1. canvas.coords(): 在更新 fish 位置时，需要更新 both leftfish 和 rightfish 的坐标。修改后的代码如下：
if dx < 0:
whichfish = left
else:
whichfish = right
canvas.coords(left, x, y)
canvas.coords(right, x, y)
2. 第一鱼不能正常移动：修改后的代码中，在 updatefish()方法中添加了以下 if 语句：
if dx == 0 and dy == 0:
return
当 dx 和 dy 都为 0 时，函数将返回，这可以防止第一条鱼不移动。
修改后的代码如下：
import time
import random
from Tkinter import *
tank = Tk()
tank.title("Fish Tank")
tankwidth = 700 # (the background image is 700 by 525)
tankheight = 525
x = tankwidth/2
y = tankheight/2
fishwidth = 78 # (the fish image is 78 by 92)
fishheight = 92
fishx = fishwidth/2
fishy = fishheight/2
dx = 0
dy = 0
canvas = Canvas(tank,width=tankwidth,height=tankheight)
canvas.grid(row=0, column=0, columnspan=3)
bg = PhotoImage(file = "tank.gif")
left = PhotoImage(file = "fishleft.gif")
right = PhotoImage(file = "fishright.gif")
background = canvas.create_image(x,y,image=bg)
rightfish = canvas.create_image(-1234,-1234, image=right)
leftfish = canvas.create_image(-1234,-1234, image=left)
def newfish():
x = random.randrange(fishx+5, tankwidth-(fishx+5)) # +5 here so even the biggest dx or dy
y = random.randrange(fishy+5, tankheight-(fishy+5)) # won't get stuck between the border
dx = random.randrange(-3,4)
dy = random.randrange(-3,4)
leftfish = canvas.create_image(x,y, image=left)
rightfish = canvas.create_image(x,y, image=right)
updatefish(leftfish,rightfish,x,y,dx,dy)
def updatefish(left,right,x,y,dx,dy):
x += dx
y += dy
if dx < 0:
whichfish = left
canvas.coords(left,x,y)
canvas.coords(right,-1234,-1234)
if dx > 0:
whichfish = right
canvas.coords(left,-1234,-1234)
canvas.coords(right,x,y)
if x < fishx or x > tankwidth-fishx:
dx = -dx
if y < fishy or y > tankheight-fishy:
dy = -dy
print x, y, dx, dy
if dx == 0 and dy == 0:
return
canvas.coords(whichfish, x,y)
canvas.after(100, updatefish, leftfish,rightfish,x,y,dx,dy)
newfish()
new = Button(tank, text="Add Another Fish", command=newfish)
new.grid(row=1,column=1,sticky="NS")
tank.mainloop()
通过以上修改，鱼缸现在可以正常运行了，鱼可以正常移动，避免撞墙。
