Python tkinter 删除 event.widget.create_line

Question

我写了一个简单的画图程序，但在做一项任务时遇到了问题。我想要能够删除已经画出的线条。当按下鼠标左键（ButtonPress-1）时，会一直画出一条线，直到松开鼠标左键（ButtonRelease-1）。而当我按下“重置”按钮时，我希望能删除这条线。目前我设置的是，当我按下“重置”时，程序会重新启动，这样的解决办法并不好。

from Tkinter import *
import tkMessageBox
import sys
import os
import subprocess

b1 = "up"
xold, yold = None, None
color= "black"
linesize = 2

def main():
    global root
    root = Tk()
    global drawing_area
    drawing_area = Canvas(root, width=1050, height=1200, background="white")
    drawing_area.pack()
    drawing_area.bind("<Motion>", motion)
    drawing_area.bind("<ButtonPress-1>", b1down)
    drawing_area.bind("<ButtonRelease-1>", b1up)

    button1 = Button(root, text = "Reset", command = restart_program, anchor = N)
    button1.configure(width = 3, background = "#FFFFFF", relief = FLAT)
    button1_window = drawing_area.create_window(640, 0, anchor=N, window=button1)

    root.geometry('1050x1200')
    root.geometry('+0+720')
    root.mainloop()

def restart_program():
    python = sys.executable
    os.execl(python, python, * sys.argv)

def b1down(event):
    global b1
    b1 = "down"

def b1up(event):
    global b1, xold, yold
    b1 = "up"
    xold = None
    yold = None

def motion(event):
    if b1 == "down":
        global xold, yold
        if xold is not None and yold is not None:
            event.widget.create_line(xold,yold,event.x,event.y,smooth=TRUE,fill = color, width=linesize)
        xold = event.x
        yold = event.y

if __name__ == "__main__":
    main()

Answer 1

如果你想删除画布上的所有物品，可以使用canvas.delete("all")这个命令。
或者，你可以给画的线条加个标签，然后通过标签来删除。

所以你可以把：

event.widget.create_line(xold,yold,event.x,event.y,smooth=TRUE,fill = color, width=linesize)

改成：

event.widget.create_line(xold,yold,event.x,event.y,smooth=TRUE,fill = color, width=linesize, tag="line")

然后调用：

canvas.delete("line")

这样就会删除所有带有这个标签的物品。

补充说明：
为了满足最初的请求，这里有一个代码示例，可以实现简单的撤销和重做功能，但请注意以下几个缺点：

1. 当你撤销更多的物品时，存储的列表会不断增长，没有大小限制。

2. 如果你撤销了一个物品，然后手动画了一个新物品，重做时仍然会把被删除的线条放回去。

   import Tkinter as tk
   import tkMessageBox
   import sys
   import os
   
   class App(tk.Tk):
       b1 = "up"
       xold, yold = None, None
       color= "black"
       linesize = 2
       counter = 1 #keeps track of the current working line, is incremented as soon as line is finished
       undone = [] #keeps a list of coordinate lists on undone items
   
       def __init__(self):
           tk.Tk.__init__(self)
           self.drawing_area = tk.Canvas(self, width=600, height=600, background="white")
           self.drawing_area.pack()
           self.drawing_area.bind("<Motion>", self.motion)
           self.drawing_area.bind("<ButtonPress-1>", self.b1down)
           self.drawing_area.bind("<ButtonRelease-1>", self.b1up)
   
           self.button1 = tk.Button(self, text = "Reset", command = self.blank_canvas, anchor = tk.N)
           self.button1.configure(width = 3, background = "#FFFFFF", relief = tk.FLAT)
           self.button1.pack(side="left")
   
           self.button2 = tk.Button(self, text = "Undo", command = self.undo, anchor = tk.N)
           self.button2.configure(width = 3, background = "#FFFFFF", relief = tk.FLAT)
           self.button2.pack(side="left")
   
           self.button3 = tk.Button(self, text = "Redo", command = self.redo, anchor = tk.N)
           self.button3.configure(width = 3, background = "#FFFFFF", relief = tk.FLAT)
           self.button3.pack(side="left")
   
       def blank_canvas(self):
           self.drawing_area.delete("line")
   
       def undo(self):
           self.counter -= 1 #decrements the counter to look at the previous item
           currentlist = [] #creates a list to store the coordinates in
           for item in self.drawing_area.find_withtag("line"+str(self.counter)): #find all sub lines from the previous line
               currentlist.append(self.drawing_area.coords(item)) #get and add the coordinates to the working list
           self.drawing_area.delete("line"+str(self.counter)) #delete all items of the current line
           self.undone.append(currentlist) #add the working list to the stored list
   
       def redo(self):
           try:
               currentlist = self.undone.pop() #fetch and remove last set of coordinates
               for coords in currentlist: #for set of coordinates redraw subline
                   self.drawing_area.create_line(coords,smooth=tk.TRUE,fill = self.color, width=self.linesize, tags=["line", "line"+str(self.counter)])
               self.counter += 1 #re increment counter
           except IndexError:
               pass #occurs if list is empty
   
       def b1down(self, event):
           self.b1 = "down"
   
       def b1up(self, event):
           self.b1 = "up"
           self.xold = None
           self.yold = None
           self.counter += 1
   
       def motion(self, event):
           if self.b1 == "down":
               if self.xold is not None and self.yold is not None:
                   event.widget.create_line(self.xold,self.yold,event.x,event.y,smooth=tk.TRUE,fill = self.color, width=self.linesize, tags=["line", "line"+str(self.counter)])
               self.xold = event.x
               self.yold = event.y
   
   if __name__ == "__main__":
       app = App()
       app.mainloop()