Tkinter程序更新屏幕，但我不明白如何在没有canvas.create命令的情况下

Question

In this code I modified from the Web, canvas.create statements are used to create the grid and the text as usual but there is no create statement in the function AddPoint which is called to update the plot so I don't see how the plot gets written. What statement causes the plot to update ?

    import Tkinter,  time, sys, math
    from random import *
    global latch
    start = 50
    end =   800
    class HPSA:

        def HPSA_grid(self):                                                                               # create a grid
            self.lines=[]
            self.lastpos=0
            self.canvas.create_rectangle(0, 0, 800, 512, fill="black")
            self.canvas.create_text(25, 20, fill="red", text=str("dB"), anchor="e")
            self.canvas.create_text(770, 500, fill="red", text=str("MHz"), anchor="e")
            # horizontal lines   
            for y in range(0,480,50):                                                                      
                    self.canvas.create_line(0, y, 800, y, fill="#444444",dash=(4, 4))
                    #self.canvas.create_line(0, y, 800, y, fill="red",dash=(4, 4))
                    self.canvas.create_text(5, y-10, fill="#999999", text=str(-y*2/10), anchor="w")
            # vertical lines  
            for x in range(start,800,80):                                                                   
                    self.canvas.create_line(x, 0, x, 512, fill="#444444",dash=(4, 4))
                    #self.canvas.create_line(x, 0, x, 512, fill=red,dash=(4, 4))
                    self.canvas.create_text(x+3, 500-10, fill="#999999", text=str(x/10), anchor="w")
            # draw start vertical line        
            self.canvas.create_line(start, 0, start, 470, fill="yellow")   
            # draw start horizontal line                                
            self.canvas.create_line(start, 470, 800, 470, fill="yellow")                                  

            self.lineRedraw=self.canvas.create_line(0, 800, 0, 0, fill="red")

           # self.lines1text=self.canvas.create_text(800-3, 10, fill="#00FF00", text=str("TEST"), anchor="e")
            for x in range(800):
                    self.lines.append(self.canvas.create_line(x, 0, x, 0, fill="#00FF00"))

        def addPoint(self,val):
            if self.xpos>start:
                pass
                self.canvas.coords(self.lines[self.xpos],(self.xpos-1,self.lastpos,self.xpos,val))
            if self.xpos<800:
                pass
                #self.canvas.coords(self.lineRedraw,(self.xpos+1,val +5,self.xpos+1,val-5))           #  ?? draws red line near val
            self.lastpos=val
            self.xpos+=1
            if self.xpos==800:
                    self.xpos=0
                    self.totalPoints+=800
                    latch = 0
                    my_screen.update()       # continuosly draws points     
        def __init__(self, my_screen):
            self.xpos=start
            self.line1avg=0
            self.data=[0]*800
            self.canvas = Tkinter.Canvas(my_screen, width=800, height=512)
            self.canvas.pack()
            self.totalPoints=0
            self.HPSA_grid()
            self.timeStart=time.time()

    my_screen = Tkinter.Tk()
    my_screen.wm_title("HPSA")
    a = HPSA(my_screen)
    my_screen.update()    # draws screen immediately
    k= .041
    for x in range (10,800,1):
      point = 250 - 250*(math.sin(k*x)/(k*x))  
      a.addPoint(point)

Answer 1

It appears that this code creates 800 points as zero-length lines when it starts, and addPoint simply adjusts the coordinates of those lines in order to draw the graph.

Here is the code that initially creates the 800 lines:

for x in range(800):
    self.lines.append(self.canvas.create_line(x, 0, x, 0, fill="#00FF00"))

Inside addPoint , it modifies the coordinates of each of those lines here:

self.canvas.coords(self.lines[self.xpos],(self.xpos-1,self.lastpos,self.xpos,val))

You could get the same effect by removing the loop that initially creates all of the lines, and then replacing that line in addPos with a call to create_line .