如何检查两个球在tkinter中是否相撞？

import tkinter as tk

class Ball:
    def __init__(self, canvas, x, y, radius, color, dx, dy):
        self.canvas = canvas
        self.id = canvas.create_oval(x-radius, y-radius, x+radius, y+radius, fill=color)
        self.radius = radius
        self.dx = dx
        self.dy = dy

    def move(self):
        self.canvas.move(self.id, self.dx, self.dy)
        pos = self.canvas.coords(self.id)
        if pos[0] <= 0 or pos[2] >= self.canvas.winfo_width():
            self.dx *= -1
        if pos[1] <= 0 or pos[3] >= self.canvas.winfo_height():
            self.dy *= -1

def check_collision(ball1, ball2):
    pos1 = ball1.canvas.coords(ball1.id)
    pos2 = ball2.canvas.coords(ball2.id)
    distance = ((pos1[0]+pos1[2])/2 - (pos2[0]+pos2[2])/2)**2 + ((pos1[1]+pos1[3])/2 - (pos2[1]+pos2[3])/2)**2
    if distance <= (ball1.radius + ball2.radius)**2:
        return True
    return False

def update():
    for ball in balls:
        ball.move()
    if check_collision(balls[0], balls[1]):
        balls[0].canvas.itemconfig(balls[0].id, fill='red')
        balls[1].canvas.itemconfig(balls[1].id, fill='red')
    else:
        balls[0].canvas.itemconfig(balls[0].id, fill='blue')
        balls[1].canvas.itemconfig(balls[1].id, fill='blue')
    root.after(10, update)

root = tk.Tk()
canvas = tk.Canvas(root, width=400, height=400)
canvas.pack()

ball1 = Ball(canvas, 100, 200, 20, 'blue', 2, 1)
ball2 = Ball(canvas, 300, 200, 30, 'blue', -1, -2)
balls = [ball1, ball2]

update()
root.mainloop()