题目要求：当炸弹碰到底部时，让程序暂停片刻，显示出一条“Boom！”的消息。

作者建议使用前面keyboard中的演示程序中的定时代码，在炸弹爆炸后暂停数秒然后再继续。

添加等待标志位waiting=False，和等待时间seconds=2

当触底时获取起始时间，clock_start=time.perf_counter()

然后进入waiting=true的判断，不断获取已经经过去的时间current=time.perf_counter()-clock_start

如果限定的时间还有剩余seconds-current>0，那么就显示BOOM！和还有多长时间产生新的炸弹

否则waiting=False退出判断，并且重新产生一个炸弹

bomb_x=random.randint(0, 500)

bomb_y=-50

#Bomb Catcher Game
#Chapter 4
import sys,random,time,pygame
from pygame.locals import *

def print_text(font,x,y,text,color=(255,255,255)):
    imgText = font.render(text,True,color)
    screen.blit(imgText,(x,y))

#main program
pygame.init()
screen=pygame.display.set_mode((600,500))
pygame.display.set_caption("Bomb Catcher Game")

font1=pygame.font.Font(None, 24)
font2=pygame.font.Font(None, 50)
pygame.mouse.set_visible(False)
white=255,255,0
red=220,50,50
yellow=230,230,50
black=0,0,0

lives=3
score=0
game_over=True
mouse_x=mouse_y=0
pos_x=300
pos_y=460
bomb_x=random.randint(0, 500)
bomb_y=-50
vel_y=0.7
waiting=False
seconds=2

#repeating loop
while True:
    for event in pygame.event.get():
        if event.type==QUIT:
            sys.exit()
        elif event.type==MOUSEMOTION:
            mouse_x,mouse_y=event.pos
            move_x,move_y=event.rel
        elif event.type==MOUSEBUTTONUP:
            if game_over:
                game_over=False
                lives=3
                score=0

    keys=pygame.key.get_pressed()
    if keys[K_ESCAPE]:
        sys.exit()

    screen.fill((0,0,100))

    if game_over: 
        print_text(font1, 100, 200, "<Click to play>")
    else:
        #move the bomb
        bomb_y+=vel_y

        #has the player miss the bomb?
        if bomb_y>500 and waiting==False:
            lives-=1
            waiting=True
            
            clock_start=time.perf_counter()#炸弹触底后进行起始计时
            if lives ==0:
                game_over=True

        #see if player has catch the bomb
        elif bomb_y>pos_y and waiting==False:
            if bomb_x>pos_x and bomb_x<pos_x+120:
                score+=10
                bomb_x=random.randint(0, 500)
                bomb_y=-50
          
        if waiting==True:
            current=time.perf_counter()-clock_start
            if seconds-current>0:
                print_text(font2, 250, 230, "BOOM!",yellow)
                print_text(font1, 250, 270, "wait for "+str(round(seconds-current,1))+" s")
            else:
                waiting=False
                bomb_x=random.randint(0, 500)
                bomb_y=-50
        
        #draw the bomb
        pygame.draw.circle(screen, black, (bomb_x-4,int(bomb_y)-4), 30,0)
        pygame.draw.circle(screen, yellow, (bomb_x,int(bomb_y)), 30,0)

        #set basket postion 
        pos_x=mouse_x
        if pos_x<0:
            pos_x=0
        elif pos_x>500:
            pos_x=500
        
        #draw the basket 
        pygame.draw.rect(screen,black,(pos_x-4,pos_y-4,120,40),0)
        pygame.draw.rect(screen,red,(pos_x,pos_y,120,40),0)
    
    #print of lives

    print_text(font1, 0, 0, "lives:"+str(lives))

    #print score
    print_text(font1, 500, 0, "score:"+str(score))

    pygame.display.update()