In this lesson, you’ll add custom events to your game to create new enemies. The design calls for enemies to appear at regular intervals. This means that, at set intervals, you need to do two things:

Create a new Enemy
Add it to all_sprites and enemies

Let’s see how to create a custom event that’s generated every few seconds. You can create a custom event by naming it:

Python
      
    
# Create the screen object
# The size is determined by the constant SCREEN_WIDTH and SCREEN_HEIGHT
screen = pygame.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))

# Create a custom event for adding a new enemy
ADDENEMY = pygame.USEREVENT + 1
pygame.time.set_timer(ADDENEMY, 250)

# Instantiate player. Right now, this is just a rectangle.
player = Player()

Next, you need to insert this new event into the event queue at regular intervals throughout the game. That’s where the time module comes in. Line 84 fires the new ADDENEMY event every 250 milliseconds, or four times per second. You call .set_timer() outside the game loop since you only need one timer, but it will fire throughout the entire game.

Add the code to handle your new event:

Python
      
    
# Main loop
while running:
    # Look at every event in the queue
    for event in pygame.event.get():
        # Did the user hit a key?
        if event.type == KEYDOWN:
            # Was it the Escape key? If so, stop the loop.
            if event.key == K_ESCAPE:
                running = False

        # Did the user click the window close button? If so, stop the loop.
        elif event.type == QUIT:
            running = False

        # Add a new enemy?
        elif event.type == ADDENEMY:
            # Create the new enemy and add it to sprite groups
            new_enemy = Enemy()
            enemies.add(new_enemy)
            all_sprites.add(new_enemy)

    # Get the set of keys pressed and check for user input
    pressed_keys = pygame.key.get_pressed()
    player.update(pressed_keys)

    # Update enemy position
    enemies.update()

For more information about the time module, check out the pygame documentation.

Custom Events