PyGame Custom Event

Question

I want to ask about the use of custom events in pygame ..
I experimented a little with pygame, so I know how usual pygame-generated events works ..
My Question is, why would anyone be interested in a user event, does it help simplify
combined pygame-events ? And how would someone implement it, and benefit from
it in a real world example .. ?

I found an example in a book I have been reading lately ..

# creating the event
CATONKEYBOARD = USEREVENT+1
my_event = pygame.event.Event(CATONKEYBOARD, message="Bad cat!")
pygame.event.post(my_event)

# handling it
for event in pygame.event.get():
    if event.type == CATONKEYBOARD:
        print event.message

I tried it, and I found that the event generates only one time (as soon as it gets posted)
Can anybody explain things to me .. ?

Thanks ..

Answer 1

You can compare events with "==". The compared events must be exactly the same with their attirubutes to be equal

Event1=pygame.event.Event(pygame.USEREVENT, attr1='Event1')
Event2=pygame.event.Event(pygame.USEREVENT, attr1='Event2')

Throw events

pygame.event.post(Event1)
pygame.event.post(Event2)

Main loop

while True:

    for event in pygame.event.get():

        if event.type == pygame.QUIT:
            break
        if event== Event1:
            print("event1")
        elif event == Event2:
            print("event2")

Answer 2

You can post custom events either manually with pygame.event.post, as shown in your example.

Also, you can use pygame.time.set_timer to post a custom event at specific time intervalls. Here's a little example I wrote for another question, where custom events are used to move objects and to control a reload timeout:

import pygame

# you'll be able to shoot every 450ms
RELOAD_SPEED = 450

# the foes move every 1000ms sideways and every 3500ms down
MOVE_SIDE = 1000
MOVE_DOWN = 3500

screen = pygame.display.set_mode((300, 200))
clock = pygame.time.Clock()

pygame.display.set_caption("Micro Invader")

# create a bunch of events 
move_side_event = pygame.USEREVENT + 1
move_down_event = pygame.USEREVENT + 2
reloaded_event  = pygame.USEREVENT + 3

move_left, reloaded = True, True

invaders, colors, shots = [], [] ,[]
for x in range(15, 300, 15):
    for y in range(10, 100, 15):
        invaders.append(pygame.Rect(x, y, 7, 7))
        colors.append(((x * 0.7) % 256, (y * 2.4) % 256))

# set timer for the movement events
pygame.time.set_timer(move_side_event, MOVE_SIDE)
pygame.time.set_timer(move_down_event, MOVE_DOWN)

player = pygame.Rect(150, 180, 10, 7)

while True:
    clock.tick(40)
    if pygame.event.get(pygame.QUIT): break
    for e in pygame.event.get():
        if e.type == move_side_event:
            for invader in invaders:
                invader.move_ip((-10 if move_left else 10, 0))
            move_left = not move_left
        elif e.type == move_down_event:
            for invader in invaders:
                invader.move_ip(0, 10)
        elif e.type == reloaded_event:
            # when the reload timer runs out, reset it
            reloaded = True
            pygame.time.set_timer(reloaded_event, 0)

    for shot in shots[:]:
        shot.move_ip((0, -4))
        if not screen.get_rect().contains(shot):
            shots.remove(shot)
        else:
            hit = False
            for invader in invaders[:]:
                if invader.colliderect(shot):
                    hit = True
                    i = invaders.index(invader)
                    del colors[i]
                    del invaders[i]
            if hit:
                shots.remove(shot)

    pressed = pygame.key.get_pressed()
    if pressed[pygame.K_LEFT]: player.move_ip((-4, 0))
    if pressed[pygame.K_RIGHT]: player.move_ip((4, 0))

    if pressed[pygame.K_SPACE]: 
        if reloaded:
            shots.append(player.copy())
            reloaded = False
            # when shooting, create a timeout of RELOAD_SPEED
            pygame.time.set_timer(reloaded_event, RELOAD_SPEED)

    player.clamp_ip(screen.get_rect())

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

    for invader, (a, b) in zip(invaders, colors): 
        pygame.draw.rect(screen, (150, a, b), invader)

    for shot in shots: 
        pygame.draw.rect(screen, (255, 180, 0), shot)

    pygame.draw.rect(screen, (180, 180, 180), player)    
    pygame.display.flip()

---

Can't I just "give in" the suitable conditions for posting the event, so that it can generate it automatically then ? I think that way it'd be more practical ...

Implementing such a function is quite easy. Just create a list of conditions and events and check every condition in your main loop:

conditions = [ # blink if player is outside screen
              (lambda: not s_r.contains(player), pygame.event.Event(E_OUTSIDE)),
               # if mouse if over player then grow and shrink player  
              (lambda: player.collidepoint(pygame.mouse.get_pos()), pygame.event.Event(E_MOUSE))]

while True:
    # generate events from conditions
    map(pygame.event.post, [e for (c, e) in conditions if c()])

    for event in pygame.event.get():
       ...

Here's the full example:

import pygame

pygame.init() 

screen = pygame.display.set_mode((300, 300)) 
s_r = screen.get_rect()
player = pygame.Rect((100, 100, 50, 50))
timer = pygame.time.Clock()
flash = 0
grow = True
color = pygame.color.Color('Black')

E_OUTSIDE = pygame.USEREVENT  + 1
E_MOUSE   = pygame.USEREVENT  + 2

conditions = [ # blink if player is outside screen
              (lambda: not s_r.contains(player), pygame.event.Event(E_OUTSIDE)),
               # if mouse if over player then grow and shrink player  
              (lambda: player.collidepoint(pygame.mouse.get_pos()), pygame.event.Event(E_MOUSE))]

while True:
    # generate events from conditions
    map(pygame.event.post, [e for (c, e) in conditions if c()])

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            raise
        elif event.type == E_OUTSIDE and not flash:
            flash = 5
        elif event.type == E_MOUSE:
            if grow: 
                player.inflate_ip(4, 4)
                grow = player.width < 75
            else: 
                player.inflate_ip(-4, -4)
                grow = player.width < 50

    flash = max(flash - 1, 0)
    if flash % 2:
        color = pygame.color.Color('White')                

    pressed = pygame.key.get_pressed()
    l, r, u, d = map(lambda x: x*4, [pressed[k] for k in pygame.K_a, pygame.K_d, pygame.K_w, pygame.K_s])
    player.move_ip((-l + r, -u + d))

    screen.fill(color)
    color = pygame.color.Color('Black')

    pygame.draw.rect(screen, pygame.color.Color('Grey'), player)

    pygame.display.flip()
    timer.tick(25)