Unit 12.2A · Term 2

Game Loop

The game loop is the heartbeat of every game. It runs continuously — handling events, updating state, and rendering visuals — dozens of times per second.

Learning Objectives

12.5.1.3 Code the game loop of the application

Lesson Presentation

12.2A-game-loop.pdf · Slides for classroom use

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Conceptual Anchor

The Film Projector Analogy

A movie projector shows 24 frames per second — each frame is a still image, but played fast they create the illusion of motion. A game loop does the same: clear screen → draw new frame → show it → repeat. The faster this cycle runs (FPS), the smoother the game feels.

Rules & Theory

The 3 Phases of a Game Loop

Phase	What Happens	Code
1. Handle Events	Process user input (keys, mouse, quit)	`pygame.event.get()`
2. Update State	Move objects, check collisions, update scores	Game logic code
3. Render (Draw)	Clear screen, draw everything, display	`screen.fill()` + `draw` + `flip()`

FPS Control with Clock

Command	Purpose
`clock = pygame.time.Clock()`	Create a Clock object
`clock.tick(60)`	Limit loop to 60 FPS maximum
`clock.get_fps()`	Get actual frames per second

Event Types

Event	When It Fires
`pygame.QUIT`	User clicks the window close button (✕)
`pygame.KEYDOWN`	A key is pressed down
`pygame.KEYUP`	A key is released
`pygame.MOUSEBUTTONDOWN`	A mouse button is pressed
`pygame.MOUSEMOTION`	The mouse is moved

Worked Examples

1 Standard Game Loop Template

import pygame
import sys

pygame.init()

# --- Setup ---
WIDTH, HEIGHT = 800, 600
FPS = 60
screen = pygame.display.set_mode((WIDTH, HEIGHT))
pygame.display.set_caption("Game Loop Demo")
clock = pygame.time.Clock()

# --- Colors ---
BLACK = (0, 0, 0)
WHITE = (255, 255, 255)

# --- Game Loop ---
running = True
while running:
    # Phase 1: HANDLE EVENTS
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_ESCAPE:
                running = False

    # Phase 2: UPDATE STATE
    # (game logic goes here)

    # Phase 3: RENDER
    screen.fill(BLACK)
    # (draw objects here)
    pygame.display.flip()

    # Control FPS
    clock.tick(FPS)

pygame.quit()
sys.exit()

2 Showing FPS Counter

import pygame, sys

pygame.init()
screen = pygame.display.set_mode((800, 600))
pygame.display.set_caption("FPS Counter")
clock = pygame.time.Clock()
font = pygame.font.SysFont("Arial", 24)

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    screen.fill((30, 30, 30))

    # Display current FPS
    fps_text = font.render(f"FPS: {clock.get_fps():.0f}",
                           True, (0, 255, 0))
    screen.blit(fps_text, (10, 10))

    pygame.display.flip()
    clock.tick(60)

pygame.quit()
sys.exit()

Common Pitfalls

No clock.tick() = Unlimited Speed

Without clock.tick(), the loop runs as fast as the CPU allows. This wastes power, heats the CPU, and makes game speed inconsistent across different computers.

Drawing BEFORE Filling

Always screen.fill() FIRST, then draw objects. If you draw first and then fill, your drawings get covered!

Tasks

Remember

Name the 3 phases of a game loop and describe what happens in each.

Apply

Write a game loop that changes the background color when the user presses the Space bar.

Apply

Add an FPS counter to any PyGame application displaying current frames per second.

Self-Check Quiz

Q1: What are the 3 phases of a game loop?

1) Handle Events — process user input. 2) Update State — move objects, check collisions. 3) Render — clear screen, draw, display.

Q2: What does clock.tick(60) do?

Limits the game loop to a maximum of 60 iterations per second (60 FPS), ensuring consistent speed and preventing CPU waste.

Q3: What event type is triggered when the user clicks ✕?

pygame.QUIT