Decoupling Input, Game Logic, And Rendering In The Game Loop For Maintainability
The Tight Coupling Problem in Game Loops
Game loops often contain input handling, logic update, and rendering tightly coupled together in a single loop. This tight integration makes the code difficult to maintain and extend over time as the codebase grows in complexity. The single game loop becomes an entangled mess of input polling, state updates, and graphic commands.
Tightly coupling the major components of a game together causes a number of software engineering problems:
- Difficult to isolate and test individual components
- Hard to reuse components in other projects
- Changes can ripple through and break seemingly unrelated parts of the code
- Limitations on distributing work across threads/processes
As new features get added, the interconnectedness of the monolithic game loop turns into friction that slows down further progress. What may have started as clean and simple code becomes an unmanageable ball of mud over time under the pressures of product requirements. Breaking this tight coupling by decoupling game logic is key for maintainability.
Benefits of Decoupled Game Architecture
By decoupling the major systems in a game engine, such as input, game logic, assets, audio, rendering, etc, we can realize several software engineering benefits:
- Isolates components for easier testing and debugging – Code that does not depend on lots of other systems can be tested independently, speeding up debug/edit/test cycles.
- Allows for better code reuse – Logic and assets created for one game can be easily adapted for use in another game built on the same architecture.
- Enables easier incremental updates – Small tweaks to one subsystem will not break unseen dependencies in other areas.
- Permits targeted optimization – System resources can be tuned for specific subsystems instead of only globally.
- Reduces ripple effect bugs – Bugs staying isolated within one subsystem without propagate through entire codebase.
- Supports distributed development – Ownership and responsibilities kept separate between subsystems.
By designing clear boundaries and interfaces between components, game logic can evolve independently. This modularity and separation of concern is essential for maintaining a codebase through the iterations of game development.
Implementing a Decoupled Game Loop
The key characteristics defining a decoupled game architecture are:
- An input system that polls hardware devices and dispatches discrete events
- Game logic systems that maintain state and respond to events by updating that state
- A renderer that draws the current frame by accessing game state data
- A loader that handles asynchronous asset management
Compared to the simpler single game loop, extra complexity is introduced by the addition of queues, dispatchers, caches, and timing logic to facilitate the communication between decoupled systems.
Pseudocode for a simplified decoupled game loop may look like:
while (gameIsRunning):
input.update()
for (each gameLogicSystem):
gameLogicSystem.update()
renderScene()
The specifics of each stage may be something like:
updateInput():
for each inputDevice in devices:
poll device
queueEvents(device.getNewEvents)
dequeue events
dispatch events to relevant systems
updateGameSystems(elapsedTime):
for each game system:
process events
update logic based on delta time
submit rendering data
renderScene():
sort/optimize rendering data
draw calls to video device
draw UI
The individual event, logic, and render updates for that frame are batched together for efficiency, but the key game systems can be developed and optimized independently.
Facilitating Communication Between Systems
In order for independently running systems to communicate effectively, a lightweight messaging API built on top of an event queue works very well to dispatch messages between handlers:
/***** Messaging API ******/
void sendMessage(string topic, Message message) {
messageQueue.push(MessageEnvelope(topic, message))
}
void registerHandler(string topic, function handler) {
handlers[topic] = handler;
}
/***** System A ********/
void OnCollisionDetected(Collision data) {
//do stuff
sendMessage("collision", data);
}
/***** System B *******/
function onCollision(Collision data) {
//react to collision
}
registerHandler("collision", onCollision)
This above publish-subscribe pattern provides a straightforward model for emitting events without coupling the emitter to any direct dependencies.
Alternatives that overload method parameters or embed dependencies directly risk increased coupling. The event queue and registered handlers act as an abstraction layer that loosens connections between systems.
Care should be take to design the message payload data structures for flexibility and forwards compatibility as the needs change over the development process.
Example Code for a Decoupled Game Loop
Here is one way to implement the structure of a decoupled game loop using classes in Python. This demonstrates the separation of input handling, game logic updates, and rendering into distinct phases.
///////////////////////////
/***** Input Handler *****/
///////////////////////////
class InputManager:
def __init__(self):
self.keyDownEvents = []
self.keyUpEvents = []
def update(self):
# Scan hardware for new events
newEvents = getInputEvents()
# Queue event data internally
for event in newEvents:
if event.type == KEY_DOWN:
keyDownEvents.put(event)
elif event.type == KEY_UP:
keyUpEvents.put(event)
def getKeyDownEvents(self):
return keyDownEvents
def getKeyUpEvents(self):
return keyUpEvents
//////////////////////////////
/***** Game Logic System *****/
//////////////////////////////
class MovementSystem:
def __init__(self):
self.positions = {}
self.messageBus = MessageBus()
def update(self, dt):
# React to inputs
for event in inputManager.getKeyDownEvents():
if event.key == LEFT:
self.positions[player].x -= 1
# Additional event handling
# Additional game logic
self.messageBus.postMessage("positionUpdated", self.positions[player])
/////////////////////////////////
/***** Rendering System *******/
/////////////////////////////////
class Renderer:
def __init__(self):
self.positions = {}
def handleEvent(self, message):
if message.type == "positionUpdated":
self.positions = message.body
def draw(self):
clear()
for entity, pos in self.positions.items():
drawSprite(entity, pos)
presentScene()
////////////////////////////////
/***** Main Loop *******/
////////////////////////////////
input = InputManager()
movement = MovementSystem()
renderer = Renderer()
while gameIsRunning:
input.update()
movement.update(dt)
renderer.draw()
This structure demonstrates how the input, logic, and rendering phases can be separated while still communicating through a shared event bus.
Optimizing Performance of a Decoupled Game Loop
While there are significant software engineering benefits to a decoupled game architecture, the downside is often decreased runtime performance due to cache misses and cost of dispatching across interfaces between systems.
Some strategies to optimize performance within a decoupled game loop are:
- Keep related data spatially close in memory to avoid cache misses during access
- Parallelize independent tasks across threads
- Reduce number of event dispatches by batching updates
- Pool memory for dispatcher internals
- Profile slow paths to locate bottlenecks
Managing the tradeoff between peak performance and maintainability is key. The flexibility gains from loosely coupled game logic may outweigh modest losses in speed.
However, for certain systems like the renderer that get called every frame, it may make sense to keep that module more tightly integrated to avoid dispatch costs. Finding the right balance based on project needs is important.
Next Steps for Learning
For readers interested to learn more about decoupled game architecture and implement their own variants, here are some suggested resources:
- Glenn Fiedler’s Game Engine Architecture Blog
- Jason Gregory’s Game Engine Architecture book
- The Entity-Component-System architectural pattern
- Retention-based entity systems
- Unreal’s Component system reference
There are many architectural patterns like ECS that complement decoupled designs as well. Learning more about these industry proven patterns will help guide optimal engineering decisions when implementing a custom engine.
Key Takeaways
- Decoupling game logic and rendering from input allows the components to scale independently and enables better code quality over time.
- Must balance flexibility gains against runtime performance considerations around caching and dispatch costs when evaluating integration points.
- Loosely coupled game architecture opens up more possibilities for innovation and custom tailoring to project needs.
