[Game Programming] Game Loop Pattern

One-line pattern summary

The core execution pattern of games that maintains a stable input-update-render loop.

Typical Unity use cases

When implementing a fixed-step simulation.
When input timing and render timing should be separated.

Parts (roles)

Input Step
Simulate Step
Render Step

Unity example (C#)

The code below is a simplified Unity example based on the scenario described above.

using UnityEngine;

public sealed class FixedStepLoop : MonoBehaviour
{
    private const float SimulationStep = 1f / 60f;
    private float accumulatedDeltaTime;

    private void Update()
    {
        accumulatedDeltaTime += Time.deltaTime;
        while (accumulatedDeltaTime >= SimulationStep)
        {
            Simulate(SimulationStep);
            accumulatedDeltaTime -= SimulationStep;
        }
        float interpolationAlpha = accumulatedDeltaTime / SimulationStep;
        Render(interpolationAlpha);
    }

    private void Simulate(float deltaTime) { }

    private void Render(float interpolationAlpha) { }
}

Advantages

A fixed update order improves simulation reproducibility and makes debugging safer.
Separating fixed-step updates from interpolation helps balance physics accuracy and visual smoothness.

Things to watch out for

On heavy frames, the while loop can grow too long and lead to a spiral of death.
If responsibilities between Update and FixedUpdate are mixed, input latency and physics errors can increase.

Interaction diagram

This is the main loop that repeats input, simulation, and render on a frame basis.

flowchart LR

  start["Frame Begin"]
  input["Poll Input"]
  fixed_step["Fixed Update / Physics"]
  game_update["Gameplay Update"]
  render["Render"]
  next["Next Frame"]

  start --> input
  input --> fixed_step
  fixed_step --> game_update
  game_update --> render
  render --> next
  next -->|loop| start