Game Loop

Game Loop

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.