Waiting for FixedUpdate Physics Cycles

In this lesson, you will learn how to synchronize your custom logic with the physics engine by using WaitForFixedUpdate within a Coroutine.

Why Physics Needs Its Own Rhythm

As you’ve learned, Update() runs every frame, while FixedUpdate() runs on a rigid, predictable timer specifically for physics calculations. Sometimes, your game logic needs to perform an action—like applying an explosive force or checking a sensor—immediately after the physics engine has finished its calculations for the current step.

If you try to run this in a standard Coroutine using yield return null, you are waiting for the next frame, which might happen before or after the physics engine has processed its math. By using new WaitForFixedUpdate(), you instruct your code to pause exactly until the end of the current FixedUpdate cycle.

The Implementation

When you yield a WaitForFixedUpdate instruction, Unity suspends your Coroutine and resumes it only after all physics calculations for that specific time step have been completed. This ensures that any data you read—such as a Rigidbody velocity or collision state—is perfectly up-to-date with the engine's internal physics step.

Here is how you implement it:

IEnumerator PhysicsSyncRoutine()
{
    while (true)
    {
        // Wait until the physics engine finishes its calculations
        yield return new WaitForFixedUpdate();

        // Now it is safe to read physics data 
        // knowing it aligns with the most recent FixedUpdate step
        Debug.Log("Physics cycle complete. Current velocity is: " + rb.velocity);
    }
}

Why Use This?

This technique is particularly useful for mechanics that depend on stable physics results, such as complex hovering systems, procedural character controllers, or physics-based triggers that require high precision. By yielding to the physics clock, you avoid the "jitter" that occurs when game logic and physics calculations drift out of sync. Using this method keeps your gameplay consistent, regardless of how much your frame rate fluctuates on the user's machine.

Key Takeaway: Use yield return new WaitForFixedUpdate() in your Coroutines to pause execution until the physics engine finishes its current cycle, ensuring your logic always works with the most accurate and stable physics state.