---
title: "waitForFence(_:)"
framework: metal
role: symbol
role_heading: Instance Method
path: "metal/mtlaccelerationstructurecommandencoder/waitforfence(_:)"
---

# waitForFence(_:)

Encodes a command that instructs the GPU to pause the acceleration structure pass until another pass updates a fence.

## Declaration

```swift
func waitForFence(_ fence: any MTLFence)
```

## Parameters

- `fence`: A fence that the pass waits for before it runs any of its commands.

## Discussion

Discussion You can synchronize memory operations of an acceleration structure pass that access resources with an MTLFence. This method instructs the GPU to wait until another pass updates fence before running the acceleration structure pass. The fence indicates when the pass can access those resources without a race condition. For more information about synchronization with fences, see: Resource synchronization Synchronizing passes with a fence Reuse a fence by waiting first and updating second When encoding an acceleration structure pass that reuses a fence, wait for other passes to update the fence before repurposing that fence to notify subsequent passes with an update: Call the waitForFence(_:) method before encoding commands that need to wait for other passes. Call the updateFence(_:) method after encoding commands that later passes depend on. The GPU driver evaluates the fences that apply to the pass and the commands that depend on those fences when your app commits the enclosing MTLCommandBuffer. warning: Don’t update a fence and then wait for the same fence within a pass because it can create a GPU deadlock.

## See Also

### Preventing resource access conflicts

- [updateFence(_:)](metal/mtlaccelerationstructurecommandencoder/updatefence(_:).md)