====== AsyncGPUReadback ======

===== Metal/iOS =====

In at least 2018.2 and possibly other versions there appears to be an undocumented bug that readback requests mess with the command buffer:

  * Readback requests inserted in the middle will split the command buffer((this is reasonable)), however the second part will have LoadAction=Clear which destroys everything rendered before the request
  * Readback requests inserted at the end will make Unity skip ''presentDrawable'' which results in the screen not being updated, and the behavior differs between devices:
    * Those configured for debugging or currently debugging: nothing renders but input is accepted and the game continues to run.
    * Others: crash caused by an OS timeout

The workaround is to move the readback request to the beginning of the next frame.

===== General =====

==== Random failures ====

In some cases<sup>[//[[:wiki:tag:research-needed|research needed]]//]</sup>, the callback will be called but when attempting to call [[https://docs.unity3d.com/ScriptReference/Rendering.AsyncGPUReadbackRequest.GetData.html|GetData]], it will throw an exception.((observed at least on 2018.2.21 when exiting play mode and sometimes while paused in editor))

The cause is unknown, it does not appear to be detectable beforehand. All [[https://docs.unity3d.com/ScriptReference/Rendering.AsyncGPUReadbackRequest.html|AsyncGPUReadbackRequest]] properties indicate that it's a valid completed request. It seems to be safe to catch and ignore this exception.