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====== 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> ((observed at least on 2018.2.21 when exiting play mode and sometimes while paused in editor)), 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.

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.

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