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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 (which is fine) however the second part will have LoadAction=Clear which effectively 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 may differ between devices:
    * those configured for debugging or at least currently debugging - can't see anything happening in the game but input is accepted and things continue to work
    * 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) but it seems to be safe to catch and ignore this exception.

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