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UNREAL_UPGRADES.md

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Here are the steps I take when upgrading uasset-rs to a new version:

  1. Install the new engine version from Epic Games Store (in this case, 5.1.1)
  2. Copy the assets from the previous version into a new project in the new engine version (i.e. assets/UE50)
  3. Open the project in the new engine version
  4. Re-save all the assets with the new engine
  5. Copy the re-saved assets into the assets directory (i.e. assets/UE51)
  6. Run the commandline tool against the test assets (cargo run --features=commandline-tool validate .\assets\UE51\)
  7. See if anything goes wrong and if so, address it

That last bullet point of course is doing a lot of work, and it's got to be considered on a case-by-case basis. I'll go through what I did for 5.1.

The immediate failure is this:

.\assets\UE51\DirectCycle\DirectCycleA.uasset: Could not parse asset: failed to parse .\assets\UE51\DirectCycle\DirectCycleA.uasset: UnsupportedUE5Version(1008)

This means that EUnrealEngineObjectUE5Version from Engine/Source/Runtime/Core/Public/UObject/ObjectVersion.h has a new value, 1008! If you open up ObjectVersion.h from the 5.1 branch, you'll see that there are two new entries that I then add to ObjectVersionUE5.

The next step is to add tests that automatically try to load the newly added assets. I go to the test_utilities and add a new test case for 5.1 with the expected ObjectVersionUE5::ADD_SOFTOBJECTPATH_LIST version. A quick cargo test will reveal these tests as failing, because the new object version requires appropriate migration code.

The FSOFTOBJECTPATH_REMOVE_ASSET_PATH_FNAMES migration you can find in FSoftObjectPath::SerializePathWithoutFixup -- it looks like a field that used to be serialized as an FName is now serialized as an FString. We don't currently do any serialization of FSoftObjectPath, so we don't need to make any changes for this.

Reviewing ADD_SOFTOBJECTPATH_LIST, we can find a reference to it in FPackageFileSummary's serialization code. This is relevant to us, as this is a structure we serialize. The format of the serialization is what is called "indirect array" serialization in uasset-rs. It means that it's serializing a list of entries by encoding a count & offest in the header of the asset, and you can seek to that offset to deserialize the relevant entries (FSoftObjectPath in this case). We don't have any immediate use for this header data, and we haven't added FSoftObjectPath deserialization yet, so we leave it as-is and just load the int32 values into AssetHeader. Run the tests again, and tada! It successfully loads all the old and the new 5.1 assets correctly.

Next up I need to do the same for 5.2 and 5.3, but not today.