new in Unreal Engine 5.4: data validation quick fixes

a few days ago I got a really cool change into Unreal, which allows you to add quick fixes to any data validation warning/error you emit:
you can get this change in…
- Git commit 0a6de35a5beb8534e19579cf1058460b4eb2bc79
- Perforce changelist 31126352
the API revolves around the UE::DataValidation::IFixer type. fixers are pieces of code that can fix your issues given their application conditions are met
- the interface you have to implement is:
  
  struct IFixer { virtual EFixApplicability GetApplicability(int32 FixIndex) const = 0; virtual FFixResult ApplyFix(int32 FixIndex) = 0; }
  - for the curious cats among us: FixIndex is an arbitrary number that can be passed to FFixToken, to handle multiple different fixes with one fixer
- first, the editor calls GetApplicability to know whether the fix can be applied
  - EFixApplicability::CanBeApplied should be returned if the fix can be applied at the moment
  - EFixApplicability::Applied should be returned if the fix has been applied and can no longer be applied anymore
  - EFixApplicability::DidNotApply should be returned if the fix was not applied and can no longer be applied anymore
- then, if GetApplicability() returns EFixApplicability::CanBeApplied, ApplyFix is called to actually run the fix
  - this function is safe to assume the caller has ensured that the fix CanBeApplied and therefore need not do any extra validity checks
  - other than that it’s free to do whatever it wants
  - after a fix is applied, the fixer returns an FFixResult which is used to display a notification to the user informing them of the changes
- fortunately you don’t have to define a new struct for each fix, and instead can use UE::DataValidation::TLambdaFixer together with the UE::DataValidation::MakeFix function for more convenience

once you have a fixer, you can display it in your (FTokenizedMessage-based) validation messages using FFixToken

minimal example:

auto Message = FTokenizedMessage::Create(
    EMessageVerbosity::Error,
    LOCTEXT(
        "FoundNoneEntries",
        "Found None entries in ability array."
        "Please remove them, otherwise they may cause crashes during runtime."
    )
);
Message->AddToken(
    UE::DataValidation::MakeFix(
        [this]
        {
            Abilities.SetNum(Algo::RemoveIf(
                Abilities,
                [](const UGameplayAbility* Ability) { return Ability == nullptr; }
            ));
            return FFixResult::Success(LOCTEXT("NoneEntriesRemoved", "None entries have been removed"));
        }
    )
    ->CreateToken(LOCTEXT("RemoveNoneEntries", "Remove the None entries"))
);
Context.AddMessage(MoveTemp(Message));

the user can then apply a fix by clicking on it in the Message Log message that contains it
fixers can be freely stacked and composed - there are a few such layers available out of the box in the engine
- the set is quite limited at the moment, but you’re free to create your own or contribute them to mainline Unreal
- UE::DataValidation::FSingleUseFixer makes it so that your fix’s applicability becomes EFixApplicability::Applied after the user applies the fix
- UE::DataValidation::FObjectSetDependentFixer makes it so that your fix becomes DidNotApply after the specified objects are deleted from memory
- UE::DataValidation::FAutoSavingFixer tells the user to save any assets modified by the fix after it’s applied
- UE::DataValidation::FValidatingFixer runs data validation on any assets modified by the fix after it’s applied
- UE::DataValidation::FMutuallyExclusiveFixSet is actually not a fixer, but a fixer builder - you give it a set of fixers, and it will make it so that when one is applied, it becomes EFixApplicability::Applied, and the rest becomes EFixApplicability::DidNotApply - thus creating a set of mutually-exclusive fixes
- you can refer to the DataValidationFixers.h header file for more documentation
all of this is going to be available in Unreal 5.4