This blog post was written in reference to Unreal Engine 5.8

Overview

Traits(UMassEntityTraitBase) are UObject type’s, declared Abstract and EditInlineNew, that allow for designers to configure Entity Template’s and configure their default values within the editor. You can add not just Fragment configurations but also specific parameters to additionally configure the Trait for a better UX.

A Trait is not an asset on its own. Traits are Instanced subobjects held by an FMassEntityConfig, and the UDataAsset in this system is UMassEntityConfigAsset, which wraps that config. So the workflow is: author a Mass Entity Config asset, then add Trait instances inline to its Traits array. Config assets can also inherit from a Parent config, which is how Trait sets get shared rather than by referencing a Trait as an asset.

Traits are also where you can add default Tags and Fragments to an Entity while its Entity Template (FMassEntityTemplateData) is being built, by overriding ::BuildTemplate() on the Trait’s type. Every Trait writes into a shared FMassEntityTemplateBuildContext, and only once all of them have contributed does Mass hand the finished composition to FMassEntityManager::CreateArchetype() and store the resulting FMassArchetypeHandle on the finalized FMassEntityTemplate. So Traits configure the Template, and the Archetype is derived from it afterwards.

void UMyTrait::BuildTemplate(FMassEntityTemplateBuildContext& BuildContext, const UWorld& World) const
{
    BuildContext.AddFragment<FTransformFragment>();

    BuildContext.AddTag<FMyTag>();
}

/**
* Adding ==================
* Fragments:
* - FMyFragment
*
* Const Shared Fragments:
* - FMyConstSharedFragment
*
* Shared Fragments:
* - FMySharedFragment
*
* Tags:
* - FMyTag1
* - FMyTag2
*
* Requires ================
* Fragments:
* - FTransformFragment
*
* Tags:
* - FMyTag3
*/
UCLASS()
class UMyTrait : public UMassEntityTraitBase
{
    GENERATED_BODY()
public:

    // UMassEntityTraitBase overrides
    virtual void BuildTemplate(FMassEntityTemplateBuildContext& BuildContext, const UWorld& World) const override
    {
        FMassEntityManager& EntityManager = UE::Mass::Utils::GetEntityManagerChecked(World);
        
        BuildContext.AddConstSharedFragment(EntityManager.GetOrCreateConstSharedFragment(FMyConstSharedFragment()));

        BuildContext.AddSharedFragment(EntityManager.GetOrCreateSharedFragment(FMySharedFragment()));

        BuildContext.AddFragment<FMyFragment>();

        BuildContext.AddTag<FMyTag1>();
        BuildContext.AddTag<FMyTag2>();
    }
    // ~UMassEntityTraitBase overrides
};