README.md

Community Mass Sample

Authors:

• Karl Mavko - @Megafunk
• Alvaro Jover - @vorixo

Our very WIP understanding of Unreal Engine 5's experimental Entity Component System (ECS) plugin with a small sample project. We are not affiliated with Epic Games and this system is actively being changed often so this information might not be totally accurate.

We are totally open to contributions, If something is wrong or you think it could be improved, feel free to open an issue or submit a pull request.

Currently built for the Unreal Engine 5 latest version binary from the Epic Games launcher. This documentation will be updated often!

Requirements:

• Unreal Engine 5.0.1 (latest version as of writing) from the Epic Games launcher
• Git version control:
  • Windows
  • Linux/Unix & macOS
• Git Large File Storage

Download instructions (Windows):

After installing the requirements from above, follow these steps:

1. Right-Click where you wish to hold your project, then press Git Bash Here.

2. Within the terminal, clone the project:

   git clone https://github.com/Megafunk/MassSample.git

3. Pull LFS:

   git lfs pull

4. Once LFS finishes, close the terminal.

Note: This project requires Git LFS for it to work properly, zip downloads won't work.

Table of Contents

1. Mass
2. Entity Component System
3. Sample Project
4. Mass Concepts
   4.1 Entities
   4.2 Fragments
         4.2.1 Shared Fragments
   4.3 Tags
   4.4 The archetype model
         4.4.1 Tags in the archetype model
         4.4.2 Fragments in the archetype model
   4.5 Processors
   4.6 Queries
         4.6.1 Access requirements
         4.6.2 Presence requirements
         4.6.3 Iterating Queries
         4.6.3 Mutating entities with Defer()
   4.7 Traits
   4.8 Observers
         4.8.1 Observing multiple Fragment/Tags
   4.10 Mulitthreading
5. Mass common operations
   5.1 Spawning an entity
   5.2 Destroying an entity
6. Mass Plugins and Modules
   6.1 MassEntity
   6.2 MassGameplay
   6.3 MassAI
7. Other Resources

1. Mass

Mass is Unreal's new in-house ECS framework! Technically, Sequencer already used one internally but it wasn't intended for gameplay code. Mass was created by the AI team at Epic Games to facilitate massive crowd simulations, but has grown to include many other features as well. It was featured in the new Matrix demo Epic released recently.

2. Entity Component System

Mass is an archetype-based Entity Componenet System. If you already know what that is you can skip ahead to the next section.

In Mass, some ECS terminology differs from the norm in order to not get confused with existing unreal code:

ECS	Mass
Entity	Entity
Component	Fragment
System	Processor

Typical Unreal Engine game code is expressed as actor objects that inherit from parent classes to change their data and functionality based on what they are. In an ECS, an entity is only composed of fragments that get manipulated by processors based on which ECS components they have.

An entity is really just a small unique identifier that points to some fragments. A Processor defines a query that filters only for entities that have specific fragments. For example, a basic "movement" Processor could query for entities that have a transform and velocity component to add the velocity to their current transform position.

Fragments are stored in memory as tightly packed arrays of other identical fragment arrangements called archetypes. Because of this, the aforementioned movement processor can be incredibly high performance because it does a simple operation on a small amount of data all at once. New functionality can easily be added by creating new fragments and processors.

Internally, Mass is similar to the existing Unity DOTS and FLECS archetype-based ECS libraries. There are many more!

3. Sample Project

Currently, the sample features the following:

• A bare minimum movement processor to show how to set up processors.
• An example of how to use Mass spawners for zonegraph and EQS.
• Mass-simulated crowd of cones that parades around the level following a ZoneGraph shape with lanes.
• Linetraced projectile simulation example.
• Simple 3d hashgrid for entities.
• Very basic Mass blueprint integration.
• Grouped niagara rendering for entities.

4. Mass Concepts

Sections

4.1 Entities
4.2 Fragments
4.3 Tags
4.4 The archetype model
4.5 Processors
4.6 Queries
4.7 Traits
4.8 Observers

4.1 Entities

Small unique identifiers that point to a combination of fragments and tags in memory. Entities are mainly a simple integer ID. For example, entity 103 might point to a single projectile with transform, velocity, and damage data.

4.2 Fragments

Data-only UScriptStructs that entities can own and processors can query on. To create a fragment, inherit from FMassFragment.

USTRUCT()
struct MASSSAMPLE_API FLifeTimeFragment : public FMassFragment
{
	GENERATED_BODY()
	float Time;
};

With FMassFragments each entity gets its own fragment data, however if we wish to share data across all our entities, we have to use a shared fragment.

4.2.1 Shared Fragments

A Shared Fragment is a type of Fragment that multiple entities can point to. This is often used for configuration common to a group of entities, like LOD or replication settings. To create a shared fragment, inherit from FMassSharedFragment.

USTRUCT()
struct MASSSAMPLE_API FClockSharedFragment : public FMassSharedFragment
{
	GENERATED_BODY()
	float Clock;
};

In the example above, all the entities containing the FClockSharedFragment will see the same Clock value. If an entity modifies the Clock value, the rest of the entities with this fragment will see the change, as this fragment is shared accross them.

Thanks to this sharing data requirement, the Mass Entity subsystem only needs to store one Shared Fragment for the entities that use it.

4.3 Tags

Empty UScriptStructs that processors can use to filter entities to process based on their presence/absence. To create a tag, inherit from FMassTag.

USTRUCT()
struct MASSSAMPLE_API FProjectileTag : public FMassTag
{
	GENERATED_BODY()
};

Note: Tags should never contain any member properties.

4.4 The archetype model

As mentioned previously, an entity is a unique combination of fragments and tags. Mass calls each of these combinations archetypes. For example, given three different combinations used by our entities, we would generate three archetypes:

The FMassArchetypeData struct represents an archetype in Mass internally.

4.4.1 Tags in the archetype model

Each archetype (FMassArchetypeData) holds a bitset (TScriptStructTypeBitSet<FMassTag>) that constains the tag presence information, whereas each bit in the bitset represents whether a tag exists in the archetype or not.

Following the previous example, Archetype 0 and Archetype 2 contain the tags: TagA, TagC and TagD; while Archetype 1 contains TagC and TagD. Which makes the combination of Fragment A and Fragment B to be split in two different archetypes.

4.4.2 Fragments in the archetype model

At the same time, each archetype holds an array of chunks (FMassArchetypeChunk) with fragment data.

Each chunk contains a subset of the entities included in our archetype where data is organized in a pseudo-struct-of-arrays way:

The following Figure represents the archetypes from the example above in memory:

By having this pseudo-struct-of-arrays data layout divided in multiple chunks, we are allowing a great number of whole-entities to fit in the CPU cache.

This is thanks to the chunk partitoning, since without it, we wouldn't have as many whole-entities fit in cache, as the following diagram displays:

In the above example, the Chunked Archetype gets whole-entities in cache, while the Linear Archetype gets all the A Fragments in cache, but cannot fit each fragment of an entity.

The Linear approach would be fast if we would only access the A Fragment when iterating entities, however, this is almost never the case. Usually, when we iterate entities we tend to access multiple fragments, so it is convenient to have them all in cache, which is what the chunk partitioning provides.

The chunk size (UE::MassEntity::ChunkSize) has been conveniently set based on next-gen cache sizes (128 bytes per line and 1024 cache lines). This means that archetypes with more bits of fragment data will contain less entities per chunk.

Note: It is relevant to note that a cache miss would be produced every time we want to access a fragment that isn't on cache for a given entity.

4.5 Processors

Processors combine multiple user-defined queries with functions that compute entities.

Processors are automatically registered with Mass and added to the EMassProcessingPhase::PrePhsysics processing phase by default. Each EMassProcessingPhase relates to an ETickingGroup, meaning that, by default, processors tick every frame in their given processing phase.

Users can configure to which processing phase their processor belongs by modifying the ProcessingPhase variable included in UMassProcessor:

EMassProcessingPhase	Related ETickingGroup	Description
PrePhysics	TG_PrePhysics	Executes before physics simulation starts.
StartPhysics	TG_StartPhysics	Special tick group that starts physics simulation.
DuringPhysics	TG_DuringPhysics	Executes in parallel with the physics simulation work.
EndPhysics	TG_EndPhysics	Special tick group that ends physics simulation.
PostPhysics	TG_PostPhysics	Executes after rigid body and cloth simulation.
FrameEnd	TG_LastDemotable	Catchall for anything demoted to the end.

In their constructor, processors can define rules for their execution order, the processing phase and which type of game clients they execute on:

UMyProcessor::UMyProcessor()
{
	// This processor is registered with mass by just existing! This is the default behaviour of all processors.
	bAutoRegisterWithProcessingPhases = true;
	// Setting the processing phase explicitly
	ProcessingPhase = EMassProcessingPhase::PrePhysics;
	// Using the built-in movement processor group
	ExecutionOrder.ExecuteInGroup = UE::Mass::ProcessorGroupNames::Movement;
	// You can also define other processors that require to run before or after this one
	ExecutionOrder.ExecuteAfter.Add(TEXT("MSMovementProcessor"));
	// This executes only on Clients and Standalone
	ExecutionFlags = (int32)(EProcessorExecutionFlags::Client | EProcessorExecutionFlags::Standalone);
}

On initialization, Mass creates a dependency graph of processors using their execution rules so they execute in order (ie: In the example above we make sure to move our entities before we call Execute in UMyProcessor).

Note: Mass ships with a series of processors that are designed to be inherited and extended with custom logic. ie: The visualization and LOD processors.

4.6 Queries

Queries (FMassEntityQuery) filter and iterate entities given a series of rules based on Fragment and Tag presence.

Processors can define multiple FMassEntityQuerys and should override the ConfigureQueries to add rules to the different queries defined in the processor's header:

void UMyProcessor::ConfigureQueries()
{
	MyQuery.AddTagRequirement<FMoverTag>(EMassFragmentPresence::All);
	MyQuery.AddRequirement<FHitLocationFragment>(EMassFragmentAccess::ReadOnly, EMassFragmentPresence::Optional);
}

Queries are executed by calling the ForEachEntityChunk member function with a lambda, passing the related UMassEntitySubsystem and FMassExecutionContext.

Processors execute queries within their Execute function:

void UMyProcessor::Execute(UMassEntitySubsystem& EntitySubsystem, FMassExecutionContext& Context)
{
	//Note that this is a lambda! If you want extra data you may need to pass it in the [] operator
	MyQuery.ForEachEntityChunk(EntitySubsystem, Context, [](FMassExecutionContext& Context)
	{
		//Loop over every entity in the current chunk and do stuff!
		for (int32 EntityIndex = 0; EntityIndex < Context.GetNumEntities(); ++EntityIndex)
		{
			// ...
		}
	});
}

Be aware that the index we employ to iterate entities, in this case EntityIndex, doesn't identify uniquely your entities along time, since chunks' disposition may change and an entity that has an index this frame, may be in a different chunk with a different index in the next frame.

Note: Queries can also be created and iterated outside processors.

4.6.1 Access requirements

Queries can define read/write access requirements for Fragments:

EMassFragmentAccess	Description
None	No binding required.
ReadOnly	We want to read the data for the fragment.
ReadWrite	We want to read and write the data for the fragment.

FMassFragments use AddRequirement to add access and presence requirement to our fragments. While FMassSharedFragments employ AddSharedRequirement.

Here are some basic examples in which we add access rules in two Fragments from a FMassEntityQuery MyQuery:

void UMyProcessor::ConfigureQueries()
{
	// Entities must have an FTransformFragment and we are reading and writing it (EMassFragmentAccess::ReadWrite)
	MyQuery.AddRequirement<FTransformFragment>(EMassFragmentAccess::ReadWrite);
		
	// Entities must have an FMassForceFragment and we are only reading it (EMassFragmentAccess::ReadOnly)
	MyQuery.AddRequirement<FMassForceFragment>(EMassFragmentAccess::ReadOnly);

	// Entities must have a common FClockSharedFragment that can be read and written
	MyQuery.AddSharedRequirement<FClockSharedFragment>(EMassFragmentAccess::ReadWrite);
}

ForEachEntityChunks can use the following two functions to access ReadOnly or ReadWrite fragment data according to the access requirement:

EMassFragmentAccess	Function	Description
ReadOnly	GetFragmentView	Returns a read only TConstArrayView containing the data of our ReadOnly fragment.
ReadWrite	GetMutableFragmentView	Returns a writable TArrayView containing de data of our ReadWrite fragment.

Find below the following two functions employed in context:

MyQuery.ForEachEntityChunk(EntitySubsystem, Context, [](FMassExecutionContext& Context)
{
	const auto TransformList = Context.GetMutableFragmentView<FTransformFragment>();
	const auto ForceList = Context.GetMutableFragmentView<FMassForceFragment>();

	for (int32 EntityIndex = 0; EntityIndex < Context.GetNumEntities(); ++EntityIndex)
	{
		FTransform& TransformToChange = TransformList[EntityIndex].GetMutableTransform();
		const FVector DeltaForce = Context.GetDeltaTimeSeconds() * ForceList[EntityIndex].Value;
		TransformToChange.AddToTranslation(DeltaForce);
	}
});

Note: Tags do not have access requirements since they don't contain data.

4.6.2 Presence requirements

Queries can define presence requirements for Fragments and Tags:

EMassFragmentPresence	Description
All	All of the required fragments/tags must be present. Default presence requirement.
Any	At least one of the fragments/tags marked any must be present.
None	None of the required fragments/tags can be present.
Optional	If fragment/tag is present we'll use it, but it does not need to be present.

4.6.2.1 Presence requirements in Tags

To add presence rules to Tags, use AddTagRequirement.

void UMyProcessor::ConfigureQueries()
{
	// Entities are considered for iteration without the need of containing the specified Tag
	MyQuery.AddTagRequirement<FOptionalTag>(EMassFragmentPresence::Optional);
	// Entities must at least have the FHorseTag or the FSheepTag
	MyQuery.AddTagRequirement<FHorseTag>(EMassFragmentPresence::Any);
	MyQuery.AddTagRequirement<FSheepTag>(EMassFragmentPresence::Any);
}

ForEachChunks can use DoesArchetypeHaveTag to determine if the current archetype contains the the Tag:

MyQuery.ForEachEntityChunk(EntitySubsystem, Context, [](FMassExecutionContext& Context)
{
	if(Context.DoesArchetypeHaveTag<FOptionalTag>())
	{
		// I do have the FOptionalTag tag!!
	}

	// Same with Tags marked with Any
	if(Context.DoesArchetypeHaveTag<FHorseTag>())
	{
		// I do have the FHorseTag tag!!
	}
	if(Context.DoesArchetypeHaveTag<FSheepTag>())
	{
		// I do have the FSheepTag tag!!
	}
});

4.6.2.2 Presence requirements in Fragments

Fragments and shared fragments can define presence rules in an additional EMassFragmentPresence parameter through AddRequirement and AddSharedRequirement, respectively.

void UMyProcessor::ConfigureQueries()
{
	// Entities are considered for iteration without the need of containing the specified Fragment
	MyQuery.AddRequirement<FMyOptionalFragment>(EMassFragmentAccess::ReadWrite, EMassFragmentPresence::Optional);
	// Entities must at least have the FHorseFragment or the FSheepFragment
	MyQuery.AddRequirement<FHorseFragment>(EMassFragmentAccess::ReadWrite, EMassFragmentPresence::Any);
	MyQuery.AddRequirement<FSheepFragment>(EMassFragmentAccess::ReadWrite, EMassFragmentPresence::Any);
}

ForEachChunks can use the length of the Optional/Any fragment's TArrayView to determine if the current chunk contains the Fragment before accessing it:

MyQuery.ForEachEntityChunk(EntitySubsystem, Context, [](FMassExecutionContext& Context)
{
	const auto OptionalFragmentList = Context.GetMutableFragmentView<FMyOptionalFragment>();
	const auto HorseFragmentList = Context.GetMutableFragmentView<FHorseFragment>();	
	const auto SheepFragmentList = Context.GetMutableFragmentView<FSheepFragment>();
	for (int32 i = 0; i < Context.GetNumEntities(); ++i)
	{
		// An optional fragment array is present in our current chunk if the OptionalFragmentList isn't empty
		if(OptionalFragmentList.Num() > 0)
		{
			// Now that we know it is safe to do so, we can compute
			OptionalFragmentList[i].DoOptionalStuff();
		}

		// Same with fragments marked with Any
		if(HorseFragmentList.Num() > 0)
		{
			HorseFragmentList[i].DoHorseStuff();
		}
		if(SheepFragmentList.Num() > 0)
		{
			SheepFragmentList[i].DoSheepStuff();
		}		
	}
});

4.6.3 Mutating entities with Defer()

Within the ForEachEntityChunk we have access to the current execution context. FMassExecutionContext enables us to get entity data and mutate their composition. The following code adds the tag FIsRedTag to any entity that has a color fragment with its Color property set to Red:

EntityQuery.ForEachEntityChunk(EntitySubsystem, Context, [&,this](FMassExecutionContext& Context)
{
	auto ColorList = Context.GetFragmentView<FSampleColorFragment>();

	for (int32 EntityIndex = 0; EntityIndex < Context.GetNumEntities(); ++EntityIndex)
	{

		if(ColorList[EntityIndex].Color == FColor::Red)
		{
			// Adding a tag to this entity after processing                                     
			Context.Defer().AddTag<FIsRedTag>(Context.GetEntity(EntityIndex));
		}
	}

});

4.6.3.1 Basic mutation operations

The following Listings define the native mutations that you can defer:

Fragments:

Context.Defer().AddFragment<FMyTag>(Entity);
Context.Defer().RemoveFragment<FMyTag>(Entity);

Tags:

Context.Defer().AddTag<FMyTag>(Entity);
Context.Defer().RemoveTag<FMyTag>(Entity);

Destroying entities:

Context.Defer().DestroyEntity(MyEntity);
Context.Defer().BatchDestroyEntities(MyEntitiesArray);

4.6.3.2 Advanced mutation operations

There is a set of FCommandBufferEntryBase commands that can be used to defer some more useful entity mutations. The following subsections provide an overview.

4.6.3.2.1 FMassCommandAddFragmentInstanceList

Defers a list of Fragment mutations over an entity using FStructViews and/or FConstStructViews.

In the example below we mutate the FHitResultFragment with HitResult data, and a FSampleColorFragment fragment with a new color.

FConstStructView HitResulStruct = FConstStructView::Make(FHitResultFragment(HitResult));
FStructView ColorStruct = FStructView::Make(FSampleColorFragment(Color));

Context.Defer().PushCommand(FMassCommandAddFragmentInstanceList(Entity, 
	{HitResulStruct, ColorStruct}
));

4.6.3.2.2 FCommandAddFragmentInstance (Singular)

Identical to FMassCommandAddFragmentInstanceList but it takes a single Fragment as input instead of a list.

FConstStructView HitResulStruct = FConstStructView::Make(FHitResultFragment(HitResult));

Context.Defer().PushCommand(FCommandAddFragmentInstance(Entity, HitResulStruct));

4.6.3.2.3 FBuildEntityFromFragmentInstances

Creates an entity given a list of initialized Fragments using FStructViews and/or FConstStructViews.

In the example below, we inline the FStructViews:

FSampleColorFragment ColorFragment;
ColorFragment.Color = FColor::Green;

FTransformFragment TransformFragment;
TransformFragment.SetTransform(SpawnTransform);

Context.Defer().PushCommand(FBuildEntityFromFragmentInstances(Entity,
	{FStructView::Make(ColorFragment),FStructView::Make(ThingyFragment)}
));

4.6.3.2.4 FBuildEntityFromFragmentInstance (singular)

Identical to FBuildEntityFromFragmentInstances but it takes a single Fragment as input instead of a list.

FConstStructView ColorStruct = FConstStructView::Make(FSampleColorFragment(HitResult));

Context.Defer().PushCommand(FBuildEntityFromFragmentInstance(Entity, ColorStruct));

4.6.3.2.5 FCommandSwapTags

Removes the first tag (FOffTag in this example) and adds the second to the entity. (FOnTag)

Context.Defer().PushCommand(FCommandSwapTags(Entity, 
	FOffTag::StaticStruct(), 
	FOnTag::StaticStruct()
	));

4.6.3.2.6 FCommandRemoveComposition

The FMassArchetypeCompositionDescriptor is a struct that defines a set of fragments and tags that make up an archetype. For example, we can get one from a given archetype handle or template. In this example we get one from a UMassEntityConfigAsset pointer.

const FMassEntityTemplate* EntityTemplate = 
	EntityConfig->GetConfig().GetOrCreateEntityTemplate(*Owner, *EntityConfig);
	
const FMassArchetypeCompositionDescriptor& Composition = EntityTemplate->GetCompositionDescriptor();
	
Context.Defer().PushCommand(FCommandRemoveComposition(Entity, Composition));

Note that the commands that mutate entities change the value of ECommandBufferOperationType in their decleration in order to pass their changes to relevant observers when commands are flushed. They also manually add their changes to the observed changes list by implementing AppendAffectedEntitiesPerType.

4.6.3.2.7 Custom mutation operations

It is possible to create custom mutations by implementing your own commands derived from FCommandBufferEntryBase.

Context.Defer().EmplaceCommand<FMyCustomComand>(...)

4.7 Traits

Traits are C++ defined objects that declare a set of Fragments, Tags and data for authoring new entities in a data-driven way.

To start using traits, create a DataAsset that inherits from MassEntityConfigAsset and add new traits to it. Each trait can be expanded to set properties if it has any.

In addition, it is possible to inherit Fragments from another MassEntityConfigAsset by setting it in the Parent field.

Between the many built-in traits offered by Mass, we can find the Assorted Fragments trait, which holds an array of FInstancedStruct that enables adding Fragments to this trait from the editor without the need of creating a new C++ Trait.

Traits are often used to add Shared Fragments in the form of settings. For example, our visualization traits save memory by sharing which mesh they are displaying, parameters etc. Configs with the same settings will share the same Shared Fragment.

4.7.1 Creating a trait

Traits are created by inheriting UMassEntityTraitBase and overriding BuildTemplate. Here is a very basic example:

UCLASS(meta = (DisplayName = "Debug Printing"))
class MASSSAMPLE_API UMSDebugTagTrait : public UMassEntityTraitBase
{
	GENERATED_BODY()
public:
	virtual void BuildTemplate(FMassEntityTemplateBuildContext& BuildContext, UWorld& World) const override
	{
		// Adding a tag
		BuildContext.AddTag<FMassSampleDebuggableTag>();
		
		// Adding a fragment
		BuildContext.AddFragment<FTransformFragment>();

		// _GetRef lets us mutate the fragment
		BuildContext.AddFragment_GetRef<FSampleColorFragment>().Color = UserSetColor;
	};

	// Editable in the editor
	UPROPERTY(EditAnywhere)
	FColor UserSetColor;
};

Note: We recommend looking at the many existing traits in this sample and the mass modules for a better overview. For the most part, they are fairly simple UObjects that occasionally have extra code to make sure the fragments are all valid and set correctly.

Shared Fragments

Here is a partial BuildTemplate example for adding a shared struct, which can do some extra work to see if a shared fragment identical to the new one already exists:

	//Create the actual fragment struct and set up the data for it however you like 
	FMySharedSettings MyFragment;
	MyFragment.MyValue = UserSetValue;

	//Get a hash of a FConstStructView of said fragment and store it
	uint32 MySharedFragmentHash = UE::StructUtils::GetStructCrc32(FConstStructView::Make(MyFragment));
	
	//Search the Mass Entity subsystem for an identical struct with the hash. If there are none, make a new one with the set fragment.
	FSharedStruct MySharedFragment = 
		EntitySubsystem->GetOrCreateSharedFragment<FMySharedSettings>(MySharedFragmentHash, MyFragment);

	//Finally, add the shared fragment to the BuildContext!
	BuildContext.AddSharedFragment(MySharedFragment);

4.7.2 Validating traits

There is also a ValidateTemplate overridable function which appears to just let you create your own validation for the trait that can raise errors or even change the buildcontext if need be. This is called after BuildTemplate is called for all of the traits of the current template.

In this snippet, we check if a field of the trait is null and print an error:

void UMSNiagaraRepresentationTrait::ValidateTemplate(FMassEntityTemplateBuildContext& BuildContext, UWorld& World) const
{
	//If our shared niagara system is null, show an error!
	if (!SharedNiagaraSystem)
	{
		UE_VLOG(&World, LogMass, Error, TEXT("SharedNiagaraSystem is null!"));
		return;
	}
}

4.8 Observers

The UMassObserverProcessor is a type of processor that operates on entities that have just performed a EMassObservedOperation over the Fragment/Tag type observed:

EMassObservedOperation	Description
Add	The observed Fragment/Tag was added to an entity.
Remove	The observed Fragment/Tag was removed from an entity.

Observers do not run every frame, but every time a batch of entities is changed in a way that fulfills the observer requirements.

For example, you could create an observer that handles entities that just had an FColorFragment added to change their color:

UMSObserverOnAdd::UMSObserverOnAdd()
{
	ObservedType = FSampleColorFragment::StaticStruct();
	Operation = EMassObservedOperation::Add;
	ExecutionFlags = (int32)(EProcessorExecutionFlags::All);
}

void UMSObserverOnAdd::ConfigureQueries()
{
	EntityQuery.AddRequirement<FSampleColorFragment>(EMassFragmentAccess::ReadWrite);
}

void UMSObserverOnAdd::Execute(UMassEntitySubsystem& EntitySubsystem, FMassExecutionContext& Context)
{
	EntityQuery.ForEachEntityChunk(EntitySubsystem, Context, [&,this](FMassExecutionContext& Context)
	{
		auto Colors = Context.GetMutableFragmentView<FSampleColorFragment>();
		for (int32 EntityIndex = 0; EntityIndex < Context.GetNumEntities(); ++EntityIndex)
		{
			// When a color is added, make it random!
			Colors[EntityIndex].Color = FColor::MakeRandomColor();
		}
	});
}

It is also possible to create queries to use during the execution process regardless the observed Fragment/Tag.

Note: Currently observers are only called during batched entity actions. This covers processors and spawners but not single entity changes from C++.

4.8.1 Observing multiple Fragment/Tags

Observers can also be used to observe multiple operations and/or types. For that, override the Register function in UMassObserverProcessor:

void UMyMassObserverProcessor::Register()
{
	check(ObservedType);
	check(MyObservedType);

	UMassObserverRegistry::GetMutable().RegisterObserver(*ObservedType, Operation, GetClass());
	UMassObserverRegistry::GetMutable().RegisterObserver(*ObservedType, MyOperation, GetClass());
	UMassObserverRegistry::GetMutable().RegisterObserver(*MyObservedType, MyOperation, GetClass());
	UMassObserverRegistry::GetMutable().RegisterObserver(*MyObservedType, Operation, GetClass());
	UMassObserverRegistry::GetMutable().RegisterObserver(*MyObservedType, EMassObservedOperation::Add, GetClass());
}

As noted above, it is possible to reuse the same EMassObservedOperation operation for multiple observed types, and vice-versa.

4.10 Multithreading

Out of the box Mass can spread out work to threads in two different ways:

• Per-Processor threading based on the processor dependency graph by setting the console variable mass.FullyParallel 1

• Per-query parrallel for calls that spread the job of one query over multiple threads by using the command argument ParallelMassQueries=1 for the given Unreal process. This is currently used nowhere in the Mass modules or sample and currently it seems to break when deferring commands from it multiple times a frame.

6. Mass Plugins and Modules

This Section overviews the three main Mass plugins and their different modules:

6.1 MassEntity
6.2 MassGameplay
6.3 MassAI

6.1 MassEntity

MassEntity is the main plugin that manages everything regarding Entity creation and storage.

6.2 MassGameplay

The MassGameplay plugin compiles a number of useful Fragments and Processors that are used in different parts of the Mass framework. It is divided into the following modules:

6.2.1 MassCommon
6.2.2 MassMovement
6.2.3 MassRepresentation
6.2.4 MassSpawner
6.2.5 MassActors
6.2.6 MassLOD
6.2.7 MassReplication
6.2.8 MassSignals
6.2.9 MassSmartObjects

6.2.1 MassCommon

Basic fragments like FTransformFragment.

6.2.2 MassMovement

Features an important UMassApplyMovementProcessor processor that moves entities based on their velocity and force. Also includes a very basic sample.

6.2.3 MassRepresentation

Processors and fragments for rendering entities in the world. They generally use an ISMC to do so.

6.2.4 MassSpawner

A highly configurable actor type that can spawn specific entities where you want.

6.2.5 MassActors

A bridge between the general UE5 actor framework and Mass. A type of fragment that turns entities into "Agents" that can exchange data in either direction (or both).

6.2.6 MassLOD

LOD Processors that can manage different kinds of levels of detail, from rendering to ticking at different rates based on fragment settings.

6.2.7 MassReplication

Replication support for Mass! Other modules override UMassReplicatorBase to replicate stuff. Entities are given a separate Network ID that gets passed over the network, rather than the EntityHandle. An example showing this is planned for much later.

6.2.8 MassSignals

A system that lets entities send named signals to each other.

6.2.9 MassSmartObjects

Lets entities "claim" SmartObjects to interact with them.

6.3 MassAI

MassAI is a plugin that provides AI features for Mass within a series of modules:

6.3.1 ZoneGraph
6.3.2 StateTree
6.3.3 ...

This Section, as the rest of the document is still work in progress.

6.3.1 ZoneGraph

In-level splines and shapes that use config defined lanes to direct zonegraph pathing things around! Think sidewalks, roads etc.

6.3.2 StateTree

A new lightweight AI statemachine that can work in conjunction with Mass Crowds. One of them is used to give movement targets to the cones in the parade in the sample.

7. Other Resources

7.1 Mass

This section compiles very useful Mass resources to complement this documentation.

Epic Games Official resources:

• [Documentation] MassEntity: Overview of Unreal Engine's MassEntity system.
• [Documentation] Mass Avoidance: Mass Avoidance is a force-based avoidance system integrated with MassEntity.
• [Documentation] Smart Objects: Smart Objects represent a set of activities in the level that can be used through a reservation system.
• [Documentation] StateTree: Overview of the Mass AI StateTree system.
• [Video] State of Unreal : Large Numbers of Entities with Mass: Mario Palermo (Global Unreal Engine 5 Lead Evangelist) showcases Mass in detail in a 30-minute video.

@quabqi's blog posts (Chinese):

• ECS of UE5: MASS framework (1): Mass memory hierarchy, entity and archetype introduction.
• ECS of UE5: MASS framework (2): Mass basic execution.
• ECS of UE5: MASS framework (3): A deep dive in MassGameplay.
• MassAI crowd drawing of UE5 CitySample: How are the pedestrians of the UE5 CitySample handled?

7.2 General Entity Component Systems (ECS)

• Sander's Entity Component System FAQ: This FAQ is for anyone interested in ECS & modern, high performance game development.