How To: Receive Processing Phase Transition Events

Documented Version: UE 5.7

Coordinating data across different tick (or processing) phases is a significant concern for your game.

To facilitate this, the Mass Processing Manager provides event hooks for Started and Finished events for each of the Mass Processing Phases defined by EMassProcessingPhase.

Document Overview

• Register for Events
  • Example Subsystem Initialization
• Unregister from Events
  • Example Subsystem Deinitialization
• Handling Events
• Flow of Events

Register for Events

In your Subsystem Initialize for example, hook into the World’s UMassSimulationSubsystem Events OnProcessingPhaseStarted and OnProcessingPhaseFinished for whichever processing phases are of interest.

Example Subsystem Initialization

if (auto SimSystem = World->GetSubsystem<UMassSimulationSubsystem>())
{
    SimSystem->GetOnProcessingPhaseStarted(EMassProcessingPhase::PrePhysics).AddUObject(this, &ThisClass::NativeOnStartPrePhysics);
    SimSystem->GetOnProcessingPhaseFinished(EMassProcessingPhase::PrePhysics).AddUObject(this, &ThisClass::NativeOnFinishPrePhysics);
}

You may prefer to use Collection.InitializeDependency in a Subsystem like:

if (auto SimSystem = Collection.InitializeDependency<UMassSimulationSubsystem>())
{ /* ... */ }

Unregister from Events

Always unregister from events! In Subsystems, do it in Deinitialize, like:

Example Subsystem Deinitialization

if (auto SimSystem = World->GetSubsystem<UMassSimulationSubsystem>())
{
    // Make sure to unregister EVERY event we registered in Initialize()
    SimSystem->GetOnProcessingPhaseStarted(EMassProcessingPhase::PrePhysics).RemoveAll(this);
    SimSystem->GetOnProcessingPhaseFinished(EMassProcessingPhase::PrePhysics).RemoveAll(this);
}

Handling Events

Example C++ Header

void NativeOnStartPrePhysics(const float DeltaSeconds) const;
void NativeOnFinishPrePhysics(const float DeltaSeconds) const;

Example C++ Source

void UXmsRepSubsystem::NativeOnStartPrePhysics(const float DeltaSeconds) const
{
    // Do stuff before any PrePhysics processors run
}

void UXmsRepSubsystem::NativeOnFinishPrePhysics(const float DeltaSeconds) const
{
    // Do stuff after all PrePhysics processors have run
}

Flow of Events

During ticks, events will trigger in order, before and after the processors that run during that phase, in a flow from top to bottom for each Tick:

Phase	Transition Event	Processor Execution
PrePhysics	Started
PrePhysics		Processors Execute
PrePhysics	Finished
StartPhysics	Started
StartPhysics		Processors Execute
StartPhysics	Finished
DuringPhysics	Started
DuringPhysics		Not supported for processor execution
DuringPhysics	Finished
EndPhysics	Started
EndPhysics		Processors Execute
EndPhysics	Finished
PostPhysics	Started
PostPhysics		Processors Execute
PostPhysics	Finished
FrameEnd	Started
FrameEnd		Processors Execute
FrameEnd	Finished