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> Unreal Engine API Reference > Plugins > ModelingComponentsEditorOnly > Scene > FLevelObjectsObserver

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FLevelObjectsObserver

[FLevelObjectsObserver](API\Plugins\ModelingComponentsEditorOnly\Scene\FLevelObjectsObserver) is a utility class that tracks the active-set of Actors in a World, in the Editor, and emits events when the set changes.

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Inheritance Hierarchy

FEditorUndoClient

FLevelObjectsObserver

References

Module	ModelingComponentsEditorOnly
Header	/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponentsEditorOnly/Public/Scene/LevelObjectsObserver.h
Include	#include "Scene/LevelObjectsObserver.h"

Syntax

class FLevelObjectsObserver : public FEditorUndoClient

Remarks

FLevelObjectsObserver is a utility class that tracks the active-set of Actors in a World, in the Editor, and emits events when the set changes. The goal is to guarantee that:

OnActorAdded will be emitted when an Actor comes into existence
OnActorRemoved will be emitted when an Actor ceases to exist (possibly after GC, though)
these signals will be unique, ie once per Actor

This is a surprisingly complicated problem in the Editor due to the many ways that Actors might be created or destroyed, in particular via Undo/Redo.

that it is not possible to guarantee that OnActorRemoved() fires before an Actor is GC'd. So, listeners should maintain TWeakObjectPtr<>'s on Actors if they need to know this information.

Variables

	Name	Description
TSet< AActor &#...	Actors
FActorAddRemove...	OnActorAdded	OnActorAdded is emitted when a new Actor is detected
FDelegateHandle	OnActorAddedHandle
FDelegateHandle	OnActorDeletedHandle
FDelegateHandle	OnActorListChangedHandle
FActorAddRemove...	OnActorRemoved	OnActorRemoved is emitted when an existing Actor is destroyed.
UWorld *	World

Functions

	Name	Description
void	HandleActorAddedEvent ( AActor* Actor )
void	HandleActorDeletedEvent ( AActor* Actor )
void	Initialize ( UWorld* WorldIn )	Begin watching WorldIn. This will emit OnActorAdded for all Actors in the World.
void	OnUntrackedLevelChange()
void	Shutdown()	Stop watching WorldIn.

Overridden from FEditorUndoClient

		Name	Description
	void	PostRedo ( bool bSuccess )	Signal that client should run any PostRedo code
	void	PostUndo ( bool bSuccess )	FEditorUndoClient implementation.

Typedefs

Name	Description
FActorAddRemoveSignature