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With this checkin, the binding system inside Level Sequences has been refactored to allow C++ custom binding classes inheriting from a new class, UMovieSceneCustomBinding. Custom bindings are held as instanced UObjects inside FMovieSceneBindingReference, alongside the Universal Object Locator. In the case where a custom binding has been specified, the UOL will be ignored. The intention with this change is to allow developers to create their own arbitrary methods of binding any UObject to a track inside Sequencer. Examples of this include: * Runtime dynamic bindings to spawned NPC or player characters (e.g. in UEFN) with a different spawned preview character * Blueprint-specified dynamic bindings * Spawnable bindings based on something other than a AActor template * Scene Graph/Prefabs Alongside the base UMovieSceneCustomBinding class, a new hierarchy of spawnable custom binding classes has been created as the basis for allowing spawnable bindings that don't require object templates, and may instead require other asset references, for example UEFN NPC character spawnables requiring an NPC Character Definition asset. This hierarchy is currently. UMovieSceneCustomBinding <- UMovieSceneSpawnableBindingBase <- UMovieSceneSpawnableActorBindingBase <- UMovieSceneSpawnableActorBinding. This hierarchy allows for spawnables of non-actor types, spawnables of actor types not based on templates, and finally UMovieSceneSpawnableActorBinding, which is a re-implementation of FMovieSceneSpawnable inside the custom binding system. Going forward, we will be moving to deprecate FMovieSceneSpawnable in UE Sequencer in favor of the custom spawnable binding types. Sequences with FMovieSceneSpawnables will continue to function as normal, but new spawnables in new sequences will use UMovieSceneSpawnableActorBinding under the hood. These will technically have FMovieScenePossessable structs created for them for now rather than FMovieSceneSpawnable structs. This should be mostly invisible to the user. In addition, I've added a separate hierarchy of custom bindings called 'Replaceable' bindings. A 'Replaceable' is a binding type that will expect in runtime to always be overridden by a different object that may not be available in editor. UMovieSceneReplaceableBinding will hold in EDITORONLY_DATA a UMovieSceneSpawnableBindingBase to provide a preview while within Sequencer. Included in this is a UMovieSceneReplaceableActorBinding, which uses the UMovieSceneSpawnableActorBinding internally to spawn its preview, and has no implementation to find the actor during runtime- it relies on the user overriding the actor binding using the existing BindingOverrides method on LevelSequenceActor. Next Steps: * UEFN custom bindings will be built upon this system, to allow UEFN users multiple ways to override bindings. * UEFN custom spawnable bindings will be built upon this system, to allow UEFN users a way to spawn actors and objects within Sequencer with their lifetime managed by Sequencer * With this change, we have a cleaner way of incorporating the 'dynamic binding' work that uses the Director Blueprint to create a spawnable or possessable. This should be incorporated into the UCustomBinding hierarchy. * Once Verse supports it, we should add Verse custom binding logic * We need to add a Universal Object Locator type that can reference any other binding in the sequence hierarchy. This will allow us to adapt the existing 'Spawnable Parent' possessable binding logic to reference any other type of binding, not just spawnable ones. * Eventually we will fully deprecate FMovieSceneSpawnable Further notes: * A previous imagined version of this system used the cache as a way to allow locators to spawn actors and register them with Sequencer. This change removes this code. * Custom Spawnable Bindings use the Binding Lifetime Track to determine their spawn lifetime rather than the old spawn track. This unifies the concept of the lifetime of an object binding within sequencer for both spawnables and possessables. * As FMovieScenePossessable and FMovieSceneBindingReferences allows multiple bindings per guid, this change now allows multiple spawnables to be spawned and controlled by a single binding track. However, some of the editor tooling code doesn't currently handle this properly and at times assumes a single binding. This should be resolved one way or the other. * The UX for this allows UCustomBindings to handle creating bindings from any dragged in object using a priority system- Sequencer will ask all classes in priority order if they support binding creation from that object. * The UX supports unique overlay icons and tooltips for different types of bindings * The UX allows each binding type to decide whether it can convert a current binding into its own type and then do so * Currently there is some manual instanced detail customization for UMovieSceneCustomSpawnableActorBinding. We use instanced customization so that the UX has the sequencer context. This should both be improved (to allow ways to choose a class/template via this UI), as well as extended so that clients outside of the Movie Scene plugin can register instanced detail customizations for their own binding types. * Where possible, Sequencer APIs around spawnables have been refactored to take guids rather than FMovieSceneSpawnable references. In particular, custom spawnables do still use the spawn register, but do not use actor spawners- instead the UMovieSceneCustomBinding class is given the responsiblity of spawning/destroying the object/actor. This created an awkward API for USD export, which had overridden the spawn register to unhide/hide actors rather than spawning/destroying them. This could use some more work. MetaHumanPerformanceExportUtils will also need to be refactored at some point to use the custom spawnable binding system. * FSequencerUtilities::CreateBinding and CreateGenericBinding has been refactored to accept binding parameters and now supports creating custom bindings including spawnables, as well as replacing existing bindings and adding additional bindings to an existing guid. [REVIEW] [at]ue-sequencer #jira UE-199299, FORT-582815, UE-209594 #rb Andrew.Rodham, daniel.coelho, Max.Chen [CL 32218187 by david bromberg in ue5-main branch]