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Game Flow Diagram#

Highlevel GameFlowChart
Related: Detailed Actor LifeCycle Diagram


Numbers signify steps not necessarily at the same class nesting

  • UGameEngine::Init

  • UGameInstance::InitializeStandalone()

    • UGameInstance::Init() - CreateUOnlineSession and Register Delegates()
  • UGameEngine::Start

  • UGameInstance::StartGameInstance()=>

    • UEngine::LoadMap()
    1. UWorld::InitializeActorsForPlay() - Call register components on all actor components in all levels. Note: Construction scripts are rerun in uncooked mode

      • UActorComponent::RegisterComponent() - Adding itself to its owner and inside owner's world, possibly creating rendering/physics state
    2. ABBGameModeBase::InitGame - Create the Game Session and register FGameDelegates (ex: PreCommitMapChangeDelegate, HandleDisconnectDelegate)

    3. Ulevel::RouteActorInitialize() -

      • a) Actor::PreInitializeComponents() - On all actors in the level

      • Side Note: AGameModeBase::PreInitializeComponents() creates

        • AGameStateBase and calls InitGameState()

        • AGameNetworkManager which handles game-specific networking management (cheat detection, bandwidth management, etc)

      • b) Iterate through Ulevel::Actors[] and call these functions on them one at a time

      • Actor::InitializeComponents()

        • UActorComponent::Activate() - Sets Component Tick To Be Enables & bIsActive = true

        • UActorComponent::InitializeComponent() - Place for components to Initialize themselves before BeginPlay (Actor or Component for anything in the world)

      • PostInitializeComponents() - Code that can run after gaurantee that all components have been initialized

      • c) Iterate through all Ulevel::ActorsToBeginPlay[] and call BeginPlay() to allows code to run with assumption that all other level actors have been PostInitializeComponents()

      • Not sure why this is here instead of the main call to BeginPlay(possibly for networked late joins?)

    4. FWorldDelegates::OnWorldInitializedActors.Broadcast(OnActorInitParams)

  1. Uworld::BeginPlay()
  • a) GameMode::StartPlay()

    • GameMode::StartMatch()

    • GameState::HandleBeginPlay()

      • AWorldSettings::NotifyBeginPlay()

      • Actor::BeginPlay(), for all actors

        • UActorComponent::RegisterAllComponentTickFunctions() - Allows components to register multiple tick functions (ex: Physics tick, cloth tick in skeletalmeshcomponent)
        • UActorComponent::BeginPlay()


  • UGameEngine::Tick()

  • Uworld::Tick()

    • FTiskTaskManagerInterface::StartFrame()

    • Queues up all the TickFunctions according to their dependency graph & TickGroup

    • Ticking within each group is done by a dependency graph (AddTickPrerequisite) of tick functions of various objects during TickFunction registration

    • This function might change what TickGroup something runs in according to the prerequisite tick function's tickgroup

    • Actor Components do not necessarily tick after their owner Actor

    • Calls RunTickGroup() for various tick groups which ticks components

    • FTickableGameObject::TickObjects() - ticks UObjects or anything that derives from FTickableGameObject (e.g. SceneCapturerCubes or LevelSequencePlayers )

  • FTicker::GetCoreTicker().Tick(FApp::GetDeltaTime()) - Ticks all objects of type FTickerObjectBase. Ex: FHttpManager, FAvfMediaPlayer, FVoiceCapture, FSteamSocketSubsystem)

  • This would be great place to add Objects that need to tick at the end of the frame that are engine/world agnostic

  • Good possible place for our own UDP network ticking replication

GameMode Flow:

  • InitGame()
  • InitGameState()
  • PostInitializeComponents()
  • ChoosePlayerStart_Implementation()
  • PostLogin()
  • HandleStartingNewPlayer_Implementation()
  • ChoosePlayerStart_Implementation()
  • SetPlayerDefaults()
  • RestartPlayerAtPlayerStart()
  • StartPlay()