为了管理时间，Unreal将游戏运行时间片分隔为"Ticks"。一个Tick是关卡中所有Actors更新的最小时间单位。一个tick一般是10ms-100ms（CPU性能越好，游戏逻辑越简单，tick的时间越短）
UE3使用的是游戏循环模型为：可变FPS决定游戏速度 详见：游戏主循环（Game Loop）
注1：能够tick的对象类型有 -- 从FTickableObject派生的Object、Actor、UActorComponent等
注2：当前Tick使用的DeltaTime为上一个Tick执行花费的逻辑时间
注3：DeltaTime=GDeltaTime*Info->TimeDilation（在UWorld::Tick中进行计算，见下面游戏循环示意代码）
可通过slomo 5命令将WorldInfo的TimeDilation变量设置成5，那么游戏将以5倍的速率运行逻辑
注4：TimeSeconds为游戏逻辑世界的运行时间，RealTimeSeconds为真实世界的运行时间，也在UWorld::Tick中进行计算
每次LoadMap加载地图关卡时，会在UWorld::BeginPlay中将TimeSeconds、RealTimeSeconds重置为0
注5：绝大部分函数是同步的，不会跨越tick来执行；UE3提供了"latent functions(延迟函数)"，在State code中需要跨越多个tick才能完成
这种类似于协程的机制，非常适合开发者使用线性代码来完成需要异步的逻辑
注6：UE3中使用appSeconds()【通过调用QueryPerformanceFrequency/QueryPerformanceCounter实现】获取当前系统时间
CPU上也有一个计数器，以机器的clock为单位，可以通过rdtsc读取，而不用中断，精度为微妙级
常见的laten函数：
// Actor类
final latent function Sleep( float Seconds );
final latent function FinishAnim( AnimNodeSequence SeqNode );
// Controller类
final latent function MoveTo(vector NewDestination, optional Actor ViewFocus, optional float DestinationOffset, optional bool bShouldWalk = (Pawn != None) ? Pawn.bIsWalking : false);
final latent function MoveToDirectNonPathPos(vector NewDestination, optional Actor ViewFocus, optional float DestinationOffset, optional bool bShouldWalk = (Pawn != None) ? Pawn.bIsWalking : false);
final latent function MoveToward(Actor NewTarget, optional Actor ViewFocus, optional float DestinationOffset, optional bool bUseStrafing, optional bool bShouldWalk = (Pawn != None) ? Pawn.bIsWalking : false);
final latent function FinishRotation();
final latent function WaitForLanding(optional float waitDuration);
// UDKBot类
final latent function WaitToSeeEnemy();
final latent function LatentWhatToDoNext();
UE3游戏循环示意代码：
while( !GIsRequestingExit )
{
EngineTick()
{
GEngineLoop.Tick();
{
void FEngineLoop::Tick()
{
// 计算GDeltaTime，对循环时间进行控制
appUpdateTimeAndHandleMaxTickRate();
{
static DOUBLE LastTime = appSeconds() - 0.0001; GLastTime = GCurrentTime; if( GIsBenchmarking || GUseFixedTimeStep ) // UDK.exe CAT2-City_20_Main.udk -benmark -dumpmovie
{
GDeltaTime = GFixedDeltaTime;
LastTime = GCurrentTime;
GCurrentTime += GDeltaTime;
}
else
{
GCurrentTime = appSeconds(); FLOAT DeltaTime = GCurrentTime - LastTime;
const FLOAT MaxTickRate = GEngine->GetMaxTickRate( DeltaTime ); // 获取配置的限制帧数
FLOAT WaitTime = ;
// Convert from max FPS to wait time.
if( MaxTickRate > )
{
WaitTime = Max( .f / MaxTickRate - DeltaTime, .f );
} if( WaitTime > )
{
// Give up timeslice for remainder of wait time.
const DOUBLE WaitStartTime = GCurrentTime;
while( GCurrentTime - WaitStartTime < WaitTime )
{
GCurrentTime = appSeconds();
appSleep( );
}
} GDeltaTime = GCurrentTime - LastTime;
LastTime = GCurrentTime;
}
}
GEngine->Tick( GDeltaTime );
{
Client->Tick( DeltaSeconds );
{
void UWindowsClient::Tick( FLOAT DeltaTime )
{
ProcessDeferredMessages(); //处理Windows延迟消息
// 更新所有的viewports
for( … ) Viewports(ViewportIndex)->Tick(DeltaTime);
//使用dxInput8读取玩家鼠标键盘输入缓冲区到UInput成员变量中，
//通过Binds配置信息查找对应的响应函数并执行
ProcessInput( DeltaTime );
}
} GWorld->Tick( LEVELTICK_All, DeltaSeconds );
{
AWorldInfo* Info = GetWorldInfo(); InTick=; // 是否在Tick过程中 // 网络复制（变量同步，远程函数调用）
NetDriver->TickDispatch( DeltaSeconds ); // Update time.
Info->RealTimeSeconds += DeltaSeconds; // Audio always plays at real-time regardless of time dilation, but only when NOT paused
if( !IsPaused() )
{
Info->AudioTimeSeconds += DeltaSeconds;
} // apply time multipliers
DeltaSeconds *= Info->TimeDilation;
// Clamp time between 2000 fps and 2.5 fps.
DeltaSeconds = Clamp(DeltaSeconds,0.0005f,0.40f);
Info->DeltaSeconds = DeltaSeconds; if (!IsPaused())
{
Info->TimeSeconds += DeltaSeconds;
} // 执行Kismet逻辑
for( … ) CurrentLevel->GameSequences(SeqIdx)->UpdateOp( DeltaSeconds ); // Tick Actors（Actor.TickGroup = TG_PreAsyncWork）即：物理仿真前类型Actor
TickGroup = TG_PreAsyncWork;
// 这一步APlayerController::Tick会被执行，然后会调用其成员变量数组Interactions(InteractionIndex)->Tick(DeltaSeconds)
// 从而会调用UInput::Tick，当DeltaSeconds!=-1.0f时，该函数会处理按下ASDW持续移动逻辑；当DeltaSeconds==-1.0f时，
// 该函数会首次将镜头参数绑定到AxisArray数组中，方便后续每帧清理镜头参数（将AxisArray 各元素置0即可）
TickActors<FDeferredTickList::FGlobalActorIterator>(this,DeltaSeconds,TickType,GDeferredList);
{
World->NewlySpawned.Reset();
for (ITER It(DeferredList); It; ++It)
{
AActor* Actor = *It;
Actor->Tick(DeltaSeconds*Actor->CustomTimeDilation,TickType);
{
UBOOL AActor::Tick( FLOAT DeltaSeconds, ELevelTick TickType )
{
// 调用uc脚本中Tick方法
eventTick(DeltaSeconds);
// 执行state code
ProcessState( DeltaSeconds );
// 执行Timers
UpdateTimers( DeltaSeconds );
}
}
// 更新Actor的组件（骨骼、粒子特效、光照等）
TickActorComponents(Actor,DeltaSeconds,TickType,&DeferredList);
}
// If an actor was spawned during the async work, tick it in the post
// async work, so that it doesn't try to interact with the async threads
if (World->TickGroup == TG_DuringAsyncWork)
{
DeferNewlySpawned(World,DeferredList);
}
else
{
TickNewlySpawned(World,DeltaSeconds,TickType);
}
}
TickAsyncWork(Info->bPlayersOnly == FALSE ? DeltaSeconds : .f); // 同步数据到物理仿真线程
// Tick Actors（Actor.TickGroup = TG_DuringAsyncWork）即：物理仿真过程中类型Actor
TickGroup = TG_DuringAsyncWork;
TickActors<FDeferredTickList::FActorDuringAsyncWorkIterator>(this,DeltaSeconds,TickType,GDeferredList);
{
... ...
}
TickDeferredComponents<FDeferredTickList::FComponentDuringAsyncWorkIterator>(DeltaSeconds,GDeferredList);
WaitForAsyncWork(); // 等待物理仿真线程结束
// Tick Actors（Actor.TickGroup = TG_PostAsyncWork）即：物理仿真后类型Actor
TickGroup = TG_PostAsyncWork;
DispatchRBCollisionNotifies(RBPhysScene);
TickActors<FDeferredTickList::FActorPostAsyncWorkIterator>(this,DeltaSeconds,TickType,GDeferredList);
{
... ...
}
TickDeferredComponents<FDeferredTickList::FComponentPostAsyncWorkIterator>(DeltaSeconds,GDeferredList); // Tick all objects inheriting from FTickableObjects.
for( INT i=; i<FTickableObject::TickableObjects.Num(); i++ )
{
FTickableObject* TickableObject = FTickableObject::TickableObjects(i);
if( TickableObject->IsTickable() )
{
TickableObject->Tick(DeltaSeconds);
}
} // Update cameras last. This needs to be done before NetUpdates, and after all actors have been ticked.
for( AController *C = this->GetFirstController(); C != NULL; C = C->NextController)
{
APlayerController* PC = C->GetAPlayerController(); // if it is a player, update the camra.
if( PC && PC->PlayerCamera )
{
PC->PlayerCamera->eventUpdateCamera(DeltaSeconds); }
} InTick = ; // Tick过程结束 // 执行垃圾回收（GC）
if(…) PerformGarbageCollection(); // GC Mark Time
else IncrementalPurgeGarbage( TRUE ); // GC Sweep Time } //按照UUTConsole、UGFxInteraction、UTGGameInteraction、UPlayerManagerInteraction顺序响应玩家输入
GameViewport->Tick(DeltaSeconds);
// 渲染上屏
RedrawViewports();
// 播放声音
Client->GetAudioDevice()->Update( !GWorld->IsPaused() );
}
// 处理Windows消息循环
appWinPumpMessages();
}
}
}
}
UE3的UGameEngine::LoadMap示意代码：
UBOOL UGameEngine::LoadMap( const FURL& URL, UPendingLevel* Pending, FString& Error )
{
// send a callback message
GCallbackEvent->Send(CALLBACK_PreLoadMap); // 清理地图关卡
CleanupPackagesToFullyLoad(FULLYLOAD_Map, GWorld->PersistentLevel->GetOutermost()->GetName()); // cleanup the existing per-game pacakges
// @todo: It should be possible to not unload/load packages if we are going from/to the same gametype.
// would have to save the game pathname here and pass it in to SetGameInfo below
CleanupPackagesToFullyLoad(FULLYLOAD_Game_PreLoadClass, TEXT(""));
CleanupPackagesToFullyLoad(FULLYLOAD_Game_PostLoadClass, TEXT(""));
CleanupPackagesToFullyLoad(FULLYLOAD_Mutator, TEXT("")); // 关闭网络连接，回收老的游戏世界GWorld
if( GWorld )
{
// close client connections
{
UNetDriver* NetDriver = GWorld->GetNetDriver();
if (NetDriver != NULL && NetDriver->ServerConnection == NULL)
{
for (INT i = NetDriver->ClientConnections.Num() - ; i >= ; i--)
{
if (NetDriver->ClientConnections(i)->Actor != NULL && NetDriver->ClientConnections(i)->Actor->Pawn != NULL)
{
GWorld->DestroyActor(NetDriver->ClientConnections(i)->Actor->Pawn, TRUE);
}
NetDriver->ClientConnections(i)->CleanUp();
}
}
} // Clean up game state.
GWorld->SetNetDriver(NULL);
GWorld->FlushLevelStreaming( NULL, TRUE );
GWorld->TermWorldRBPhys();
GWorld->CleanupWorld(); // send a message that all levels are going away (NULL means every sublevel is being removed
// without a call to RemoveFromWorld for each)
GCallbackEvent->Send(CALLBACK_LevelRemovedFromWorld, (UObject*)NULL); // Disassociate the players from their PlayerControllers.
for(FLocalPlayerIterator It(this);It;++It)
{
if(It->Actor)
{
if(It->Actor->Pawn)
{
GWorld->DestroyActor(It->Actor->Pawn, TRUE);
}
GWorld->DestroyActor(It->Actor, TRUE);
It->Actor = NULL;
}
} GWorld->RemoveFromRoot();
GWorld = NULL;
} // Clean up the previous level out of memory.
UObject::CollectGarbage( GARBAGE_COLLECTION_KEEPFLAGS, TRUE ); // 加载新的地图
UPackage* WorldPackage = LoadPackage(MapOuter, *URL.Map, LOAD_None); // 创建和初始化新的游戏世界GWorld
GWorld = FindObjectChecked<UWorld>( WorldPackage, TEXT("TheWorld") );
GWorld->AddToRoot();
GWorld->Init(); // 将GPendingLevel的网络连接移交给新的GWorld
if( Pending )
{
check(Pending==GPendingLevel); // Hook network driver up to level.
GWorld->SetNetDriver(Pending->NetDriver);
if( GWorld->GetNetDriver() )
{
GWorld->GetNetDriver()->Notify = GWorld;
UPackage::NetObjectNotifies.AddItem(GWorld->GetNetDriver());
} // Setup level.
GWorld->GetWorldInfo()->NetMode = NM_Client;
} // 设置新的GWorld的GameInfo并初始化游戏 MaxPlayers，TimeLimit等重要参数在该函数中被赋值
GWorld->SetGameInfo(URL);
{
AWorldInfo* Info = GetWorldInfo(); if( IsServer() && !Info->Game )
{
Info->Game = (AGameInfo*)SpawnActor( GameClass );
}
} // Initialize gameplay for the level.
GWorld->BeginPlay(URL);
{
AWorldInfo* Info = GetWorldInfo(); // 重置TimeSeconds、RealTimeSeconds和AudioTimeSeconds为0
if (bResetTime)
{
GetWorldInfo()->TimeSeconds = 0.0f;
GetWorldInfo()->RealTimeSeconds = 0.0f;
GetWorldInfo()->AudioTimeSeconds = 0.0f;
} // Init level gameplay info.
if( !HasBegunPlay() )
{
// Enable actor script calls.
Info->bBegunPlay = ;
Info->bStartup = ; // 调用GameInfo中的InitGame函数
if (Info->Game != NULL && !Info->Game->bScriptInitialized)
{
Info->Game->eventInitGame( Options, Error );
}
}
} // 创建LocalPlayer的本地Controller
// 单机模式下使用该创建的Controller（GWorld->IsServer()为true）
// 联网模式下Loading地图时使用该创建的Controller，但当玩家成功加入游戏后
// 客户端从服务器同步自动创建新的Controller(UActorChannel::ReceivedBunch函数中)，
// 然后调用UNetConnection::HandleClientPlayer销毁此处为LocalPlayer创建的Controller，并将LocalPlayer设置给新的Controller
for(FLocalPlayerIterator It(this);It;++It)
{
It->SpawnPlayActor(URL.String(),Error2);
} // send a callback message
GCallbackEvent->Send(CALLBACK_PostLoadMap); return TRUE;
}
更多请参加：