UE中委托的使用很廣泛，許多Event的觸發都有對應的虛函數和委托，虛函數不用講，只能在派生類中使用，而委托可以在別的類或者藍圖中使用，就應用范圍而言，委托的使用更靈活。以AActor的
/** * Event when this actor overlaps another actor, for example a player walking into a trigger. * For events when objects have a blocking collision, for example a player hitting a wall, see 'Hit' events. * @note Components on both this and the other Actor must have bGenerateOverlapEvents set to true to generate overlap events. */ virtual void NotifyActorBeginOverlap(AActor* OtherActor); /** * Event when this actor overlaps another actor, for example a player walking into a trigger. * For events when objects have a blocking collision, for example a player hitting a wall, see 'Hit' events. * @note Components on both this and the other Actor must have bGenerateOverlapEvents set to true to generate overlap events. */ UFUNCTION(BlueprintImplementableEvent, meta=(DisplayName = "ActorBeginOverlap"), Category="Collision") void ReceiveActorBeginOverlap(AActor* OtherActor); /** * Event when an actor no longer overlaps another actor, and they have separated. * @note Components on both this and the other Actor must have bGenerateOverlapEvents set to true to generate overlap events. */ virtual void NotifyActorEndOverlap(AActor* OtherActor);
為例，重疊（overlapped)事件發生時，就會觸發這組函數（Begin/EndOverlap），但僅限於在派生類中進行重載。其實，AActor提供了另外一種更加靈活的方式，那就是：委托（Delegate）
/** * Called when another actor begins to overlap this actor, for example a player walking into a trigger. * For events when objects have a blocking collision, for example a player hitting a wall, see 'Hit' events. * @note Components on both this and the other Actor must have bGenerateOverlapEvents set to true to generate overlap events. */ UPROPERTY(BlueprintAssignable, Category="Collision") FActorBeginOverlapSignature OnActorBeginOverlap;
當然也有對應的End形式。查看FActorBeginOverlapSignature的定義：
// Delegate signatures
DECLARE_DYNAMIC_MULTICAST_DELEGATE_FiveParams( FTakeAnyDamageSignature, AActor*, DamagedActor, float, Damage, const class UDamageType*, DamageType, class AController*, InstigatedBy, AActor*, DamageCauser );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_NineParams( FTakePointDamageSignature, AActor*, DamagedActor, float, Damage, class AController*, InstigatedBy, FVector, HitLocation, class UPrimitiveComponent*, FHitComponent, FName, BoneName, FVector, ShotFromDirection, const class UDamageType*, DamageType, AActor*, DamageCauser );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams( FActorBeginOverlapSignature, AActor*, OverlappedActor, AActor*, OtherActor );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams( FActorEndOverlapSignature, AActor*, OverlappedActor, AActor*, OtherActor );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_FourParams( FActorHitSignature, AActor*, SelfActor, AActor*, OtherActor, FVector, NormalImpulse, const FHitResult&, Hit );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam( FActorBeginCursorOverSignature, AActor*, TouchedActor );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam( FActorEndCursorOverSignature, AActor*, TouchedActor );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams( FActorOnClickedSignature, AActor*, TouchedActor , FKey, ButtonPressed );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams( FActorOnReleasedSignature, AActor*, TouchedActor , FKey, ButtonReleased );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams( FActorOnInputTouchBeginSignature, ETouchIndex::Type, FingerIndex, AActor*, TouchedActor );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams( FActorOnInputTouchEndSignature, ETouchIndex::Type, FingerIndex, AActor*, TouchedActor );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams( FActorBeginTouchOverSignature, ETouchIndex::Type, FingerIndex, AActor*, TouchedActor );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams( FActorEndTouchOverSignature, ETouchIndex::Type, FingerIndex, AActor*, TouchedActor );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam(FActorDestroyedSignature, AActor*, DestroyedActor );
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams(FActorEndPlaySignature, AActor*, Actor , EEndPlayReason::Type, EndPlayReason);
DECLARE_DELEGATE_SixParams(FMakeNoiseDelegate, AActor*, float /*Loudness*/, class APawn*, const FVector&, float /*MaxRange*/, FName /*Tag*/);
其中DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams，表明這是一個多播。在UE中，委托有單播和多播兩種方式，另外還有委托的函數帶不帶返回值（_RetVal后綴），單播和多播均為無返回值（函數返回類型為void)。
一、單播：
使用如下定義（這里只使用無參和一個參數兩種類型，別的類似）：
//聲明委托（單播），無參數 DECLARE_DELEGATE(MyDelegate) DECLARE_DELEGATE_OneParam(MyIntDelegate, int32)
這樣就聲明了兩種委托類型，MyDelegate（無參數），MyIntDelegate(帶有一個int32參數類型），用法如下：
1）聲明和實現回調函數，注意：一定要加UFUNCTION修飾函數聲明，因為委托是參與UE的反射系統中的，即UE引擎要知道有這個函數，這在下面的例子中會看到，聲明和實現如下：
UFUNCTION()
void IntFunction(int32 Value)
{
GLog->Log(ELogVerbosity::Warning, "IntFunction: " + FString::FromInt(Value));
}
UFUNCTION()
void SecondIntFunction(int32 Value)
{
GLog->Log(ELogVerbosity::Warning, "SecondIntFunction: " + FString::FromInt(Value));
}
UFUNCTION()
void ThirdIntFunction(int32 Value)
{
GLog->Log(ELogVerbosity::Warning, "ThirdIntFunction: " + FString::FromInt(Value));
}
UFUNCTION()
void SomeFunction()
{
GLog->Log(ELogVerbosity::Warning, "SomeFunction: ");
}
使用方法：
隨便找個地方測試，這里在BeginPlay()中測試：
MyIntDelegate IntDelegate;
IntDelegate.BindUFunction(this, FName("IntFunction"));
IntDelegate.BindUFunction(this, FName("SecondIntFunction"));
單播，只能觸發一個委托，因此上面雖然綁定了兩個函數IntFunction和SecondIntFuction,但是只有第2個函數(SecondIntFunction)得到調用
IntDelegate.Execute(999);
在引擎的日志中顯示如下：
注意到一個有趣的地方，我們綁定函數時，不是通過函數的名字來綁定的，而是通過函數的名字對應的FName來綁定的，這是說明（或許稱推測更准確），這些函數在UE引擎中通過某種方式（VM更有可能）有函數和其對應名稱字符串的一個映射，這也是前面在函數聲明處添加UFUNCTION修飾的原因（告訴UHT要把這個函數添加進反射系統）。另外一個有趣的地方是調用委托（IntDelegate.Execute(999))時，VC會自動推導出參數類型（VS2017測試）為int32。
二、多播：多播時可以觸發多個委托的函數，多播帶有Multicast這樣的字符串，這很容易和單播區分出來。多播就意味着可以觸發多個事件，如下：
GLog->Log(ELogVerbosity::Warning, TEXT("將要進行多播……。"));
MyIntMulticastDelegate IntMulticastDelegate;
IntMulticastDelegate.AddUFunction(this, FName("IntFunction"));
IntMulticastDelegate.AddUFunction(this, FName("SecondIntFunction"));
IntMulticastDelegate.AddUFunction(this, FName("ThirdIntFunction"));
IntMulticastDelegate.Broadcast(123);
注意，多播要使用Broadcast觸發，Broadcast就意味着廣播，廣播嘛，大家只要願意，都能收到，這也正是觀察者(Observer）模式。
運行結果：
三、擴展：
既然引入時使用了重疊的例子，這里把這個事件再注解下，老了，記性不好：（
void AActor::NotifyActorBeginOverlap(AActor* OtherActor)
{
// call BP handler
ReceiveActorBeginOverlap(OtherActor);
}
該函數是虛函數，調用一個ReceiveActorBeginOverlap函數，ReceiveActorBeginOverlap原型在引入中有原型，看下說明知道，這里其實就是調用藍圖中對應的OnActorBeginOverlap結點（NODE）。所以重載時要注意，如果需要調用藍圖中的事件（通常如此）要顯式調用Super::NotifyActorBeginOverlap(OtherActor)。調用的位置，即先於自定義代碼，或者在自定義代碼后調用，要根據自己的情況，例如，如果在派生類中做些初始化，以供藍圖使用，則應先調用自己的代碼，后調用父類，否則，無所謂。
四，進一步說明重疊事件：
在前方中提及，AActor是不參與物體的collide的，這里又有重載，又有多播，這不是打自己的臉？其實不是。AActor是容器！AActor是容器！AActor是容器！所以功能的實現都是通過組件實現的。在前方中說明碰撞(collide，當然包括重疊和Hit，唔，，Hit叫撞擊聽着不順就Hit好了）是在UPrimitiveComponent中實現，這里把它扒出來，晾晾。
// @todo, don't need to pass in Other actor?
void UPrimitiveComponent::BeginComponentOverlap(const FOverlapInfo& OtherOverlap, bool bDoNotifies)
{
// If pending kill, we should not generate any new overlaps
if (IsPendingKill())
{
return;
}
// UE_LOG(LogActor, Log, TEXT("BEGIN OVERLAP! Self=%s SelfComp=%s, Other=%s, OtherComp=%s"), *GetNameSafe(this), *GetNameSafe(MyComp), *GetNameSafe(OtherComp->GetOwner()), *GetNameSafe(OtherComp));
UPrimitiveComponent* OtherComp = OtherOverlap.OverlapInfo.Component.Get();
if (OtherComp)
{
bool const bComponentsAlreadyTouching = IsOverlappingComponent(OtherOverlap);
if (!bComponentsAlreadyTouching && CanComponentsGenerateOverlap(this, OtherComp))
{
AActor* const OtherActor = OtherComp->GetOwner();
AActor* const MyActor = GetOwner();
const bool bNotifyActorTouch = bDoNotifies && !MyActor->IsOverlappingActor(OtherActor);
// Perform reflexive touch.
OverlappingComponents.Add(OtherOverlap); // already verified uniqueness above
OtherComp->OverlappingComponents.AddUnique(FOverlapInfo(this, INDEX_NONE)); // uniqueness unverified, so addunique
if (bDoNotifies)
{
// first execute component delegates
if (!IsPendingKill())
{
OnComponentBeginOverlap.Broadcast(this, OtherActor, OtherComp, OtherOverlap.GetBodyIndex(), OtherOverlap.bFromSweep, OtherOverlap.OverlapInfo);
}
if (!OtherComp->IsPendingKill())
{
// Reverse normals for other component. When it's a sweep, we are the one that moved.
OtherComp->OnComponentBeginOverlap.Broadcast(OtherComp, MyActor, this, INDEX_NONE, OtherOverlap.bFromSweep, OtherOverlap.bFromSweep ? FHitResult::GetReversedHit(OtherOverlap.OverlapInfo) : OtherOverlap.OverlapInfo);
}
// then execute actor notification if this is a new actor touch
if (bNotifyActorTouch)
{
// First actor virtuals
if (IsActorValidToNotify(MyActor))
{
MyActor->NotifyActorBeginOverlap(OtherActor);
}
if (IsActorValidToNotify(OtherActor))
{
OtherActor->NotifyActorBeginOverlap(MyActor);
}
// Then level-script delegates
if (IsActorValidToNotify(MyActor))
{
MyActor->OnActorBeginOverlap.Broadcast(MyActor, OtherActor);
}
if (IsActorValidToNotify(OtherActor))
{
OtherActor->OnActorBeginOverlap.Broadcast(OtherActor, MyActor);
}
}
}
}
}
}
代碼有點長，似乎超過50，想必Epic不會找我麻煩，只看后一段好了。
簡單來講就是，如果Actor有效(Valid)則調用自己的Notify對應的事件，然后再廣播！順便做個好人，也幫對方觸發一下事件，也幫對方發個小廣告，為什么這么做？唔，，，思考一下，很快就明白，在Sweep方式下，被重疊的物體是通常都是被動的，如果它再主動檢查，很耗資源，所以，它就在那里吃瓜好了，等我碰到你再幫你觸發事件，再幫你發你要發的消息（小廣告），反正如果大伙都不動，肯定不會發生碰撞（沒有進行物理模擬情況下，有物理模擬，唔，同樣的結論似乎也成立）。
五、進進進進一步說明重疊事件：剛才突發奇想，打開物理模擬后又如何？簡單測試，隨便弄個球好了，球打開物理模擬，設置對pawn對象overlap,勾選generated overlap event，然后在level blueprint中隨便打印個字符串，我選擇的是輸出：overlap字符串。pawn一碰到球，產生了同樣的事件，結論成立。
六、感謝Epic提供的源代碼，引用我最喜歡的侯捷老師一句話：源碼面前，了無秘密。話說侯老師，你不准備寫一本《UE4引擎深入淺出》嗎？多少money我都支持，擦，，，，MFC深入淺出我買了二本，第一本大多數人都沒見過，淺藍色封面的，可惜送人了。嘻嘻。