CommonDelegate解决方案

实现业务时，经常需要用到全局级别的 CommonDelegate；

定义如下 UTestCommonDelegate，即可定义出 UTestCommonDelegate::GetDelegate()->OnTestEvent 这样的一个 Delegate；

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// TestCommonDelegate

UCLASS()
class UTestCommonDelegate : public UCommonDelegateBase
{
	GENERATED_BODY()

public:
	UFUNCTION()
	static UTestCommonDelegate* GetDelegate()
    {
        return GetCommonDelegate<UTestCommonDelegate>();
    }
    
public:
    DECLARE_DELEGATE_AnyParams(FTestEvent, int32 /*IntVal*/);
	FTestEvent OnTestEvent;
}

CommonDelegateBase

首先定义 UCommonDelegateBase 作为所有 Delegate 的基类；

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// CommonDelegateBase

UCLASS(Abstract)
class UCommonDelegateBase : public UObject
{
	GENERATED_BODY()
};

定义 GlobalCommonDelegates 用于存储全局的所有 Delegates；

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USTRUCT()
struct FCommonDelegates
{
	GENERATED_BODY()

public:
	TMap<TSubclassOf<UCommonDelegateBase>, UCommonDelegateBase*>* GetDelegateMap()
    {
        return &DelegateMap;
    }
    
private:
	UPROPERTY()
	TMap<TSubclassOf<UCommonDelegateBase>, UCommonDelegateBase*> DelegateMap;
};


extern FCommonDelegates GlobalCommonDelegates;

在 GetDelegate 中尝试寻找或创建 Delegate，将其 AddToRoot；

同时提供 DeleteAllCommonDelegates 在合适的时候（比如切换 World）调用，清空所有数据；

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template<class T>
typename TEnableIf<TIsDerivedFrom<T, UCommonDelegateBase>::Value, T*>::Type
GetCommonDelegate()
{
    auto DelegateMap = GlobalCommonDelegates.GetDelegateMap();
    if (DelegateMap == nullptr) return nullptr;

    if (auto Delegate = DelegateMap->FindRef(T::StaticClass()); IsValid(Delegate))
    {
        return Cast<T>(Delegate);
    }

    auto NewDelegate = NewObject<T>();
    NewDelegate->AddToRoot();
    DelegateMap->FindOrAdd(T::StaticClass()) = NewDelegate;
    return NewDelegate;
}

void DeleteAllCommonDelegates()
{
    auto DelegateMap = GlobalCommonDelegates.GetDelegateMap();
    if (DelegateMap == nullptr) return nullptr;

    for (auto DelegatePair : *DelegateMap)
    {
        if (IsValid(DelegatePair.Value))
        {					
            DelegatePair.Value->RemoveFromRoot();
        }
    }

    DelegateMap->Empty();
}

DeclareAnyParams

一般的 DECLARE_DELEGATE 需要写明 Params 的个数；比如 DECLARE_MULTICAST_DELEGATE_OneParam，这里限定了 OneParam；

实际上，可以看到在 Engine\Source\Runtime\Core\Public\Delegates\Delegate.h 中，支持添加 AnyParams：

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// Delegate.h

#define FUNC_DECLARE_DELEGATE( DelegateName, ReturnType, ... ) \
	typedef TDelegate<ReturnType(__VA_ARGS__)> DelegateName;

#define FUNC_DECLARE_MULTICAST_DELEGATE( MulticastDelegateName, ReturnType, ... ) \
	typedef TMulticastDelegate<ReturnType(__VA_ARGS__)> MulticastDelegateName;

#define FUNC_DECLARE_EVENT( OwningType, EventName, ReturnType, ... ) \
	class EventName : public TMulticastDelegate<ReturnType(__VA_ARGS__)> \
	{ \
		friend class OwningType; \
	};

于是可以对非 DYNAMIC 的 DELEGATE 添加声明，

直接在 Engine\Source\Runtime\Core\Public\Delegates\DelegateCombinations.h 中补充：

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#define DECLARE_DELEGATE_AnyParams( DelegateName, ... ) FUNC_DECLARE_DELEGATE( DelegateName, void, __VA_ARGS__ )
#define DECLARE_MULTICAST_DELEGATE_AnyParams( DelegateName, ... ) FUNC_DECLARE_MULTICAST_DELEGATE( DelegateName, void, __VA_ARGS__ )
#define DECLARE_EVENT_AnyParams( OwningType, EventName, ... ) FUNC_DECLARE_EVENT( OwningType, EventName, void, __VA_ARGS__ )
#define DECLARE_DELEGATE_RetVal_AnyParams( ReturnValueType, DelegateName, ... ) FUNC_DECLARE_DELEGATE( DelegateName, ReturnValueType, __VA_ARGS__ )