本文章內容基於UnrealEngine5.1版本
最近在看5.0新導入的MassFramework，看到裡面出現了FInstanceStruct這個詞引起了我的興趣。基本上這東西可以讓我們動態選擇struct，而在Unreal中是把這種組裝用的property稱為Fragment。這個概念我第一次看到是在Unreal官方釋出的Lyra範例專案中看到，只是當時實驗了一下，由於他是使用UObject搭上reflection macro來實現，我沒辦法把一個UObject直接放進DataTable的Row中做編輯。這帶來的結果，就是當我在製作Plugin的時候，若需要用到DataTable時，必須要預先預想好所有使用者可能會想要有的欄位：哪些要直接放到TableRow中、哪些要指向外部的asset讓使用者自己在自己定義的BP或DataAsset中擴充。然而，可以想像的到，多了一個uasset就代表編輯的時候要另外開檔案，這對企畫人員非常的不友善，而且檔案一多，在維護上也會有不小壓力。這次看到了FInstanceStruct，馬上直覺想到這東西或許可以解決這個困擾我很久的問題。稍微實驗了一下，馬上驗證了上面提到的問題可以靠FInstanceStruct的機制來解決，因此決定花一點時間整理二種方法的優缺點。
接著，讓我們來看看這二種方法個別怎麼實現吧。
使用InstanceStruct製作Fragment的例子
InstanceStruct是定義在StructUtils這個Plugin中，記得要先去開起來。雖然在5.1還是Experimental，不過它是MassEntity的相依功能，而這個Plugin已經進Beta，因此看起來是不太需要擔心，目前我用起來感覺還蠻穩定的。
在把Plugin開起來之後，首先我們需要要定義一個BaseFragment：
// Fragment for GameItem Table
USTRUCT(BlueprintType)
struct MYGAME_API FHorizonGameItemTableRowFragment
{
GENERATED_BODY()
public:
FHorizonGameItemTableRowFragment() {}
};接著就可以在TableRow中宣告這個property，而這個property需要的meta如下：
BaseStruct用來限定Editor中可以顯示哪些Struct出來，而ExcludeBaseStruct可以把這個BaseFragment從選項中排除。
UPROPERTY(EditAnywhere, BlueprintReadOnly, Category = "CustomData",
meta = (BaseStruct = "/Script/MyGame.HorizonGameItemTableRowFragment", ExcludeBaseStruct))
FInstancedStruct CustomDataFragment;然後就可以根據需求繼承這個BaseFragment並加入任何需要的資料。
USTRUCT()
struct MYGAME_API FHorizonGameItemTableRowFragment_Test : public FHorizonGameItemTableRowFragment
{
GENERATED_BODY()
public:
FHorizonGameItemTableRowFragment_Test() {}
UPROPERTY(EditAnywhere, BlueprintReadOnly, Category = "Data")
FText TestText;
UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Data")
int32 TestInt = 1;
};實際在DataTable中使用起來的體驗如下圖，我們可以自由選到所有繼承自BaseFragment的struct，裡面對應可編輯的欄位也會動態反射顯示出來：
接著可以用以下方法拿出我們所定義的Fragment：
bool UMyDemoLibrary::GetItemFragment(const FInstancedStruct& InInstancedStruct, FHorizonGameItemTableRowFragment_Test& OutFragment)
{
bool bResult = false;
const auto pFragment = InInstancedStruct.GetPtr<FHorizonGameItemTableRowFragment_Test>();
if(pFragment)
{
OutFragment = *pFragment;
bResult = true;
}
return bResult;
}
FHorizonGameItemTableRowFragment_Test fragment;
bool bHasFragment = GetItemFragment(InInstancedStruct, fragment);
if(bHasFragment)
{
fragment.TestText;
fragment.TestInt;
}不過這方法的缺點是，相關的擴充功能只能在C++中進行，因為我們無法用BP去剛剛宣告出來的BaseFragment。
使用UObject製作Fragment的例子
基本概念跟上面的例子差不多，只是我們需要換成使用UObject宣告BaseFragment，另外要記得UCLASS裡要加上DefaultToInstanced跟EditInlineNew。
UCLASS(DefaultToInstanced, EditInlineNew, Blueprintable, BlueprintType, Abstract)
class UHorizonGameItemDataAssetFragment : public UObject
{
GENERATED_BODY()
};宣告使用，可以在BP或DataAsset中宣告，這邊宣告在DataAsset中：
UCLASS(Blueprintable)
class HORIZONGAMEITEM_API UHorizonGameItemDataAsset : public UDataAsset
{
GENERATED_BODY()
public:
UPROPERTY(EditAnywhere, BlueprintReadOnly, Category = Display, Instanced)
TObjectPtr<UHorizonGameItemDataAssetFragment> Fragment;
};接著就可以繼承下來做資料的擴充
UCLASS()
class UHorizonGameItemDataAssetFragment_Test : public UHorizonGameItemDataAssetFragment
{
GENERATED_BODY()
public:
UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Data")
int32 TestInt = 0;
UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Data")
FText TestText;
};當我們把他宣告在DataAsset中時，就可以根據狀況選擇需要的Fragment，另外除了C++外，也可以純使用BP下來做欄位的擴充，裡面的可編輯欄位也能動態的反射顯示出來。
使用法方就是把DataAsset讀進來後，直接對Fragment做Cast就行：
UHorizonGameItemDataAsset* pDataAsset = Cast<UHorizonGameItemDataAsset>(InSoftObjectPath.TryLoad());
auto pFragment = Cast<UHorizonGameItemDataAssetFragment_Test>(pDataAsset->Fragment);
if(pFragment)
{
pFragment->TestText;
pFragment->TestInt;
}結論
二種製作Fragment的方法各有優缺點，就資料擴充性而言由於InstanceStruct只能在C++中進行擴充，而UObject則可以同時在C++/BP中做，這讓UObject版本帶來某方面的優勢，然而，這方法的缺點是需要遊戲設計者另外製做出BP或DataAsset才能夠做資料的編輯，也就是說會多一個uasset，這在某些需要定義大量道具、武器裝備數據的應用是無法接受的；相反的InstanceStruct則可以很自然的作為TableRow的擴充嵌入DataTable。關於這點，我們可以看下圖，CustomDataFragment使用的是InstanceStruct方法，而CustomDataAsset則是使用UObject的方法，這個DataAsset中包含了Fragment的定義。
因此哪種方法比較好，可能要看遊戲的設計而決定。例如UE5導入的MassFramework，裡面的FMassFragment是基於InstanceStruct的設計；而在Unreal官方釋出的Lyra範例專案中，他的裝備系統是基於UObject Fragment方法的設計。另外若是多個TableRow需要「共用」參數的話，或許UObject Fragment也是個不錯的解決方案。我自己是在資料表中同時提供二種方式，讓使用者自行決定要使用哪種方式做擴充。
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