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03-UnrealEngine/Gameplay/UObject/UnrealSpecifiers/Meta/Blueprint/ExposeOnSpawn/ExposeOnSpawn.md

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+# ExposeOnSpawn
+
+- **功能描述：** 使该属性在ContructObject或SpawnActor等创建对象的时候暴露出来。
+- **使用位置：** UPROPERTY
+- **引擎模块：** Blueprint
+- **元数据类型：** bool
+- **常用程度：** ★★★★★
+
+使该属性在ContructObject或SpawnActor等创建对象的时候暴露出来。
+
+- 具体来说，通过在源码搜索，这个标记在UK2Node_AddComponent，UK2Node_ConstructObjectFromClass，UK2Node_SpawnActor，UK2Node_LatentGameplayTaskCall的时候用到。
+- 在C++里设置的效果等同于在蓝图里勾上ExposeOnSpawn。
+- 该meta的设置也会同时设置到PropertyFlags里的CPF_ExposeOnSpawn
+
+## 测试代码：
+
+```cpp
+
+UCLASS(BlueprintType)
+class INSIDER_API UMyProperty_ExposeOnSpawn :public UObject
+{
+	GENERATED_BODY()
+public:
+	//	(Category = MyProperty_ExposeOnSpawn, ModuleRelativePath = Property/Blueprint/MyProperty_ExposeOnSpawn.h)
+	//	PropertyFlags:	CPF_Edit | CPF_BlueprintVisible | CPF_ZeroConstructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic 
+
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+	FString MyString = TEXT("First");
+
+	//	(Category = MyProperty_ExposeOnSpawn, ExposeOnSpawn = , ModuleRelativePath = Property/Blueprint/MyProperty_ExposeOnSpawn.h)
+	//	PropertyFlags:	CPF_Edit | CPF_BlueprintVisible | CPF_ZeroConstructor | CPF_ExposeOnSpawn | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic 
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, meta = (ExposeOnSpawn))
+	FString MyString_ExposeOnSpawn = TEXT("Second");
+};
+```
+
+## 测试效果：
+
+可见MyString_ExposeOnSpawn 暴露了出来，而MyString 没有。
+
+![Untitled](Untitled.png)
+
+## 原理：
+
+在UHT的时候会分析如果包含ExposeOnSpawn就会同步设置CPF_ExposeOnSpawn。
+
+而在IsPropertyExposedOnSpawn这个函数里具体判断是否要暴露，这个函数被上述的4个函数节点引用。源码里举UK2Node_ConstructObjectFromClass里的CreatePinsForClass作为例子，可见只有bIsExposedToSpawn 的时候才会为蓝图节点开始创建额外的Pin引脚。
+
+```cpp
+if (propertySettings.MetaData.ContainsKey(UhtNames.ExposeOnSpawn))
+{
+	propertySettings.PropertyFlags |= EPropertyFlags.ExposeOnSpawn;
+}
+
+bool UEdGraphSchema_K2::IsPropertyExposedOnSpawn(const FProperty* Property)
+{
+	Property = FBlueprintEditorUtils::GetMostUpToDateProperty(Property);
+	if (Property)
+	{
+		const bool bMeta = Property->HasMetaData(FBlueprintMetadata::MD_ExposeOnSpawn);
+		const bool bFlag = Property->HasAllPropertyFlags(CPF_ExposeOnSpawn);
+		if (bMeta != bFlag)
+		{
+			const FCoreTexts& CoreTexts = FCoreTexts::Get();
+
+			UE_LOG(LogBlueprint, Warning
+				, TEXT("ExposeOnSpawn ambiguity. Property '%s', MetaData '%s', Flag '%s'")
+				, *Property->GetFullName()
+				, bMeta ? *CoreTexts.True.ToString() : *CoreTexts.False.ToString()
+				, bFlag ? *CoreTexts.True.ToString() : *CoreTexts.False.ToString());
+		}
+		return bMeta || bFlag;
+	}
+	return false;
+}
+
+void UK2Node_ConstructObjectFromClass::CreatePinsForClass(UClass* InClass, TArray<UEdGraphPin*>* OutClassPins)
+{
+	for (TFieldIterator<FProperty> PropertyIt(InClass, EFieldIteratorFlags::IncludeSuper); PropertyIt; ++PropertyIt)
+{
+	FProperty* Property = *PropertyIt;
+	UClass* PropertyClass = CastChecked<UClass>(Property->GetOwner<UObject>());
+	const bool bIsDelegate = Property->IsA(FMulticastDelegateProperty::StaticClass());
+	const bool bIsExposedToSpawn = UEdGraphSchema_K2::IsPropertyExposedOnSpawn(Property);
+	const bool bIsSettableExternally = !Property->HasAnyPropertyFlags(CPF_DisableEditOnInstance);
+
+	if(	bIsExposedToSpawn &&
+		!Property->HasAnyPropertyFlags(CPF_Parm) && 
+		bIsSettableExternally &&
+		Property->HasAllPropertyFlags(CPF_BlueprintVisible) &&
+		!bIsDelegate &&
+		(nullptr == FindPin(Property->GetFName()) ) &&
+		FBlueprintEditorUtils::PropertyStillExists(Property))
+	{
+		if (UEdGraphPin* Pin = CreatePin(EGPD_Input, NAME_None, Property->GetFName()))
+		{
+}
+```