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

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+# AllowPrivateAccess
+
+- **功能描述：** 允许一个在C++中private的属性，可以在蓝图中访问。
+- **使用位置：** UPROPERTY
+- **元数据类型：** bool
+- **关联项：** [BlueprintProtected](../BlueprintProtected/BlueprintProtected.md)
+- **常用程度：** ★★★★★
+
+允许一个在C++中private的属性，可以在蓝图中访问。
+
+AllowPrivateAccess的意义是允许这个属性在C++是private的，不允许C++子类访问，但又允许其暴露到蓝图中访问。
+
+## 测试代码：
+
+```cpp
+public:
+	//CPF_BlueprintVisible | CPF_ZeroConstructor | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic 
+	UPROPERTY(BlueprintReadWrite)
+	int32 MyNativeInt_NativePublic;
+private:
+	//CPF_ZeroConstructor | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPrivate 
+	//error : BlueprintReadWrite should not be used on private members
+	UPROPERTY()
+	int32 MyNativeInt_NativePrivate;
+
+	//(AllowPrivateAccess = TRUE, Category = MyFunction_Access, ModuleRelativePath = Function/MyFunction_Access.h)
+	//CPF_BlueprintVisible | CPF_ZeroConstructor | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPrivate 
+	UPROPERTY(BlueprintReadWrite, meta = (AllowPrivateAccess = true))
+	int32 MyNativeInt_NativePrivate_AllowPrivateAccess;
+```
+
+在MyNativeInt_NativePrivate上如果尝试加上BlueprintReadWrite或BlueprintReadOnly都会触发UHT编译报错。
+
+## 蓝图里的效果：
+
+默认情况下MyNativeInt_NativePrivate_AllowPrivateAccess在蓝图里的访问权限和MyNativeInt_NativePublic一致。
+
+如果读者想要修改改属性在蓝图中的访问权限，则可以配合加上BlueprintProtected和BlueprintPrivate。
+
+![Untitled](Untitled.png)
+
+## 原理：
+
+UHT在识别属性的BlueprintReadWrite或BlueprintReadOnly标识符的时候，会同时检测是否有AllowPrivateAccess，没有的话会触发报错。
+
+因此AllowPrivateAccess的意义其实只是在阻止UHT的报错，这层检测报错过了之后，属性上的BlueprintReadWrite或BlueprintReadOnly就会被识别并发挥作用，从而可以在蓝图中访问。
+
+```cpp
+	private static void BlueprintReadWriteSpecifier(UhtSpecifierContext specifierContext)
+	{
+			bool allowPrivateAccess = context.MetaData.TryGetValue(UhtNames.AllowPrivateAccess, out string? privateAccessMD) && !privateAccessMD.Equals("false", StringComparison.OrdinalIgnoreCase);
+			if (specifierContext.AccessSpecifier == UhtAccessSpecifier.Private && !allowPrivateAccess)
+			{
+				context.MessageSite.LogError("BlueprintReadWrite should not be used on private members");
+			}
+	}
+	
+	private static void BlueprintReadOnlySpecifier(UhtSpecifierContext specifierContext)
+	{
+			bool allowPrivateAccess = context.MetaData.TryGetValue(UhtNames.AllowPrivateAccess, out string? privateAccessMD) && !privateAccessMD.Equals("false", StringComparison.OrdinalIgnoreCase);
+			if (specifierContext.AccessSpecifier == UhtAccessSpecifier.Private && !allowPrivateAccess)
+			{
+				context.MessageSite.LogError("BlueprintReadOnly should not be used on private members");
+			}
+	}
+
+```