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03-UnrealEngine/Gameplay/UObject/UnrealSpecifiers/Meta/Material/MaterialControlFlow/MaterialControlFlow.md

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+# MaterialControlFlow
+
+- **功能描述：** 标识该UMaterialExpression为一个控制流节点，当前在材质蓝图右键菜单中隐藏。
+- **使用位置：** UCLASS
+- **引擎模块：** Material
+- **元数据类型：** bool
+- **限制类型：** UMaterialExpression子类
+- **常用程度：** ★
+
+标识该UMaterialExpression为一个控制流节点，当前在材质蓝图右键菜单中隐藏。
+
+一般是在IfThenElse，ForLoop这种节点上，当前引擎实现还未完善，因此该功能禁用。
+
+## 源码例子：
+
+```cpp
+UCLASS(collapsecategories, hidecategories=Object, MinimalAPI)
+class UMaterialExpressionIf : public UMaterialExpression
+{}
+
+UCLASS(collapsecategories, hidecategories = Object, MinimalAPI, meta=(MaterialControlFlow))
+class UMaterialExpressionIfThenElse : public UMaterialExpression
+{}
+```
+
+## 测试效果：
+
+可以找到If节点，但是无法调用IfThenElse节点。
+
+![Untitled](Untitled.png)
+
+## 原理：
+
+在遍历所有可用FMaterialExpression的时候，如果有MaterialControlFlow则略过。当前引擎下AllowMaterialControlFlow一直未false，未实现。
+
+```cpp
+
+// r.MaterialEnableControlFlow is removed and the feature is forced disabled as how control flow should be
+// implemented in the material editor is still under discussion.
+inline bool AllowMaterialControlFlow()
+{
+	return false;
+}
+
+void MaterialExpressionClasses::InitMaterialExpressionClasses()
+{
+		static const auto CVarMaterialEnableNewHLSLGenerator = IConsoleManager::Get().FindTConsoleVariableDataInt(TEXT("r.MaterialEnableNewHLSLGenerator"));
+		const bool bEnableControlFlow = AllowMaterialControlFlow();
+		const bool bEnableNewHLSLGenerator = CVarMaterialEnableNewHLSLGenerator->GetValueOnAnyThread() != 0;
+	
+		for( TObjectIterator<UClass> It ; It ; ++It )
+		{
+			if( Class->IsChildOf(UMaterialExpression::StaticClass()) )
+			{
+					// Hide node types related to control flow, unless it's enabled
+					if (!bEnableControlFlow && Class->HasMetaData("MaterialControlFlow"))
+					{
+									continue;
+					}
+					
+					if (!bEnableNewHLSLGenerator && Class->HasMetaData("MaterialNewHLSLGenerator"))
+					{
+									continue;
+					}
+					
+					// Hide node types that are tagged private
+					if(Class->HasMetaData(TEXT("Private")))
+					{
+									continue;
+					}
+				AllExpressionClasses.Add(MaterialExpression);
+			}
+		}
+}
+```