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

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+# ForceAsFunction
+
+- **功能描述：** 把C++里用BlueprintImplementableEvent或NativeEvent定义的事件强制改为函数在子类中覆写。
+- **使用位置：** UFUNCTION
+- **引擎模块：** Blueprint
+- **元数据类型：** bool
+- **常用程度：** ★★★
+
+把C++里用BlueprintImplementableEvent或NativeEvent定义的事件强制改为函数在子类中覆写。
+
+什么情况下需要把Event改成函数？
+
+- 变成函数后，在实现的时候就可以定义内部的局部变量。当然也就失去了调用Delay等延时函数的能力。
+- 事件不能有输出的参数，但是如果想要一个有输出的函数在蓝图类里覆写（得BlueprintImplementableEvent或NativeEvent），则默认的以事件方式重载是不行的。因此这个时候把这个事件强迫改为函数的形式，就可以正常的覆写。
+- 带有输出或返回参数的Event会默认被改为function，即使没有加上ForceAsFunction。
+
+## 测试代码：
+
+```cpp
+
+UCLASS(Blueprintable, BlueprintType)
+class INSIDER_API AMyFunction_ForceAsFunction :public AActor
+{
+public:
+	GENERATED_BODY()
+public:
+//FUNC_Native | FUNC_Event | FUNC_Public | FUNC_BlueprintCallable | FUNC_BlueprintEvent 
+	UFUNCTION(BlueprintCallable, BlueprintNativeEvent)
+	void MyNativeEvent_Default(const FString& name);
+
+	//FUNC_Event | FUNC_Public | FUNC_BlueprintCallable | FUNC_BlueprintEvent 
+	UFUNCTION(BlueprintCallable, BlueprintImplementableEvent)
+	void MyImplementableEvent_Default(const FString& name);
+
+public:
+	//(ForceAsFunction = , ModuleRelativePath = Function/MyFunction_ForceAsFunction.h)
+	//FUNC_Native | FUNC_Event | FUNC_Public | FUNC_BlueprintCallable | FUNC_BlueprintEvent 
+	UFUNCTION(BlueprintCallable, BlueprintNativeEvent, meta = (ForceAsFunction))
+	void MyNativeEvent_ForceAsFunction(const FString& name);
+
+	////(ForceAsFunction = , ModuleRelativePath = Function/MyFunction_ForceAsFunction.h)
+	//FUNC_Event | FUNC_Public | FUNC_BlueprintCallable | FUNC_BlueprintEvent 
+	UFUNCTION(BlueprintCallable, BlueprintImplementableEvent, meta = (ForceAsFunction))
+	void MyImplementableEvent_ForceAsFunction(const FString& name);
+
+public:
+	//FUNC_Native | FUNC_Event | FUNC_Public | FUNC_HasOutParms | FUNC_BlueprintCallable | FUNC_BlueprintEvent 
+	UFUNCTION(BlueprintCallable, BlueprintNativeEvent)
+	bool MyNativeEvent_Output(const FString& name, int32& OutValue);
+
+	//FUNC_Event | FUNC_Public | FUNC_HasOutParms | FUNC_BlueprintCallable | FUNC_BlueprintEvent 
+	UFUNCTION(BlueprintCallable, BlueprintImplementableEvent)
+	bool MyImplementableEvent_Output(const FString& name, int32& OutValue);
+
+	//(ForceAsFunction = , ModuleRelativePath = Function/MyFunction_ForceAsFunction.h)
+	//FUNC_Native | FUNC_Event | FUNC_Public | FUNC_HasOutParms | FUNC_BlueprintCallable | FUNC_BlueprintEvent 
+	UFUNCTION(BlueprintCallable, BlueprintNativeEvent, meta = (ForceAsFunction))
+	bool MyNativeEvent_Output_ForceAsFunction(const FString& name, int32& OutValue);
+
+	//(ForceAsFunction = , ModuleRelativePath = Function/MyFunction_ForceAsFunction.h)
+	//FUNC_Event | FUNC_Public | FUNC_HasOutParms | FUNC_BlueprintCallable | FUNC_BlueprintEvent 
+	UFUNCTION(BlueprintCallable, BlueprintImplementableEvent, meta = (ForceAsFunction))
+	bool MyImplementableEvent_Output_ForceAsFunction(const FString& name, int32& OutValue);
+};
+```
+
+## 蓝图中效果：
+
+在函数上覆写的时候，会发现只有MyNativeEvent_Default和MyImplementableEvent_Default被默认覆写为事件，其他都以函数的方式被覆写。
+
+图里展示了MyImplementableEvent_ForceAsFunction被改为函数后，可以在内部定义局部变量。
+
+也展示了MyNativeEvent_Output这种拥有输出参数的事件被覆写成函数后的函数体。
+
+但无论是覆写为事件还是函数，被调用的时候用法并无区别。
+
+![Untitled](Untitled.png)
+
+## 原理：
+
+判断一个函数是否是事件的逻辑为以下函数：
+
+主要看第二if和最后判断，BlueprintImplementableEvent或NativeEvent的函数上都会加上FUNC_BlueprintEvent标签，因此如果带有ForceAsFunction元数据或者有输出参数（返回值也算），就只能显示为函数。
+
+```cpp
+bool UEdGraphSchema_K2::FunctionCanBePlacedAsEvent(const UFunction* InFunction)
+{
+	// First check we are override-able, non-static, non-const and not marked thread safe
+	if (!InFunction || !CanKismetOverrideFunction(InFunction) || InFunction->HasAnyFunctionFlags(FUNC_Static|FUNC_Const) || FBlueprintEditorUtils::HasFunctionBlueprintThreadSafeMetaData(InFunction))
+	{
+		return false;
+	}
+
+	// Check if meta data has been set to force this to appear as blueprint function even if it doesn't return a value.
+	if (InFunction->HasAllFunctionFlags(FUNC_BlueprintEvent) && InFunction->HasMetaData(FBlueprintMetadata::MD_ForceAsFunction))
+	{
+		return false;
+	}
+
+	// Then look to see if we have any output, return, or reference params
+	return !HasFunctionAnyOutputParameter(InFunction);
+}
+```