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

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+# ExposedAsyncProxy
+
+- **功能描述：** 在 Async Task 节点中公开此类的一个代理对象。
+- **使用位置：** UCLASS
+- **引擎模块：** Blueprint
+- **元数据类型：** string="abc"
+- **限制类型：** Async Blueprint node
+- **关联项：** [HideSpawnParms](../Param/HideSpawnParms/HideSpawnParms.md), [HasDedicatedAsyncNode](../HasDedicatedAsyncNode/HasDedicatedAsyncNode.md), [HideThen](../HideThen/HideThen.md)
+- **常用程度：** ★★★
+
+在UK2Node_BaseAsyncTask中使用，用来为蓝图异步节点暴露一个异步对象引脚，以支持对这个异步行为的进一步操作。
+
+在源码里的用处一是UBlueprintAsyncActionBase的子类，二是UGameplayTask子类，皆是分别会有另外的UK2Node_BaseAsyncTask以及UK2Node_LatentGameplayTaskCall来解析类的声明定义并包装生成相应的异步蓝图节点。
+
+基类都是继承自UBlueprintAsyncActionBase。利用ExposedAsyncProxy 指定异步任务对象的名字。在异步蓝图节点上继续返回异步对象，可以在之后支持取消该异步操作。
+
+## 测试代码：
+
+UCancellableAsyncAction是引擎提供的继承自UBlueprintAsyncActionBase的一个便利的子类。UMyFunction_Async 定义了一个蓝图异步节点DelayLoop。
+
+```cpp
+DECLARE_DYNAMIC_MULTICAST_DELEGATE(FDelayOutputPin);
+UCLASS(Blueprintable, BlueprintType,meta = (ExposedAsyncProxy = MyAsyncObject))
+class INSIDER_API UMyFunction_Async :public UBlueprintAsyncActionBase
+{
+public:
+	GENERATED_BODY()
+public:
+	UPROPERTY(BlueprintAssignable)
+	FDelayOutputPin Loop;
+
+	UPROPERTY(BlueprintAssignable)
+	FDelayOutputPin Complete;
+
+	UFUNCTION(BlueprintCallable, meta = (BlueprintInternalUseOnly = "true", WorldContext = "WorldContextObject"), Category = "Flow Control")
+	static UMyFunction_Async* DelayLoop(const UObject* WorldContextObject, const float DelayInSeconds, const int Iterations);
+
+	virtual void Activate() override;
+
+	UFUNCTION()
+	static void Test();
+private:
+	const UObject* WorldContextObject = nullptr;
+	float MyDelay = 0.f;
+	int MyIterations = 0;
+	bool Active = false;
+
+	UFUNCTION()
+	void ExecuteLoop();
+
+	UFUNCTION()
+	void ExecuteComplete();
+};
+```
+
+## 默认的蓝图节点是：
+
+如果UMyFunction_Async 直接继承自UBlueprintAsyncActionBase，并且没有设置ExposedAsyncProxy，则生成的蓝图异步节点为为下图。
+
+![Untitled](Untitled.png)
+
+而如果继承自UCancellableAsyncAction (提供了Cancel方法)，并且设置ExposedAsyncProxy 为自己想要的AsyncObject引脚名称。
+
+```cpp
+UCLASS(Abstract, BlueprintType, meta = (ExposedAsyncProxy = AsyncAction), MinimalAPI)
+class UCancellableAsyncAction : public UBlueprintAsyncActionBase
+{
+	UFUNCTION(BlueprintCallable, Category = "Async Action")
+	ENGINE_API virtual void Cancel();
+}
+
+DECLARE_DYNAMIC_MULTICAST_DELEGATE(FDelayOutputPin);
+UCLASS(Blueprintable, BlueprintType,meta = (ExposedAsyncProxy = MyAsyncObject))
+class INSIDER_API UMyFunction_Async :public UCancellableAsyncAction 
+{}
+```
+
+## 修改后的效果如下图：
+
+![Untitled](Untitled%201.png)
+
+## 该Meta在源码中发生的位置：
+
+```cpp
+void UK2Node_BaseAsyncTask::AllocateDefaultPins()
+{
+	bool bExposeProxy = false;
+	bool bHideThen = false;
+	FText ExposeProxyDisplayName;
+	for (const UStruct* TestStruct = ProxyClass; TestStruct; TestStruct = TestStruct->GetSuperStruct())
+	{
+		bExposeProxy |= TestStruct->HasMetaData(TEXT("ExposedAsyncProxy"));
+		bHideThen |= TestStruct->HasMetaData(TEXT("HideThen"));
+		if (ExposeProxyDisplayName.IsEmpty())
+		{
+			ExposeProxyDisplayName = TestStruct->GetMetaDataText(TEXT("ExposedAsyncProxy"));
+		}
+	}
+
+	if (bExposeProxy)
+	{
+		UEdGraphPin* ProxyPin = CreatePin(EGPD_Output, UEdGraphSchema_K2::PC_Object, ProxyClass, FBaseAsyncTaskHelper::GetAsyncTaskProxyName());
+		if (!ExposeProxyDisplayName.IsEmpty())
+		{
+			ProxyPin->PinFriendlyName = ExposeProxyDisplayName;
+		}
+	}
+
+}
+```