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03-UnrealEngine/Gameplay/UObject/UnrealSpecifiers/Meta/Actor/ChildCanTick/ChildCanTick.md

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+# ChildCanTick
+
+- **功能描述：** 标记允许其蓝图子类可以接受响应Tick事件
+- **使用位置：** UCLASS
+- **引擎模块：** Actor
+- **元数据类型：** bool
+- **限制类型：** Actor或ActorComponent子类
+- **关联项：** [ChildCannotTick](../ChildCannotTick.md)
+- **常用程度：** ★★★
+
+要在蓝图中重载Tick事件函数并只会在编译的时候触发判断。
+
+```cpp
+//(BlueprintType = true, ChildCannotTick = , IncludePath = Class/Blueprint/MyActor_ChildTick.h, IsBlueprintBase = true, ModuleRelativePath = Class/Blueprint/MyActor_ChildTick.h)
+UCLASS(Blueprintable,meta=(ChildCanTick))
+class INSIDER_API AMyActor_ChildCanTick : public AActor
+{
+	GENERATED_BODY()
+public:
+	AMyActor_ChildCanTick()
+	{
+		PrimaryActorTick.bCanEverTick = false;
+	}
+};
+
+//(BlueprintType = true, ChildCanTick = , IncludePath = Class/Blueprint/MyActor_ChildTick.h, IsBlueprintBase = true, ModuleRelativePath = Class/Blueprint/MyActor_ChildTick.h)
+UCLASS(Blueprintable,meta=(ChildCanTick))
+class INSIDER_API UMyActorComponent_ChildCanTick : public UActorComponent
+{
+	GENERATED_BODY()
+public:
+};
+
+//(BlueprintType = true, ChildCannotTick = , IncludePath = Class/Blueprint/MyActor_ChildTick.h, IsBlueprintBase = true, ModuleRelativePath = Class/Blueprint/MyActor_ChildTick.h)
+UCLASS(Blueprintable,meta=(ChildCannotTick))
+class INSIDER_API AMyActor_ChildCannotTick : public AActor
+{
+	GENERATED_BODY()
+public:
+};
+
+//(BlueprintType = true, ChildCannotTick = , IncludePath = Class/Blueprint/MyActor_ChildTick.h, IsBlueprintBase = true, ModuleRelativePath = Class/Blueprint/MyActor_ChildTick.h)
+UCLASS(Blueprintable,meta=(ChildCannotTick))
+class INSIDER_API UMyActorComponent_ChildCannotTick : public UActorComponent
+{
+	GENERATED_BODY()
+public:
+};
+```
+
+蓝图Actor或ActorComponent里测试：
+
+也注意到这个判断跟蓝图中是否开启Tick并没有关系。
+
+![Untitled](Untitled.png)
+
+![Untitled](Untitled%201.png)
+
+而AMyActor_ChildCanTick类里虽然已经手动关闭了PrimaryActorTick.bCanEverTick，但是在子类里依然可以正常的Tick（在编译的时候内部可以正常的再重新开启bCanEverTick）。
+
+![Untitled](Untitled%202.png)
+
+## 源码里判断的逻辑：
+
+开启bCanEverTick=true的条件有3，一是EngineSettings->bCanBlueprintsTickByDefault，二是父类是AActor或UActorComponent本身，三是C++基类上有ChildCanTick的标记。
+
+```cpp
+void FKismetCompilerContext::SetCanEverTick() const
+{
+// RECEIVE TICK
+if (!TickFunction->bCanEverTick)
+{
+	// Make sure that both AActor and UActorComponent have the same name for their tick method
+	static FName ReceiveTickName(GET_FUNCTION_NAME_CHECKED(AActor, ReceiveTick));
+	static FName ComponentReceiveTickName(GET_FUNCTION_NAME_CHECKED(UActorComponent, ReceiveTick));
+
+	if (const UFunction* ReceiveTickEvent = FKismetCompilerUtilities::FindOverriddenImplementableEvent(ReceiveTickName, NewClass))
+	{
+		// We have a tick node, but are we allowed to?
+
+		const UEngine* EngineSettings = GetDefault<UEngine>();
+		const bool bAllowTickingByDefault = EngineSettings->bCanBlueprintsTickByDefault;
+
+		const UClass* FirstNativeClass = FBlueprintEditorUtils::FindFirstNativeClass(NewClass);
+		const bool bHasCanTickMetadata = (FirstNativeClass != nullptr) && FirstNativeClass->HasMetaData(FBlueprintMetadata::MD_ChildCanTick);
+		const bool bHasCannotTickMetadata = (FirstNativeClass != nullptr) && FirstNativeClass->HasMetaData(FBlueprintMetadata::MD_ChildCannotTick);
+		const bool bHasUniversalParent = (FirstNativeClass != nullptr) && ((AActor::StaticClass() == FirstNativeClass) || (UActorComponent::StaticClass() == FirstNativeClass));
+
+		if (bHasCanTickMetadata && bHasCannotTickMetadata)
+		{
+			// User error: The C++ class has conflicting metadata
+			const FString ConlictingMetadataWarning = FText::Format(
+				LOCTEXT("HasBothCanAndCannotMetadataFmt", "Native class %s has both '{0}' and '{1}' metadata specified, they are mutually exclusive and '{1}' will win."),
+				FText::FromString(FirstNativeClass->GetPathName()),
+				FText::FromName(FBlueprintMetadata::MD_ChildCanTick),
+				FText::FromName(FBlueprintMetadata::MD_ChildCannotTick)
+			).ToString();
+			MessageLog.Warning(*ConlictingMetadataWarning);
+		}
+
+		if (bHasCannotTickMetadata)
+		{
+			// This could only happen if someone adds bad metadata to AActor or UActorComponent directly
+			check(!bHasUniversalParent);
+
+			// Parent class has forbidden us to tick
+			const FString NativeClassSaidNo = FText::Format(
+				LOCTEXT("NativeClassProhibitsTickingFmt", "@@ is not allowed as the C++ parent class {0} has disallowed Blueprint subclasses from ticking.  Please consider using a Timer instead of Tick."),
+				FText::FromString(FirstNativeClass->GetPathName())
+			).ToString();
+			MessageLog.Warning(*NativeClassSaidNo, FindLocalEntryPoint(ReceiveTickEvent));
+		}
+		else
+		{
+			if (bAllowTickingByDefault || bHasUniversalParent || bHasCanTickMetadata)
+			{
+				// We're allowed to tick for one reason or another
+				TickFunction->bCanEverTick = true;
+			}
+			else
+			{
+				// Nothing allowing us to tick
+				const FString ReceiveTickEventWarning = FText::Format(
+					LOCTEXT("ReceiveTick_CanNeverTickFmt", "@@ is not allowed for Blueprints based on the C++ parent class {0}, so it will never Tick!"),
+					FText::FromString(FirstNativeClass ? *FirstNativeClass->GetPathName() : TEXT("<null>"))
+				).ToString();
+				MessageLog.Warning(*ReceiveTickEventWarning, FindLocalEntryPoint(ReceiveTickEvent));
+
+				const FString ReceiveTickEventRemedies = FText::Format(
+					LOCTEXT("ReceiveTick_CanNeverTickRemediesFmt", "You can solve this in several ways:\n  1) Consider using a Timer instead of Tick.\n  2) Add meta=({0}) to the parent C++ class\n  3) Reparent the Blueprint to AActor or UActorComponent, which can always tick."),
+					FText::FromName(FBlueprintMetadata::MD_ChildCanTick)
+				).ToString();
+				MessageLog.Warning(*ReceiveTickEventRemedies);
+			}
+		}
+	}
+}
+}
+```

03-UnrealEngine/Gameplay/UObject/UnrealSpecifiers/Meta/Actor/ChildCannotTick.md

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+# ChildCannotTick
+
+- **功能描述：** 用于Actor或ActorComponent子类，标记允许其蓝图子类不可以接受响应Tick事件，哪怕父类可以Tick
+- **使用位置：** UCLASS
+- **元数据类型：** bool
+- **限制类型：** Actor类
+- **关联项：** [ChildCanTick](ChildCanTick/ChildCanTick.md)
+- **常用程度：** ★★★