Blueprint events in C++.md

2021-03-13_11:48:31

Blueprint events in C++

A BlueprintImplementableEvent is implemented in a Blueprint subclass by right-click in the Event Graph add selecting Add Event > Event <FUNCTION_NAME>. This will create a red event start node with an output execution pin.

A complete example of a Blueprint configurable C++ Actor class.

Declaration:

UCLASS(Blueprintable)
class AMyActor : public AActor
{
    GENERATED_BODY()
public:
    UFUNCTION(BlueprintCallable)
    void MyFunction();

    UFUNCTION(BlueprintImplementableEvent)
    void MyBlueprintFunction();

    UFUNCTION(BlueprintNativeEvent)
    void MyOptionalBlueprintFunction();
}

Implementation:

void AMyActor::MyFunction()
{
    UE_LOG(
        LogTemp, Display,
        TEXT("This function can be called from a Blueprint Visual Script."));
}

/*
This function should not be implemented in C++, it should be implemented in Blueprint by a Blueprint class having this class as its parent.
void AMyActor::MyBlueprintFunction()
{
}
*/

void MyActor::MyOptionalBlueprintFunction_Implementation()
{
    UE_LOG(
        LogTemp, Display,
        TEXT("The Blueprint subclass can chose to override this."));
}

Reference parameters

Parameters can be passed by reference to a UFUNCTION. Such parameters become output pins in Visual Scripts.

UFUNCTION(BlueprintCallable)
void Add(FMyNumeric Lhs, FMyNumeric Rhs, FMyNumeric& Result);

We can add UPARAM(ref) to make the parameter an InOut parameter. Such parameters become input pins in Visual Scripts.

UFUNCTION(BlueprintCallable)
void InitializeMyStruct(UPARAM(ref) FMyStruct& MyStruct);