Output Delegates · Xander's Notebook

If a UFUNCTION takes a dynamic delegate as a parameter, Blueprints show it as an extra red pin on the node. You can drag off that pin to create a custom event that you call whenever your (async) work is finished.

Declaring the delegate and the function

// A delegate without parameters. Use DECLARE_DYNAMIC_DELEGATE_OneParam etc. to pass data back.
DECLARE_DYNAMIC_DELEGATE(FOnSomethingHappenedSignature);

UCLASS()
class UMyBlueprintLibrary : public UBlueprintFunctionLibrary
{
    GENERATED_BODY()

public:
    UFUNCTION(BlueprintCallable)
    static void DoSomething(const FOnSomethingHappenedSignature& OnSomethingHappened)
    {
        AsyncTask(ENamedThreads::AnyBackgroundThreadNormalTask, [OnSomethingHappened]()
        {
            // ... heavy work on a background thread ...

            // Go back to the game thread before calling into Blueprints
            AsyncTask(ENamedThreads::GameThread, [OnSomethingHappened]()
            {
                OnSomethingHappened.ExecuteIfBound();
            });
        });
    }
};

Blueprint code must run on the game thread. Executing the delegate directly from a background thread can cause random crashes, so always dispatch back to ENamedThreads::GameThread first.

Passing data back

DECLARE_DYNAMIC_DELEGATE_OneParam(FOnResultSignature, int32, Result);

UFUNCTION(BlueprintCallable)
static void CalculateSomething(const FOnResultSignature& OnResult);

The delegate is passed by value into the lambda. A dynamic delegate only holds a weak reference to the object it is bound to, so ExecuteIfBound safely does nothing if that Blueprint was destroyed in the meantime.

If you need multiple outputs (e.g. On Success and On Failure) or a node that shows its output pins without dragging off events, look into UBlueprintAsyncActionBase. It creates a proper latent-style async node.