How to use async task in Unreal Engine

Unreal Engine has a useful feature called AsyncTask that allows you to execute code asynchronously via Task Graph system. It functions by running specific code on a specified thread and is primarily used when the task is too heavy to be executed instantly without blocking the game thread.

AsyncTask(ENamedThreads::AnyThread, []()
{
	// This code will run asynchronously, without freezing the game thread
});

You can also create nested calls to asynchronous tasks, for example:

AsyncTask(ENamedThreads::AnyThread, []()
{
	// This code will run asynchronously, without freezing the game thread
		
	AsyncTask(ENamedThreads::GameThread, []()
	{
		// This code will be executed on the game thread
	});
});

#include "Async/Async.h"

DECLARE_DYNAMIC_DELEGATE_OneParam(FAsyncDelegateExample, const TArray<float>&, OutData);

// Class, meta, etc

UFUNCTION(BlueprintCallable)
static void AddNumbersAsync(TArray<float> InData, const FAsyncDelegateExample& Result)
{
	AsyncTask(ENamedThreads::AnyThread, [InData = MoveTemp(InData), Result]() mutable
	{
		// Just for example
		for (float& InDataElement : InData)
		{
			InDataElement += 10;
		}

		AsyncTask(ENamedThreads::GameThread, [OutData = MoveTemp(InData), Result]() mutable
		{
			Result.ExecuteIfBound(OutData);
		});
	});
}

Useful notes:

1. GameThread has a dedicated thread, whereas AnyThread, AnyHiPriThread, etc., use available threads from the pool.
2. There’s Async template function that is more flexible in some cases compared to AsyncTask (e.g., you can choose to execute the task in the thread pool, or to execute a callback when the task is done by the returned TFuture object), but is more limited to choosing the thread for the Task Graph system.
3. Async provides more flexibility compared to AsyncTask, especially with thread selection and callback handling.
4. Starting from UE 5.0, consider using the new Task Graph system for improved performance and flow in gameplay tasks: Tasks System docs .