Multi-Threading: How to Create Threads in UE4

//~~~~~ Multi Threading ~~~
class FPrimeNumberWorker : public FRunnable
{	
	/** Singleton instance, can access the thread any time via static accessor, if it is active! */
	static  FPrimeNumberWorker* Runnable;
	
	/** Thread to run the worker FRunnable on */
	FRunnableThread* Thread;
	
	/** The Data Ptr */
	TArray<uint32>* PrimeNumbers;
	
	/** The PC */
	AVictoryGamePlayerController* ThePC;
	
	/** Stop this thread? Uses Thread Safe Counter */
	FThreadSafeCounter StopTaskCounter;
	
	//The actual finding of prime numbers
	int32 FindNextPrimeNumber();
	
private:
	int32				PrimesFoundCount;
public:
	
	int32				TotalPrimesToFind;

	//Done?
	bool IsFinished() const
	{
		return PrimesFoundCount >= TotalPrimesToFind;
	}
	
	//~~~ Thread Core Functions ~~~
	
	//Constructor / Destructor
	FPrimeNumberWorker(TArray<uint32>& TheArray, const int32 IN_PrimesToFindPerTick, AVictoryGamePlayerController* IN_PC);
	virtual ~FPrimeNumberWorker();

	// Begin FRunnable interface.
	virtual bool Init();
	virtual uint32 Run();
	virtual void Stop();
	// End FRunnable interface
	
	/** Makes sure this thread has stopped properly */
	void EnsureCompletion();
	
	
	
	//~~~ Starting and Stopping Thread ~~~
	
	
	
	/* 
		Start the thread and the worker from static (easy access)! 
		This code ensures only 1 Prime Number thread will be able to run at a time. 
		This function returns a handle to the newly started instance.
	*/
	static FPrimeNumberWorker* JoyInit(TArray<uint32>& TheArray, const int32 IN_TotalPrimesToFind, AVictoryGamePlayerController* IN_PC);

	/** Shuts down the thread. Static so it can easily be called from outside the thread context */
	static void Shutdown();

        static bool IsThreadFinished();

};

//***********************************************************
//Thread Worker Starts as NULL, prior to being instanced
//		This line is essential! Compiler error without it
FPrimeNumberWorker* FPrimeNumberWorker::Runnable = NULL;
//***********************************************************

FPrimeNumberWorker::FPrimeNumberWorker(TArray<uint32>& TheArray, const int32 IN_TotalPrimesToFind, AVictoryGamePlayerController* IN_PC)
	: ThePC(IN_PC)
	, TotalPrimesToFind(IN_TotalPrimesToFind)
	, StopTaskCounter(0)
	, PrimesFoundCount(0)
{
	//Link to where data should be stored
	PrimeNumbers = &TheArray;
	
	Thread = FRunnableThread::Create(this, TEXT("FPrimeNumberWorker"), 0, TPri_BelowNormal); //windows default = 8mb for thread, could specify more
}

FPrimeNumberWorker::~FPrimeNumberWorker()
{
	delete Thread;
	Thread = NULL;
}

//Init
bool FPrimeNumberWorker::Init()
{
	//Init the Data 
	PrimeNumbers->Empty();
	PrimeNumbers->Add(2);
	PrimeNumbers->Add(3);
	
	if(ThePC) 
	{
		ThePC->ClientMessage("**********************************");
		ThePC->ClientMessage("Prime Number Thread Started!");
		ThePC->ClientMessage("**********************************");
	}
	return true;
}

//Run
uint32 FPrimeNumberWorker::Run()
{
	//Initial wait before starting
	FPlatformProcess::Sleep(0.03);
	
	//While not told to stop this thread 
	//		and not yet finished finding Prime Numbers
	while (StopTaskCounter.GetValue() == 0 && ! IsFinished())
	{
		PrimeNumbers->Add(FindNextPrimeNumber());
		PrimesFoundCount++;
		
		//***************************************
		//Show Incremental Results in Main Game Thread!
		
		//	Please note you should not create, destroy, or modify UObjects here.
		//	  Do those sort of things after all thread are completed.
		
		//	  All calcs for making stuff can be done in the threads
		//	     But the actual making/modifying of the UObjects should be done in main game thread.
		ThePC->ClientMessage(FString::FromInt(PrimeNumbers->Last()));
		//***************************************
		
		//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
		//prevent thread from using too many resources
		//FPlatformProcess::Sleep(0.01);
		//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	}
	
	//Run FPrimeNumberWorker::Shutdown() from the timer in Game Thread that is watching
        //to see when FPrimeNumberWorker::IsThreadFinished()
	
	return 0;
}

//stop
void FPrimeNumberWorker::Stop()
{
	StopTaskCounter.Increment();
}

FPrimeNumberWorker* FPrimeNumberWorker::JoyInit(TArray<uint32>& TheArray, const int32 IN_TotalPrimesToFind, AVictoryGamePlayerController* IN_PC)
{
	//Create new instance of thread if it does not exist
	//		and the platform supports multi threading!
	if (!Runnable && FPlatformProcess::SupportsMultithreading())
	{
		Runnable = new FPrimeNumberWorker(TheArray,IN_TotalPrimesToFind,IN_PC);			
	}
	return Runnable;
}

void FPrimeNumberWorker::EnsureCompletion()
{
	Stop();
	Thread->WaitForCompletion();
}

void FPrimeNumberWorker::Shutdown()
{
	if (Runnable)
	{
		Runnable->EnsureCompletion();
		delete Runnable;
		Runnable = NULL;
	}
}

bool FPrimeNumberWorker::IsThreadFinished()
{
	if(Runnable) return Runnable->IsFinished();
	return true;
}
int32 FPrimeNumberWorker::FindNextPrimeNumber()
{
	//Last known prime number  + 1
	int32 TestPrime = PrimeNumbers->Last();
	
	bool NumIsPrime = false;
	while( ! NumIsPrime)
	{
		NumIsPrime = true;
		
		//Try Next Number
		TestPrime++;
		
		//Modulus from 2 to current number - 1 
		for(int32 b = 2; b < TestPrime; b++)
		{
			//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
			//prevent thread from using too many resources
			//FPlatformProcess::Sleep(0.01);
			//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	
			if(TestPrime % b == 0) 
			{
				NumIsPrime = false;
				break;
				//~~~
			}
		}
	}
	
	//Success!
	return TestPrime;
}

//In the .h for the player controller
// this is the actual data
TArray<uint32> PrimeNumbers;

//player controller .cpp
//Multi-threading, returns handle that could be cached.
//		use static function FPrimeNumberWorker::Shutdown() if necessary
FPrimeNumberWorker::JoyInit(PrimeNumbers, 50000, this);

FPlatformProcess::Sleep(seconds);

#include "Async.h"
...
AsyncTask(ENamedThreads::GameThread, []() {
     // code to execute on game thread here
 });

/**
 * Gets single thread interface pointer used for ticking this runnable when multi-threading is disabled.
 * If the interface is not implemented, this runnable will not be ticked when FPlatformProcess::SupportsMultithreading() is false.
 *
 * @return Pointer to the single thread interface or nullptr if not implemented.
 */
virtual class FSingleThreadRunnable* GetSingleThreadInterface( )
{
	return nullptr;
}