NetworkDS
UE4.22
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UGameInstance::HandleNetworkError

当网络链接出现错误时，会调用该函数 (服务端拒绝玩家加入时也会调用)。
调用栈为（可以继承 UEngine 来替换 Engine 实现）：

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UEngine::HandleNetworkError -> UEngine::HandleNetworkFailure_NotifyGameInstance -> GameInstance->HandleNetworkError(FailureType, bIsServer);

VS 断点调用栈：

UEngine::HandleNetworkFailure实现为：

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// Runtime\Source\Runtime\Engine\Private\UnrealEngine.cpp
void UEngine::HandleNetworkFailure(UWorld *World, UNetDriver *NetDriver, ENetworkFailure::Type FailureType, const FString& ErrorString)
{
  UE_LOG(LogNet, Log, TEXT("NetworkFailure: %s, Error: '%s'"), ENetworkFailure::ToString(FailureType), *ErrorString);

  if (!NetDriver)
  {
    return;
  }

  // Only handle failure at this level for game or pending net drivers.
  FName NetDriverName = NetDriver->NetDriverName;
  if (NetDriverName == NAME_GameNetDriver || NetDriverName == NAME_PendingNetDriver)
  {
    // If this net driver has already been unregistered with this world, then don't handle it.
    if (World)
    {
      if (!FindNamedNetDriver(World, NetDriverName))
      {
        // This netdriver has already been destroyed (probably waiting for GC)
        return;
      }
    }

    // Give the GameInstance a chance to handle the failure.
    HandleNetworkFailure_NotifyGameInstance(World, NetDriver, FailureType);

    ENetMode FailureNetMode = NetDriver->GetNetMode();  // NetMode of the driver that failed
    bool bShouldTravel = true;

    switch (FailureType)
    {
    case ENetworkFailure::FailureReceived:
      break;
    case ENetworkFailure::PendingConnectionFailure:
      // TODO stop the connecting movie
      break;
    case ENetworkFailure::ConnectionLost:
      // Hosts don't travel when clients disconnect
      bShouldTravel = (FailureNetMode == NM_Client);
      break;
    case ENetworkFailure::ConnectionTimeout:
      // Hosts don't travel when clients disconnect
      bShouldTravel = (FailureNetMode == NM_Client);
      break;
    case ENetworkFailure::NetGuidMismatch:
    case ENetworkFailure::NetChecksumMismatch:
      // Hosts don't travel when clients have actor issues
      bShouldTravel = (FailureNetMode == NM_Client);
      break;
    case ENetworkFailure::NetDriverAlreadyExists:
    case ENetworkFailure::NetDriverCreateFailure:
    case ENetworkFailure::OutdatedClient:
    case ENetworkFailure::OutdatedServer:
    default:
      break;
    }

    if (bShouldTravel)
    {
      CallHandleDisconnectForFailure(World, NetDriver);
    }
  }
}

void UEngine::HandleNetworkFailure_NotifyGameInstance(UWorld *World, UNetDriver *NetDriver, ENetworkFailure::Type FailureType)
{
  bool bIsServer = true;

  if (NetDriver != nullptr)
  {
    bIsServer = NetDriver->GetNetMode() != NM_Client;
  }

  if (World != nullptr && World->GetGameInstance() != nullptr)
  {
    World->GetGameInstance()->HandleNetworkError(FailureType, bIsServer);
  }
  else
  {
    // Since the UWorld passed in might be null, as well as the NetDriver's UWorld,
    // go through the world contexts until we find the one with this net driver.
    for (auto& Context : WorldList)
    {
      if (Context.PendingNetGame != nullptr &&
        Context.PendingNetGame->NetDriver == NetDriver &&
        Context.OwningGameInstance != nullptr)
      {
        // Use the GameInstance from the current context.
        Context.OwningGameInstance->HandleNetworkError(FailureType, bIsServer);
      }
    }
  }
}

ENetworkFailure 的声明：

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// Runtime\Engine\Classes\Engine\EngineBaseType.h
/** Types of network failures broadcast from the engine */
UENUM(BlueprintType)
namespace ENetworkFailure
{
	enum Type
	{
		/** A relevant net driver has already been created for this service */
		NetDriverAlreadyExists,
		/** The net driver creation failed */
		NetDriverCreateFailure,
		/** The net driver failed its Listen() call */
		NetDriverListenFailure,
		/** A connection to the net driver has been lost */
		ConnectionLost,
		/** A connection to the net driver has timed out */
		ConnectionTimeout,
		/** The net driver received an NMT_Failure message */
		FailureReceived,
		/** The client needs to upgrade their game */
		OutdatedClient,
		/** The server needs to upgrade their game */
		OutdatedServer,
		/** There was an error during connection to the game */
		PendingConnectionFailure,
		/** NetGuid mismatch */
		NetGuidMismatch,
		/** Network checksum mismatch */
		NetChecksumMismatch
	};
}