hzFishy's Game Dev Notes

PIE sessions

Process

0 - Start

When pressing play to start a PIE session(s) the editor will set data in PlaySessionRequest

In UEditorEngine::Tick, if PlaySessionRequest.IsSet() the engine will call UEditorEngine::StartQueuedPlaySessionRequest which calls UEditorEngine::StartQueuedPlaySessionRequestImpl

The latter will run checks such as:

  • See if edited maps are saved
  • See if we run in one process or multiple
  • Broadcasting PIE editor delegates (FEditorDelegates)

Finally, it checks the enum value of PlaySessionRequest->SessionDestination

  • If EPlaySessionDestinationType::InProcess: Create one-or-more PIE/SIE sessions inside of the current process. Calls UEditorEngine::StartPlayInEditorSession
  • If EPlaySessionDestinationType::NewProcess: Create one-or-more PIE session by launching a new process on the local machine. Calls UEditorEngine::StartPlayInNewProcessSession
  • If EPlaySessionDestinationType::Launcher: Create a Play Session via the Launcher which may be on a local or remote device. Calls UEditorEngine::StartPlayUsingLauncherSession

1.1 - UEditorEngine::StartPlayInEditorSession

UEditorEngine::StartPlayInEditorSession with the play settings.

Inside a lot of processes and checks are done, such as:

  • Broadcasting PIE editor delegates (FEditorDelegates)
  • Transaction checks
  • For all? plugins check if we are in a state where a new PIE session can start
  • Registry checks and flushing async loading processes
  • Auto-compile dirty blueprints (if needed), show a dialog window if any errors occurred (ignored if in demo mode)
  • Flush all audio sources from the editor world

Then we finally enter the PIE and multiplayer area. (See PlayLevel.cpp l2774 as for UE5.4) Some operations are done to see if we are simulating, need a dedicated server separated or in the same world (listen server).

The following part will only consider the Listen Server + Client(s) route and tha twe are running in one process If using the listen server PIE net mode, the engine will count how many instances we want NumRequestedInstances. The listen server counts as 1 client, so when looping the first instance will be using EPlayNetMode::PIE_ListenServer, all the following instances will be using EPlayNetMode::PIE_Client. UEditorEngine::CreateNewPlayInEditorInstance gets called

? - UEditorEngine::CreateNewPlayInEditorInstance

Client instance gets setup, there is a special login flow if using the online subsystem (PlayInEditorSessionInfo->bUsingOnlinePlatform).

if not using a OSS the engine calls OnLoginPIEComplete_Deferred.

?+1 - UEditorEngine::OnLoginPIEComplete_Deferred

Not called directly after the call, can be called after multiple frames.

Checks if PIE logging was successful. Calls UEditorEngine::CreateInnerProcessPIEGameInstance that calls UGameInstance::StartPlayInEditorGameInstance.

For the listen server it ends here. The editor doesn’t reload the current map or load a new map.

But for the client, this will call an absoluteUEngine::Browse

The URLwill contain the map already opened in the editor with the following IP address and port: 127.0.0.1:7777. BUT since its using the following FURL constructor FURL(FURL* Base, const TCHAR* TextURL, ETravelType Type) the Map property will be replaced with UGameMapsSettings::GetGameDefaultMap(). (so this means the PIE client will “initially” not travel to the listen server map)

Inside UEngine::Browse, URL.IsInternal() && GIsClient will succeed. Later on this will call UPendingNetGame::InitNetDriver. Which calls UIpNetDriver::InitConnect.

end of the client only par?

At the end if the conditions are met UEditorEngine::OnAllPIEInstancesStarted is called.