Converting an unruly dependency injection model with a service locator - c++

Question

I've been using DI for a game engine project for a while and I just hit a wall; given the below order of creation: The job system does not depend on anything and everything depends on the file logger. It makes sense to create the job system, then the file logger, then pass the created references for each dependency down to its dependents' constructors.
App::App(const std::string& cmdString)
: EngineSubsystem()
, _theJobSystem{std::make_unique<JobSystem>(-1, static_cast<std::size_t>(JobType::Max), new std::condition_variable)}
, _theFileLogger{std::make_unique<FileLogger>(*_theJobSystem.get(), "game")}
, _theConfig{std::make_unique<Config>(KeyValueParser{cmdString})}
, _theRenderer{std::make_unique<Renderer>(*_theJobSystem.get(), *_theFileLogger.get(), *_theConfig.get())}
, _theInputSystem{std::make_unique<InputSystem>(*_theFileLogger.get(), *_theRenderer.get())}
, _theUI{std::make_unique<UISystem>(*_theFileLogger.get(), *_theRenderer.get(), *_theInputSystem.get())}
, _theConsole{std::make_unique<Console>(*_theFileLogger.get(), *_theRenderer.get())}
, _theAudioSystem{std::make_unique<AudioSystem>(*_theFileLogger.get()) }
, _theGame{std::make_unique<Game>()}
{
SetupEngineSystemPointers();
SetupEngineSystemChainOfResponsibility();
LogSystemDescription();
}
void App::SetupEngineSystemPointers() {
g_theJobSystem = _theJobSystem.get();
g_theFileLogger = _theFileLogger.get();
g_theConfig = _theConfig.get();
g_theRenderer = _theRenderer.get();
g_theUISystem = _theUI.get();
g_theConsole = _theConsole.get();
g_theInputSystem = _theInputSystem.get();
g_theAudioSystem = _theAudioSystem.get();
g_theGame = _theGame.get();
g_theApp = this;
}
void App::SetupEngineSystemChainOfResponsibility() {
g_theConsole->SetNextHandler(g_theUISystem);
g_theUISystem->SetNextHandler(g_theInputSystem);
g_theInputSystem->SetNextHandler(g_theApp);
g_theApp->SetNextHandler(nullptr);
g_theSubsystemHead = g_theConsole;
}
As you can see, passing the different subsystems around to the other subsystem constructors is starting to get messy. In particular when dealing with jobs, logging, console commands, UI, configuration, and audio (and physics, not pictured).
(Side note: These are going to eventually be replaced with interfaces created via factories for cross-compatibility, i.e. the Renderer is strictly a DirectX/Windows-only renderer but I want to eventually support OpenGL/Linux; that's why everything is passed around as references and created as pointers instead of a concrete types)
I've run in to situations where pretty much all the subsystems are in some way dependent on every other subsystem.
But, due to construction-order problems, Dependency Injection does not work because one or more of the required-to-exist subsystems hasn't been constructed yet. Same problem with two-phase construction: the subsystem may not have been initialized by the time it's needed further downstream.
I looked in to the service locator pattern and this question deems it a bad idea, but the game industry likes using bad ideas (like global variables to every subsystem for game-specific code to use) if they work.
Would converting to a service locator fix this problem?
What other implementations do you know of that could also fix the issue?

Answer 1

I ultimately went with the ServiceLocator pattern, deriving every subsystem that was a dependency as a Service:
App::App(const std::string& cmdString)
: EngineSubsystem()
, _theConfig{std::make_unique<Config>(KeyValueParser{cmdString})}
{
SetupEngineSystemPointers();
SetupEngineSystemChainOfResponsibility();
LogSystemDescription();
}
void App::SetupEngineSystemPointers() {
ServiceLocator::provide(*static_cast<IConfigService*>(_theConfig.get()));
_theJobSystem = std::make_unique<JobSystem>(-1, static_cast<std::size_t>(JobType::Max), new std::condition_variable);
ServiceLocator::provide(*static_cast<IJobSystemService*>(_theJobSystem.get()));
_theFileLogger = std::make_unique<FileLogger>("game");
ServiceLocator::provide(*static_cast<IFileLoggerService*>(_theFileLogger.get()));
_theRenderer = std::make_unique<Renderer>();
ServiceLocator::provide(*static_cast<IRendererService*>(_theRenderer.get()));
_theInputSystem = std::make_unique<InputSystem>();
ServiceLocator::provide(*static_cast<IInputService*>(_theInputSystem.get()));
_theAudioSystem = std::make_unique<AudioSystem>();
ServiceLocator::provide(*static_cast<IAudioService*>(_theAudioSystem.get()));
_theUI = std::make_unique<UISystem>();
_theConsole = std::make_unique<Console>();
_theGame = std::make_unique<Game>();
g_theJobSystem = _theJobSystem.get();
g_theFileLogger = _theFileLogger.get();
g_theConfig = _theConfig.get();
g_theRenderer = _theRenderer.get();
g_theUISystem = _theUI.get();
g_theConsole = _theConsole.get();
g_theInputSystem = _theInputSystem.get();
g_theAudioSystem = _theAudioSystem.get();
g_theGame = _theGame.get();
g_theApp = this;
}
void App::SetupEngineSystemChainOfResponsibility() {
g_theConsole->SetNextHandler(g_theUISystem);
g_theUISystem->SetNextHandler(g_theInputSystem);
g_theInputSystem->SetNextHandler(g_theRenderer);
g_theRenderer->SetNextHandler(g_theApp);
g_theApp->SetNextHandler(nullptr);
g_theSubsystemHead = g_theConsole;
}