Background/foreground app events on iOS do not work when running under the event loop

[frenzibyte]

Our application generally powers up as follows:

• Call SDL_RunApp with a main function
• Within main function, initialise everything and call SDL_SetiOSAnimationCallback to hand off the game loop to the display link initialised by SDL for correct frame pacing.
• Return from the main function.

SDL seems to be aware of this kind of logic already:

SDL/src/video/uikit/SDL_uikitappdelegate.m

Lines 416 to 419 in 7c5c88a

	// exit, passing the return status from the user's application
	/* We don't actually exit to support applications that do setup in their
	* main function and then allow the Cocoa event loop to run. */
	// exit(exit_status);

But going a few lines back, it's doing this right after coming back from our main function:

SDL/src/video/uikit/SDL_uikitappdelegate.m

Line 409 in 7c5c88a

SDL_SetiOSEventPump(false);

Which in turn removes the LifecycleObserver and not listen to any notification regarding the state of the app (i.e. applicationWillResignActive/applicationDidBecomeActive), causing the app to not receive such events.

[slouken]

I'm not sure what the right solution is here. Feel free to create a PR with something that works for you!

[frenzibyte]

I will have to begin by asking why iOS event pumps are disabled after the main function in the first place, and/or why the lifecycle observer is using that as a condition on whether to run.

This is all as well as why the lifecycle observer apparently has a fallback condition wherein if the app was running using SDL's main callbacks then the lifecycle observer is attached regardless of iOS's event pump state.

[frenzibyte]

After further consideration, given our specific use case of SDL on iOS here, I think it may be a better option for us to roll out our own application delegate class and use SDL_SetMainReady instead (albeit we will have to copy certain handlings). This can be closed if it's on the verge of being by-design.

[icculus]

I'm sort of assuming this is a bug in SDL, but I don't know the details of this code at all.

I assume this code path is trying to make it so SDL_PumpEvents does nothing because it would run the CFRunLoop, but setting an animation callback means we basically live in a runloop and don't have to (or shouldn't) manually run it...but I don't know why we turn off the observer.

Also: the iOS-specific main callbacks codepath literally calls SDL_PumpEvents, so it must be safe to do so even with an animation callback...?