Added love.graphics.isActive. love.graphics function calls (and method calls on objects created via love.graphics) are only guaranteed to work when isActive is true, otherwise bad things might happen (the program crashing, for example.)

It's usually only false when the app is inactive on iOS, and when the window hasn't been created yet.

--HG--
branch : minor

src/modules/event/sdl/Event.cpp

@@ -69,6 +69,30 @@ static void normalizedToPixelCoords(double *x, double *y)
 }
 #endif
 
+// SDL's event watch callbacks trigger when the event is actually posted inside
+// SDL, unlike with SDL_PollEvents. This is useful for some events which require
+// handling inside the function which triggered them on some backends.
+static int SDLCALL watchAppEvents(void * /*udata*/, SDL_Event *event)
+{
+	graphics::Graphics *gfx = Module::getInstance<graphics::Graphics>(Module::M_GRAPHICS);
+
+	switch (event->type)
+	{
+	// On iOS, calling any OpenGL ES function after the function which triggers
+	// SDL_APP_DIDENTERBACKGROUND is called will kill the app, so we handle it
+	// with an event watch callback, which will be called inside that function.
+	case SDL_APP_DIDENTERBACKGROUND:
+	case SDL_APP_WILLENTERFOREGROUND:
+		if (gfx)
+			gfx->setActive(event->type == SDL_APP_WILLENTERFOREGROUND);
+		break;
+	default:
+		break;
+	}
+
+	return 1;
+}
+
 const char *Event::getName() const
 {
 	return "love.event.sdl";
@@ -78,10 +102,13 @@ Event::Event()
 {
 	if (SDL_InitSubSystem(SDL_INIT_EVENTS) < 0)
 		throw love::Exception("%s", SDL_GetError());
+
+	SDL_AddEventWatch(watchAppEvents, this);
 }
 
 Event::~Event()
 {
+	SDL_DelEventWatch(watchAppEvents, this);
 	SDL_QuitSubSystem(SDL_INIT_EVENTS);
 }