Question

For the online mode for my game, I am using the context property of GKScore, and as all devices which support Game Center can update to iOS 5 (which is when the context property was added), I am requiring that the context property is available to play online. However, I am having issues implementing this runtime-check. I was assuming that I could use [GKScore instancesRespondToSelector:@selector(setContext:)] to check its existence, but this returns false on the iOS 5 and 5.1 simulators, as well as for @selector(context). Why on earth is this happening, and what is the cleanest and correct way to perform this check, please?

Answer 1

I can’t fully explain this, but an instantiated object of class GKScore will return YES to repondsToSelector(context), even while the class says it won’t. If no other solution works, construct a GKScore object just to query it.

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I wondered if [[GKScore alloc] init] actually returns an object with type other than GKScore. This can happen.

GKScore *instantiatedScore = [[GKScore alloc] init]; // Add autorelease if using manual reference counting.
NSString* className = NSStringFromClass([instantiatedScore class]);
NSLog(@"instantiatedScore class name = %@", className);

But, it doesn’t, according to this output:

instantiatedScore class name = GKScore

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I wondered if the compiler directives in the GKSCore.h header file might affect this. It defines two properties that are only available in iOS 5.0 or greater: context and shouldSetDefaultLeaderboard. Maybe those compiler directives mean that the class can’t guarantee it will support those two properties.

Under this hypothesis [GKScore instancesRepondToSelector:@selector(category)] should return YES, but [GKScore instancesRepondToSelector:@selector(shouldSetDefaultLeaderboard)] should return NO.

GKScore *instantiatedScore = [[GKScore alloc] init]; // Add autorelease if using manual reference counting.
NSLog(@"GKScore category = %d", [GKScore instancesRespondToSelector:@selector(category)]);
NSLog(@"instantiatedScore category = %d", [instantiatedScore respondsToSelector:@selector(category)]);

NSLog(@"GKScore context = %d", [GKScore instancesRespondToSelector:@selector(context)]);
NSLog(@"instantiatedScore context = %d", [instantiatedScore respondsToSelector:@selector(context)]);

NSLog(@"GKScore shouldSetDefaultLeaderboard = %d", [GKScore instancesRespondToSelector:@selector(shouldSetDefaultLeaderboard)]);
NSLog(@"instantiatedScore shouldSetDefaultLeaderboard = %d", [instantiatedScore respondsToSelector:@selector(shouldSetDefaultLeaderboard)]);

But, the output is weirder than that:

GKScore category = 0
instantiatedScore category = 1
GKScore context = 0
instantiatedScore context = 1
GKScore shouldSetDefaultLeaderboard = 1
instantiatedScore shouldSetDefaultLeaderboard = 1