Question

There are 2 main considerations to take into account when thinking about backward compatibility :

• build target
• current SDK for compilation

If I compile with iOS SDK 3.1.3, [UIScreen mainScreen].scale will raise an error.

To deal with this, I can write :

CGLoat scale = 1.0;
#if __IPHONE_OS_VERSION_MAX_ALLOWED >= 40000
scale = [UIScreen mainScreen].scale;
#endif 

But… if my minimum version target is let’s say 3.1.3, this call will crash a 3.1.3 device, even if it compiles fine.

So I must write :

CGFloat scale = 1.0;
#if __IPHONE_OS_VERSION_MAX_ALLOWED >= 40000
    if ([UIScreen instancesRespondToSelector:@selector(scale)])
        scale = [UIScreen mainScreen].scale;
#endif

Saying that, I wonder if there is a way to implement

- (CGFloat)scale;

into a UIScreen category, inside which I could manage this kind of problem, and then the caller would just have to write :

CGFloat scale = [UIScreen mainScreen].scale;

without having to think about its used SDK nor its min target.

If possible, how would you write such a category so that it compiles fine with a 3.1.3 SDK, a 4.3 SDK, and runs fine on a 3.1.3 device as on a 4.3 device?

Answer 1

You could do this with some method-swizzling, but I really wouldn’t recommend it (gets confusing really fast).

I’d prefix your category method instead, something like foo_scale. This way you won’t run into name conflicts and you still only have one method to call.