Question

enum {
   UIViewAnimationOptionLayoutSubviews            = 1 <<  0,
   UIViewAnimationOptionAllowUserInteraction      = 1 <<  1,
   UIViewAnimationOptionBeginFromCurrentState     = 1 <<  2,
   UIViewAnimationOptionRepeat                    = 1 <<  3,
   UIViewAnimationOptionAutoreverse               = 1 <<  4,
   UIViewAnimationOptionOverrideInheritedDuration = 1 <<  5,
   UIViewAnimationOptionOverrideInheritedCurve    = 1 <<  6,
   UIViewAnimationOptionAllowAnimatedContent      = 1 <<  7,
   UIViewAnimationOptionShowHideTransitionViews   = 1 <<  8,

   UIViewAnimationOptionCurveEaseInOut            = 0 << 16,
   UIViewAnimationOptionCurveEaseIn               = 1 << 16,
   UIViewAnimationOptionCurveEaseOut              = 2 << 16,
   UIViewAnimationOptionCurveLinear               = 3 << 16,

   UIViewAnimationOptionTransitionNone            = 0 << 20,
   UIViewAnimationOptionTransitionFlipFromLeft    = 1 << 20,
   UIViewAnimationOptionTransitionFlipFromRight   = 2 << 20,
   UIViewAnimationOptionTransitionCurlUp          = 3 << 20,
   UIViewAnimationOptionTransitionCurlDown        = 4 << 20,
};
typedef NSUInteger UIViewAnimationOptions;

What exactly means this expression: UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse.

Value of UIViewAnimationOptionRepeat is equal to 8(in bin 1000), UIViewAnimationOptionAutoreverse is equal to 16(in bin 10000). So expression UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse should generate as I think 16(bin 10000) -> UIViewAnimationOptionReverse.

Answer 1

UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse is known as a “mask”.

If you have a variable of type UIViewAnimationOptions, say:

 UIViewAnimationOptions a;

you can apply the mask to it like this:

 bool b = a && (UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse)

to determine if a “contains” either flags. If

a == 0x0000001;

then

b == false;

if

a == 0x0101001;  //-- completely arbitrary mask

then

 b == true;

So you are not actually interested in what UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse evaluates to, but only in the result of logically and-ing a value of that type to the flags you are interested in checking.