iOS中的按位OR

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;

这个表达式的确切含义是：UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse。

UIViewAnimationOptionRepeat的值等于8(在仓位10000中)，UIViewAnimationOptionAutoreverse等于16(在仓位1000中)。所以我认为表达式UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse应该生成16( -> 10000) UIViewAnimationOptionReverse。

Answer 1

UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse被称为“面具”。

如果你有一个UIViewAnimationOptions类型的变量，就说：

 UIViewAnimationOptions a;

您可以按如下方式对其应用遮罩：

 bool b = a && (UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse)

来确定a是否“包含”这两个标志。如果

a == 0x0000001;

然后

b == false;

如果

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

然后

 b == true;

因此，您实际上对UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse的计算结果不感兴趣，而只对将该类型的值与您感兴趣的检查标志进行逻辑and运算的结果感兴趣。

Answer 2

操作|由真值表定义

   | 0 | 1  
---+---+---
 0 | 0 | 1
 1 | 1 | 1

也就是说，仅当x == 0和y == 0都存在时才使用x | y == 0。|运算符同时处理机器字的所有位。所以

  001000   (8)
| 010000  (16)
  ------------
  011000  (24)

Answer 3

这些位是或的：

UIViewAnimationOptionRepeat      = 1 << 3 = 8  = 01000 in binary
UIViewAnimationOptionAutoreverse = 1 << 4 = 16 = 10000 in binary

   01000
OR 10000
--------
   11000

二进制中的11000是16 +8= 24 -设置了第三和第四位的整数(从0开始计数)。