在Objective-C中声明和检查/比较（bitmask-）枚举 [英] Declaring and checking/comparing (bitmask-)enums in Objective-C

问题描述

<$ p

$ p> view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;


我有一个具有多个状态的自定义 UIView ，我在一个枚举中定义，如下所示：

 枚举DownloadViewStatus {
 FileNotDownloaded，
 FileDownloading，
 FileDownloaded 
}; 
  

对于每个创建的子视图，我设置其标签： subview1.tag = FileNotDownloaded;

然后，我有一个自定义设置器的视图状态，以下：

  for（UIView * subview in self.subviews）{
 if（subview.tag == viewStatus） 
 subview.hidden = NO; 
 else 
 subview.hidden = YES; 
} 
  

但是，我想要做什么允许：

  subview1.tag = FileNotDownloaded | FileDownloaded; 
  

所以我的 subview1 显示在两个状态我的看法目前，由于 | 运算符似乎添加了两个枚举值，所以它们不显示在这两个状态中的任何一个中。

有没有办法这样做？

解决方案

声明位图：

或者分配绝对值（ 1 ， 2 ， 4 ，...）可以声明 bitmasks （如何调用）：

  typedef枚举：NSUInteger {
 FileNotDownloaded =（1< 0），// => 00000001 
 FileDownloading =（1< 1），// => 00000010 
 FileDownloaded =（1< 2）// => 00000100 
} DownloadViewStatus; 
  

或使用现代ObjC的 NS_OPTIONS / NS_ENUM 宏：

  typedef NS_OPTIONS（NSUInteger，DownloadViewStatus）{
 FileNotDownloaded =（1< 0），// => 00000001 
 FileDownloading =（1< 1），// => 00000010 
 FileDownloaded =（1< 2）// => 00000100 
}; 
  

（请参阅 Abizern的答案关于后者的更多信息）

位掩码的概念是（通常）定义每个具有单个位集的枚举值。 p>

因此 OR 这两个值执行以下操作：

  DownloadViewStatus status = FileNotDownloaded | FileDownloaded; // => 00000101 
  

相当于：

  00000001 // FileNotDownloaded 
 | 00000100 // FileDownloaded 
 ---------- 
 = 00000101 //（FileNotDownloaded | FileDownloaded）
  

比较位掩码：

检查位掩码时需要记住一点：

检查确切的相等性：

我们假设状态是这样初始化的：

  DownloadViewStatus status = FileNotDownloaded | FileDownloaded; // => 00000101 
  

如果要检查状态您可以使用

  BOOL equals =（status == FileNotDownloaded）; // =>假
  

相当于：

  00000101 //（FileNotDownloaded | FileDownloaded）
 == 00000100 // FileDownloaded 
 ----------- 
 = 00000000 //检查会员资格： 
 
 
 
 
 < >如果要检查状态仅包含  FileNotDownloaded ，则需要使用： 
  BOOL包含=（status& FileNotDownloaded）！= 0; // => true 
 
 00000101 //（FileNotDownloaded | FileDownloaded）
& 00000100 // FileDownloaded 
 ----------- 
 = 00000100 // FileDownloaded 
！= 00000000 // 0 
 -------- --- 
 = 00000001 // 1 =>真的
  
看到微妙的差异（以及为什么你当前的if表达可能是错误的） / p> 
You know in Cocoa there is this thing, for example you can create a UIView and do:
view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
I have a custom UIView with multiple states, which I have defined in an enum like this:
enum DownloadViewStatus {
  FileNotDownloaded,
  FileDownloading,
  FileDownloaded
};
For each created subview, I set its tag: subview1.tag = FileNotDownloaded;

Then, I have a custom setter for the view state which does the following:
for (UIView *subview in self.subviews) {
  if (subview.tag == viewStatus)
    subview.hidden = NO;
  else
    subview.hidden = YES;
}
But what I am trying to do, is to allow this:
subview1.tag = FileNotDownloaded | FileDownloaded;
So my subview1 shows up in two states of my view. Currently, it doesn't show up in any of those two states since the | operator seems to add the two enum values. 

Is there a way to do that?
解决方案
Declaring Bitmasks:

Alternatively to assigning absolute values (1, 2, 4, …) you can declare bitmasks (how these are called) like this:
typedef enum : NSUInteger {
  FileNotDownloaded = (1 << 0), // => 00000001
  FileDownloading   = (1 << 1), // => 00000010
  FileDownloaded     = (1 << 2)  // => 00000100
} DownloadViewStatus;
or using modern ObjC's NS_OPTIONS/NS_ENUM macros:
typedef NS_OPTIONS(NSUInteger, DownloadViewStatus) {
  FileNotDownloaded = (1 << 0), // => 00000001
  FileDownloading   = (1 << 1), // => 00000010
  FileDownloaded    = (1 << 2)  // => 00000100
};
(see Abizern's answer for more info on the latter)

The concept of bitmasks is to (usually) define each enum value with a single bit set.

Hence ORing two values does the following:
DownloadViewStatus status = FileNotDownloaded | FileDownloaded; // => 00000101
which is equivalent to:
  00000001 // FileNotDownloaded
| 00000100 // FileDownloaded
----------
= 00000101 // (FileNotDownloaded | FileDownloaded)


Comparing Bitmasks:

One thing to keep in mind when checking against bitmasks:

Checking for exact equality:

Let's assume that status is initialized like this:
DownloadViewStatus status = FileNotDownloaded | FileDownloaded; // => 00000101
If you want to check if status equals FileNotDownloaded, you can use:
BOOL equals = (status == FileNotDownloaded); // => false
which is equivalent to:
   00000101 // (FileNotDownloaded | FileDownloaded)
== 00000100 // FileDownloaded
-----------
=  00000000 // false


Checking for "membership":

If you want to check if status merely contains FileNotDownloaded, you need to use:
BOOL contains = (status & FileNotDownloaded) != 0; // => true

   00000101 // (FileNotDownloaded | FileDownloaded)
&  00000100 // FileDownloaded
-----------
=  00000100 // FileDownloaded
!= 00000000 // 0
-----------
=  00000001 // 1 => true
See the subtle difference (and why your current "if"-expression is probably wrong)?

                        
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