C语言 在 Objective-C 中声明和检查/比较（位掩码-）枚举

Declaring and checking/comparing (bitmask-)enums in Objective-C

提问by thibaultcha

You know in Cocoa there is this thing, for example you can create a UIViewand do:

你知道在 Cocoa 中有这个东西，例如你可以创建一个UIView并执行：

view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;

I have a custom UIViewwith multiple states, which I have defined in an enumlike this:

我有一个UIView具有多个状态的自定义，我定义enum如下：

enum DownloadViewStatus {
  FileNotDownloaded,
  FileDownloading,
  FileDownloaded
};

For each created subview, I set its tag: subview1.tag = FileNotDownloaded;

对于每个创建的子视图，我设置了它tag：subview1.tag = FileNotDownloaded;

Then, I have a custom setter for the view state which does the following:

然后，我有一个用于视图状态的自定义设置器，它执行以下操作：

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:

但我想做的是允许：

subview1.tag = FileNotDownloaded | FileDownloaded;

So my subview1shows 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.

所以我subview1的观点以两种状态出现。目前，它不会出现在这两种状态中的任何一种，因为|运算符似乎添加了两个枚举值。

Is there a way to do that?

有没有办法做到这一点？

回答by Regexident

Declaring Bitmasks:

声明位掩码：

Alternatively to assigning absolute values (1, 2, 4, …) you can declare bitmasks(how these are called) like this:

作为分配绝对值（1, 2, 4, ...）的替代方法，您可以像这样声明位掩码（如何调用它们）：

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

or using modern ObjC's NS_OPTIONS/NS_ENUMmacros:

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
};

(see Abizern's answerfor more info on the latter)

（有关后者的更多信息，请参阅Abilern 的回答）

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

位掩码的概念是（通常）用单个位集定义每个枚举值。

Hence ORing two values does the following:

因此，OR两个值执行以下操作：

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

which is equivalent to:

这相当于：

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

Comparing Bitmasks:

比较位掩码：

One thing to keep in mind when checking against bitmasks:

检查位掩码时要记住的一件事：

Checking for exact equality:

检查完全相等：

Let's assume that status is initialized like this:

让我们假设 status 是这样初始化的：

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

If you want to check if statusequalsFileNotDownloaded, you can use:

如果要检查是否status等于FileNotDownloaded，可以使用：

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

which is equivalent to:

这相当于：

   00000101 // (FileNotDownloaded | FileDownloaded)
== 00000100 // FileDownloaded
-----------
=  00000000 // false

Checking for "membership":

检查“会员”：

If you want to check if statusmerely containsFileNotDownloaded, you need to use:

如果要检查是否status仅包含 containsFileNotDownloaded，则需要使用：

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)?

看到细微的区别（以及为什么您当前的“if”表达式可能是错误的）？

回答by Abizern

While @Regexident has provided an excellent answer - I must mention the modern Objective-C way of declaring Enumerated options with NS_OPTIONS:

虽然@Regexident 提供了一个很好的答案 - 我必须提到用以下方式声明枚举选项的现代 Objective-C 方式NS_OPTIONS：

typedef NS_OPTIONS(NSUInteger, DownloadViewStatus) {
  FileNotDownloaded = 0,
  FileDownloading   = 1 << 0,
  FileDownloaded    = 1 << 1
};

Further Reference:

进一步参考：

• NSHipster
• I Am The Walrus

• NSHipster
• 我是海象

回答by Renetik

Useful function you can use for bitmask checking to improve readability.

可用于位掩码检查以提高可读性的有用函数。

BOOL bitmaskContains(NSUInteger bitmask, NSUInteger contains) {
    return (bitmask & contains) != 0;
}

回答by mah

enum DownloadViewStatus {
  FileNotDownloaded = 1,
  FileDownloading = 2,
  FileDowloaded = 4
};

This will let you perform bitwise OR's and AND's effectively.

这将使您有效地执行按位 OR 和 AND。