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

问题描述

你知道在 Cocoa 中有这个东西，例如你可以创建一个 UIView 并执行:

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

view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;

我有一个具有多个状态的自定义 UIView，我在 enum 中定义了它，如下所示:

I have a custom UIView with multiple states, which I have defined in an enum like this:

enum DownloadViewStatus {
  FileNotDownloaded,
  FileDownloading,
  FileDownloaded
};

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

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

然后，我有一个自定义的视图状态设置器，它执行以下操作:

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;
}

但是我正在尝试做的，是允许这样做:

subview1.tag = FileNotDownloaded | FileDownloaded;

所以我的 subview1 出现在我视图的两种状态中.目前，它不会出现在这两种状态中的任何一种，因为 | 运算符似乎将两个枚举值相加.

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.

有没有办法做到这一点?

Is there a way to do that?

推荐答案

声明位掩码:

除了分配绝对值(1、2、4、...)，您还可以声明 位掩码(这些是如何命名的)像这样:

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;

或使用现代 ObjC 的 NS_OPTIONS/NS_ENUM 宏:

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
};

(有关后者的更多信息，请参阅 Abilern 的回答)

(see Abizern's answer for more info on the latter)

位掩码的概念是(通常)用单个位集定义每个枚举值.

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

因此 OR 对两个值执行以下操作:

Hence ORing two values does the following:

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

相当于:

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

比较位掩码:

检查位掩码时要记住的一件事:

Comparing Bitmasks:

One thing to keep in mind when checking against bitmasks:

假设状态初始化如下:

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

如果要检查 status equals FileNotDownloaded 是否可以使用:

If you want to check if status equals FileNotDownloaded, you can use:

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

相当于:

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

检查会员":

如果你想检查status是否仅仅contains FileNotDownloaded，你需要使用:

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

看到细微的差别(以及为什么您当前的if"表达式可能是错误的)?

See the subtle difference (and why your current "if"-expression is probably wrong)?

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