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 OR
ing 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)?