thibaultCha thibaultCha - 5 months ago 18
Objective-C Question

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

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?

Answer

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)?