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

Question

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 1

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)?