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    <h2>Declaring Bitmasks:</h2> <p>Alternatively to assigning absolute values (<code>1</code>, <code>2</code>, <code>4</code>, …) you can declare <strong><a href="http://en.wikipedia.org/wiki/Bitmask" rel="noreferrer">bitmasks</a></strong> (how these are called) like this:</p> <pre><code>typedef enum : NSUInteger { FileNotDownloaded = (1 &lt;&lt; 0), // =&gt; 00000001 FileDownloading = (1 &lt;&lt; 1), // =&gt; 00000010 FileDownloaded = (1 &lt;&lt; 2) // =&gt; 00000100 } DownloadViewStatus; </code></pre> <p>or using modern ObjC's <code>NS_OPTIONS</code>/<code>NS_ENUM</code> macros:</p> <pre><code>typedef NS_OPTIONS(NSUInteger, DownloadViewStatus) { FileNotDownloaded = (1 &lt;&lt; 0), // =&gt; 00000001 FileDownloading = (1 &lt;&lt; 1), // =&gt; 00000010 FileDownloaded = (1 &lt;&lt; 2) // =&gt; 00000100 }; </code></pre> <p>(see <a href="https://stackoverflow.com/a/16168299/227536">Abizern's answer</a> for more info on the latter)</p> <p>The concept of bitmasks is to (usually) define each enum value with a single bit set.</p> <p>Hence <code>OR</code>ing two values does the following:</p> <pre><code>DownloadViewStatus status = FileNotDownloaded | FileDownloaded; // =&gt; 00000101 </code></pre> <p>which is equivalent to:</p> <pre><code> 00000001 // FileNotDownloaded | 00000100 // FileDownloaded ---------- = 00000101 // (FileNotDownloaded | FileDownloaded) </code></pre> <h2>Comparing Bitmasks:</h2> <p>One thing to keep in mind when checking against bitmasks:</p> <h3>Checking for exact equality:</h3> <p>Let's assume that status is initialized like this:</p> <pre><code>DownloadViewStatus status = FileNotDownloaded | FileDownloaded; // =&gt; 00000101 </code></pre> <p>If you want to check if <code>status</code> <strong>equals</strong> <code>FileNotDownloaded</code>, you can use:</p> <pre><code>BOOL equals = (status == FileNotDownloaded); // =&gt; false </code></pre> <p>which is equivalent to:</p> <pre><code> 00000101 // (FileNotDownloaded | FileDownloaded) == 00000100 // FileDownloaded ----------- = 00000000 // false </code></pre> <h3>Checking for "membership":</h3> <p>If you want to check if <code>status</code> merely <strong>contains</strong> <code>FileNotDownloaded</code>, you need to use:</p> <pre><code>BOOL contains = (status &amp; FileNotDownloaded) != 0; // =&gt; true 00000101 // (FileNotDownloaded | FileDownloaded) &amp; 00000100 // FileDownloaded ----------- = 00000100 // FileDownloaded != 00000000 // 0 ----------- = 00000001 // 1 =&gt; true </code></pre> <p>See the subtle difference (and why your current "if"-expression is probably wrong)?</p>
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    1. PODeclaring and checking/comparing (bitmask-)enums in Objective-C
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    1. CO@Abizern: Thanks! Thought this question deserved a bit more explanation than had previously been provided.
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    2. COexcellent answer!
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    3. COYeah but you're formatting the binary values as hexadecimal values (preceded by 0x). Bitmasks work on the bit level. Simple mistake, I'm sure you didn't even notice it. But someone might look at that and incorrectly assume that you can have a max of 8 options per enum, when you can actually have a maximum of 32 distinct options. Correction: `FileNotDownloaded = (0x1 << 0), // => %...00000001` etc.
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      1. USMichael Zimmerman
 

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