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    <p>I should note first that this isn't a good thing to do in any situation other than testing; even then, proceed with care -- AliSoftware provides some details and example code in comments below. See also the interesting answers at <a href="https://stackoverflow.com/q/13856037">Can I declare a <code>dispatch_once_t</code> predicate as a member variable instead of static?</a>, including some important information <a href="https://stackoverflow.com/a/19845164/">from the horse's mouth</a>.</p> <p><code>dispatch_once_t</code> is a <code>typedef</code>d <code>long</code>. Its false value is 0. If you reset that flag to 0, <code>dispatch_once()</code> will run again. Your problem is "just" how to change the value of a static variable from another compilation unit. For this, I think you need a debug/unit test hook, like so:</p> <p>MakeWhoopie.h</p> <pre><code>#import &lt;Foundation/Foundation.h&gt; void makeWhoopie(void); #ifdef DEBUG void resetDispatchOnce(void); #endif </code></pre> <p>MakeWhoopie.m</p> <pre><code>#include "MakeWhoopie.h" static dispatch_once_t * once_token_debug; void makeWhoopie(void) { static dispatch_once_t once_token; once_token_debug = &amp;once_token; // Store address of once_token // to access it in debug function. dispatch_once(&amp;once_token, ^{ NSLog(@"That's what you get, folks."); }); NSLog(@"Making whoopie."); } #ifdef DEBUG void resetDispatchOnce(void) { *once_token_debug = 0; } #endif </code></pre> <p>(You could also move <code>once_token</code> up to file level and change it directly.)</p> <p>Trying this out:</p> <pre><code>#import &lt;Foundation/Foundation.h&gt; #import "MakeWhoopie.h" int main(int argc, const char * argv[]) { @autoreleasepool { makeWhoopie(); makeWhoopie(); resetDispatchOnce(); makeWhoopie(); } return 0; } </code></pre> <p>Results in:</p> <blockquote> <p>2012-06-07 18:45:28.134 ResetDispatchOnce[8628:403] That's what you get, folks.<br> 2012-06-07 18:45:28.163 ResetDispatchOnce[8628:403] Making whoopie.<br> 2012-06-07 18:45:28.164 ResetDispatchOnce[8628:403] Making whoopie.<br> 2012-06-07 18:45:28.165 ResetDispatchOnce[8628:403] That's what you get, folks.<br> 2012-06-07 18:45:28.165 ResetDispatchOnce[8628:403] Making whoopie. </p> </blockquote>
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    1. COThe point of `dispatch_once` is that it is thread-safe. The problem by setting `*once_token_debug = 0` this way is that it's not thread-safe at all in cases another thread uses `dispatch_once(&onceToken, …)` while you set the `onceToken` by yourself. How come we can prevent such issue?
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    2. CO@AliSoftware: The scenario in the question is unit testing, and that's the _only_ use I propose for this procedure. The token is being reset _between_ tests, to create a clean slate just as would be created between runs of the larger program. Threading is not an issue because the program is not running between tests.
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    3. COI understood this, but everybody have to be aware of that and not being tempted to use this in a non-threadsafe manner elsewhere. And there may be some cases even in Unit Tests that this is not thread-safe, especially if one badly codes its test and does asynchronous actions that continue running even after the tests ends (like a call to `dispatch_after(10s, ^{ /* sthg that uses sharedInstance */ }` if the test reaches its timeout it will fail and be stopped and reach `tearDown` but the block using sharedInstance will still be dispatched in a near future… after the tearDown.
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