title: What is dispatch_once_t
body: <p>Can someone tell me what is dispatch_once_t and it's significance.</p>
  <p>this is what apple writes in its documentation</p>
  <blockquote>
  <p>A predicate for use with the dispatch_once function</p>
  </blockquote>
  <p>which either I am unable to understand.</p>
  <pre><code>+ (instancetype)sharedInstance {
      static JitsiMeet *sharedInstance = nil;
      static dispatch_once_t onceToken;
  
      dispatch_once(&amp;onceToken, ^{
          sharedInstance = [[self alloc] init];
      });
  
      return sharedInstance;
  }
  </code></pre>
  <p>Can someone please explain me? keeping above code as a reference?</p>