title: What is the meaning of following swift code?
body: <p>Can anyone tell me why we use <code>guard let self = self</code> ??</p>
  
  <p>I have seen this code while reading about GCD, I couldn't figure out what that particular line does.</p>
  
  <pre><code>DispatchQueue.global(qos: .userInitiated).async { [weak self] in
      guard let self = self else {
          return
      }
  
      // ...
  }
  </code></pre>