title: How to fix 'Missing argument for parameter 'completion' in call' error in Swift
body: <p>I am trying to run a function in the background thread to keep my app responsive to user actions but I am getting this error. -> 'Missing argument for parameter 'completion' in call'</p>
  
  <p>What is the correct way to write to call this function to avoid the error?</p>
  
  <pre><code>func loadWebsites(completion: @escaping ((_ result: [Website]) -&gt; Void)) {
      DispatchQueue.global(qos: .background).async {
          // Do some time consuming task in this background thread
          // Mobile app will remain to be responsive to user actions
          print("Performing time consuming task in this background thread")
          let url1 = URL(string: "\(self.nikeWebsite)")
          let request1 = URLRequest(url: url1!)
          let url2 = URL(string: "\(self.finishlineWebsite)")
          let request2 = URLRequest(url: url2!)
          let url3 = URL(string: "\(self.eastbayWebsite)")
          let request3 = URLRequest(url: url3!)
  
          let website1:Website = Bundle.main.loadNibNamed("Website", owner: self, options: nil)?.first as! Website
          if self.nikeWebsite == "https://firebasestorage.googleapis.com/v0/b/sneakem-2e19a.appspot.com/o/notavailable.png?alt=media&amp;token=9f82b093-04c6-499d-bb80-f260708fb8b9" {
              website1.webView.navigationDelegate = self
          } else {
              website1.webView.load(request1)
              website1.webView.navigationDelegate = self
              website1.webView.allowsBackForwardNavigationGestures = true
          }
  
          let website2:Website = Bundle.main.loadNibNamed("Website", owner: self, options: nil)?.first as! Website
          if self.finishlineWebsite == "https://firebasestorage.googleapis.com/v0/b/sneakem-2e19a.appspot.com/o/notavailable.png?alt=media&amp;token=9f82b093-04c6-499d-bb80-f260708fb8b9" {
              website2.webView.navigationDelegate = self
          } else {
              website2.webView.load(request2)
              website2.webView.navigationDelegate = self
              website2.webView.allowsBackForwardNavigationGestures = true
          }
  
          let website3:Website = Bundle.main.loadNibNamed("Website", owner: self, options: nil)?.first as! Website
          if self.eastbayWebsite == "https://firebasestorage.googleapis.com/v0/b/sneakem-2e19a.appspot.com/o/notavailable.png?alt=media&amp;token=9f82b093-04c6-499d-bb80-f260708fb8b9" {
              website3.webView.navigationDelegate = self
          } else {
              website3.webView.load(request3)
              website3.webView.navigationDelegate = self
              website3.webView.allowsBackForwardNavigationGestures = true
          }
  
          DispatchQueue.main.async {
              completion([website1, website2, website3])
          }
      }
  }
  </code></pre>