title: PHPhotoLibrary - photoLibraryDidChange called multiple times in Swift
body: <p>I have the following code:</p>
  
  <pre><code>override func viewDidLoad() {
      ........
      PHPhotoLibrary.shared().register(self)
  }
  </code></pre>
  
  <p>and I've implemented the <code>PHPhotoLibraryDidChangeObserver</code>. </p>
  
  <p>My problem is, that the method <code>photoLibraryDidChange</code> gets called multiple times if something in the device's gallery changes.<br>
  Here is the code for that function:</p>
  
  <pre><code>func photoLibraryDidChange(_ changeInstance: PHChange) {
      DispatchQueue.main.async(execute: {
          self.updateFetchResult();
      })
  }
  </code></pre>
  
  <p>Do you know why this method gets executed <code>multiple</code> times?</p>