title: iOS 15 warning: NSKeyedUnarchiver _warnAboutPlistType:missingInAllowedClasses:
body: <p>Xcode 13 and iOS 15 began warning about missingAllowedClasses when using custom DataTransformers. There is very little documentation on custom DataTransformers, so I thought I would post a question and answer it here.</p>
  <pre><code>[general] *** -[NSKeyedUnarchiver _warnAboutPlistType:missingInAllowedClasses:] allowed unarchiving safe plist type ''NSString' (0x1dc9a7660) [/System/Library/Frameworks/Foundation.framework]', even though it was not explicitly included in the client allowed classes set: '{(
      &quot;'NSArray' (0x1dc99c838) [/System/Library/Frameworks/CoreFoundation.framework]&quot;
  )}'. This will be disallowed in the future.
  </code></pre>
  <p>Notice in the warning message, it specifies 'NSArray' and the type missing, 'NSString'.</p>