Cross-platform apps that sync with open-standard 3rd-party servers: <a href="http:&#x2F;&#x2F;2doapp.com" rel="nofollow">http:&#x2F;&#x2F;2doapp.com</a> (CalDav), <a href="http:&#x2F;&#x2F;www.notebooksapp.com" rel="nofollow">http:&#x2F;&#x2F;www.notebooksapp.com</a> (WebDAV), <a href="https:&#x2F;&#x2F;www.omnigroup.com&#x2F;omnifocus" rel="nofollow">https:&#x2F;&#x2F;www.omnigroup.com&#x2F;omnifocus</a> (WebDAV).<p>Apple&#x27;s Pages&#x2F;Keynote&#x2F;Numbers support WebDAV sync and 3rd-party file-share apps, e.g. <a href="https:&#x2F;&#x2F;panic.com&#x2F;transmit-ios&#x2F;" rel="nofollow">https:&#x2F;&#x2F;panic.com&#x2F;transmit-ios&#x2F;</a>, so there is precedent.