Xcode离线文档下载地址
https://developer.apple.com/library/downloads/docset-index.dvtdownloadableindex-
去掉NavigationBar底部的黑线
[self.navigationController.navigationBar setBackgroundImage:[[UIImage alloc] init] forBarMetrics:UIBarMetricsDefault];
self.navigationController.navigationBar.shadowImage = [[UIImage alloc] init];```
