Development cannot be enabled while your device is locked.

问题：Development cannot be enabled while your device is locked.

分析原因：由于第一次连接MAC时，在iOS设备上点击了"不信任"选项，所以XCode
8不能访问手机，所以才出现这个问题

解决方法：

英文好的同学，可以看到原文章
http://stackoverflow.com/questions/26791477/xcode-device-locked-when-iphone-is-unlocked
以下我说下方法

这是APPLE官网的解释
https://support.apple.com/en-us/HT202778
重点是这段

Change your settings for trusted computers

Your iOS device remembers the computers you've chosen to trust.

If you don't want to trust a computer or other device anymore, change the privacy settings on your iPhone, iPad, or iPod touch: In iOS 8 or later, tap Settings > General > Reset > Reset Location & Privacy. Now when you connect to formerly trusted computers,
the Trust alert will ask you whether you trust that computer. 

意思是说重置隐私设置

点击 设置 --> 通用 --> 重置  --> 重置位置和隐私 

然后关闭xcode 8, 拔掉数据线，重启xocde 8开启项目， 重新插上数据线，此时手机上会显示mac与手机设备的连接访问权限，选择“信任”，