Q:

How to deal with `Use of undeclared identifier 'UIDevice'`

I followed the MD file step by step.  But there is still an error in the compilation.  What can I do? 
In the FaceppClient.m file :
if ([[[UIDevice currentDevice] systemVersion] compare: @"5.0" options: NSNumericSearch ] ! = NSOrderedAscending)
    _ios50orNewer = true;

Xcode outputs this error :

Use of undeclared identifier 'UIDevice'

I added #import "UIDevice.h" in the file, but Xcode outputs "UIDevice.h not found". 
I added the UIKit framework to the Project, but I still have the compilation issue. 
What can I do then? 

A:

Try adding this line at the top of your .m file : 
#import <UIKit/UIKit.h>

Adding the framework to the project is useful, but you also have to include the needed headers in your code. 

A:

And for Swift 5 users who got here, try 
import UIKit

