If UIKit is imported in the PCH file, then why is it imported again in each file?

Question

If #import <UIKit/UIKit.h> and #import <Foundation/Foundation.h> are both found in our `ProjectName.pch' file, making them globally imported, why are they automatically added to the header file when Xcode creates a new class?

Answer 1

The .pch is there as a compile time optimisation, personally I recommend making sure that classes can still build without a pch (so still import into h/m's manually), so that it can A) build without and B) so that if you re-use code, you can easily see its dependencies.

In general, newly-generated iOS projects come with this functionality, which is called a precompiled header or prefix header, and is a file that has the extension .pch.

You can throw all the headers you want in there and Xcode will pre-compile it before it builds anything else, and use it to compile the other compilation units in your project (e.g. .m files).

Using a precompiled header may or may not increase compile time; in general, it reduces compile time, as long as you have a lot of common headers and/or a lot of source files.

However, it's not necessarily good practice to treat the pre-compiled header like a big dumping ground, as your compilation units can form implicit dependencies on all sorts of stuff when you may want to enforce loose coupling between components.

Answer 2

Only Apple knows for sure.

Best guess: Apple can not be sure that the imports are in the pch file for all projects or that there is even a pch file. This by having these implicit imports compiling can be guaranteed.