Question

On a new project I have started, XCode has decided that it will compile every file in the project every time I run it, rather than just compiling files that change (and files dependent on those). As there are getting to be more and more files in the project, this becomes a bigger and bigger burden in both time and battery life.

It is possible that I have changed a setting somewhere that affected this; or, maybe not. What are some project settings that I should be looking at?

Answer 1

If Xcode recompiles most or all of your source files whenever you do a build, that usually means that those files are all directly or indirectly dependent on some header file that has changed. Here are some things to look for:

1. Do your source files tend to #import some top-level header file that itself recursively imports a bunch of lower-level header files? If any file in that tree of dependent headers is modified, it will force recompilation of any .m file that imports the top-level header file. You may be able to reduce these dependencies by importing headers for lower level submodules, or better yet, for just the specific headers you need for each file. (Note: Some libraries that aren’t designed to be used this way can make this approach challenging or impossible in some cases.)

2. Some third party development tools and static libraries run scripts that generate or modify code as part of their build process. If your source files are dependent on a header file that’s generated by a script, they’ll be recompiled every time the script regenerates that header file. Even if the code generated by the script doesn’t change, dependent source files will be recompiled if the last-modified date of the header file changes. It may take some clever hacking to eliminate redundant compilation if this is your issue.

3. Don’t forget to check your precompiled header (.pch) file to see what’s being imported there. The contents of that file are effectively injected at the top of every .m file in your project at compile time.

4. Try to minimize dependencies by moving as many #import statements as possible out of your .h files and into your .m files. You can generally get away with just importing the headers for your class’s superclass and any protocols your class implements in its .h file. You can use forward declarations instead of #import statements for any other classes, data types, or protocols you use in your class’s @interface.