Despite the "Unknown class MyClass in Interface Builder file." error printed at runtime, this issue has nothing to do with Interface Builder, but rather with the linker, which is not linking a class because no code uses it directly.
When the .nib data (compiled from the .xib) is loaded at runtime, MyClass is referenced using a string, but the linker doesn't analyze code functionality, just code existence, so it doesn't know that. Since no other source files references that class, the linker optimizes it out of existence when making the executable. So when Apple's code tries to load such a class, it can't find the code associated with it, and prints the warning.
By default, Objective-C targets will have -all_load -ObjC flags set by default, which will keep all of the symbols. But I had started with a C++ target, and didn't have that. Nevertheless, I found a way around this, which keeps the linker aggressive.
The hack I was originally using was to add an empty static routine like:
+(void)_keepAtLinkTime;
which does nothing, but that I would call once, such as:
int main( int argc, char** argv )
{
[MyClass _keepAtLinkTime];
// Your code.
}
This would force the linker to keep the whole class, and the error disappears.
As jlstrecker pointed out in the comments, we do not really need to add a _keepAtLinkTime method. Simply calling an existing one, such as:
[MyClass class];
does the trick (as long as you derive from an NSObject).
Of course, you can call this in any location of your code. I guess it could even be in unreachable code. The idea is to fool the linker into thinking that MyClass is used somewhere so that it isn't so aggressive in optimizing it out.
Xcode 6.3.2 & Swift 1.2
Swift definition of view. Be sure to override init(coder aDecoder: NSCoder). Objective-C definition of view controller. And, a nib in a pear tree.
Add Module Name to Nib details inspector where you pick your class.
Also if using a static library, don't forget to add the -ObjC flag to Other Linker Flags in your projects Build Settings tab.
If you have failed to do so, "Unknown class ... in Interface Builder file" will occur when trying to load a view controller from the static library in IB.
Just another case I experienced the error will show.
Best Answer
This warning also occurs when I build and run a brand new empty project in Xcode 9.1.
Replacing the default NSWindow with an empty custom class of NSWindow with no changes does remove the error, but that behaviour makes no sense.
Therefore, I'm filing it as a bug.