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Swift to Objective-C header does not contain Swift classes

Posted by: admin November 30, 2017 Leave a comment

Questions:

I’m attempting to use Swift classes in my Objective-C code, however my Swift classes don’t seem to appear in the generated header. As a result, my build fails with “Use of undeclared identifier ‘HelloWorld'”.

I used the templates to create a project called TestApp.

I have the following Build Settings in my target:

  • Product Name : TestApp
  • Product Module Name : TestAppModule
  • Defines Module : Yes

Apple’s documentation says to use #import <TestApp/TestAppModule-Swift.h> but this doesn’t work.

Instead, I’m using #import "TestAppModule-Swift.h" in my “.m” file. It seems to find this.

I’m able to navigate to it, and it looks like this…

// Generated by Swift version 1.0 (swift-600.0.34.4.5)

#if defined(__has_include) && __has_include(<swift/objc-prologue.h>)
# include <swift/objc-prologue.h>
#endif

...etc...

but no classes defined in there.

I have a Swift file in the project that looks like this…

class HelloWorld {    
    func hello() {
        println("hello world")
    }
}

Why isn’t this working using the standard header file location #import <TestApp/TestAppModule-Swift.h>?

How can I get my swift classes in that header file, so I won’t get the “undeclared identifier” error?

Answers:

Here’s how I have gotten it to work. You can see a more large-scale answer here.

Change this:

class HelloWorld {    
    func hello() {
        println("hello world")
    }
}

To:

@objc class HelloWorld { 

    class func newInstance() -> HelloWorld {
        return HelloWorld()
    }

    func hello() {
        println("hello world")
    }
}

Then, In your ObjC file:

#import "TestApp-Swift.h"

And call like this:

HelloWorld * helloWorld = [HelloWorld newInstance];
[helloWorld hello];

Questions:
Answers:

tl;dr Ensure you have a bridging header if you’re doing any cross-calling between Objective-C and Swift.

I had the exact same problem: I could see the -Swift.h file in DerivedData but it made no mention of my Swift classes. I was importing the header file correctly, the Defines Module setting was YES, and the Product Module Name was correct. I tried deleting and re-adding the Swift files, clean buiild, quitting XCode, etc, with no luck.

Then I realised I had no -Bridging-Header.h file in my project, presumably due to the way I’d cobbled it together from a previous project. Shouldn’t be a problem because I was not (yet) calling Objective-C from Swift. But when I added a bridging header, and referred to its path in the build settings (Swift Compiler – Code Generation -> Objective-C Bridging Header), it magically fixed the problem – my -Swift.h file was suddenly full of SWIFT_CLASS() goodness!

So I’m guessing the bridging header is fundamental to the process, even if you’re NOT using Objective-C from Swift.


UPDATE: I finally understand this. It is related to public/internal access modifiers. Not sure if I missed this originally or if it’s an addition to the Apple docs, but it now clearly states:-

By default, the generated header contains interfaces for Swift
declarations marked with the public modifier. It also contains those
marked with the internal modifier if your app target has an
Objective-C bridging header.

Questions:
Answers:

It is proper to use #import "TestAppModule-Swift.h" in your .m files. If you need to reference a class in a .h, use the @class forward declaration.

Further, if you want to use a Swift class from Objective-C, the Swift class must be marked with the @objc attribute. Xcode will only include classes with that attributed in the generated header. See also this documentation.

Questions:
Answers:

Class should be declared as @objc public class

Questions:
Answers:

A more convenient way would be to inherit from NSObject. Like so:

class HelloWorld: NSObject {    
    func hello() {
        println("hello world")
    }
}

Questions:
Answers:

In my case the class was not being compiled, because I first added it to my test target only… After adding it to my main target (Build Phases -> Compile Sources), it was actual compiled and added to the header file.

So much for TDD 😉

Questions:
Answers:

In my case, by following Apple guidelines, it did not work until I ran the project. The xcode editor kept flagging the unknown swift class, until i clicked “run”. The build succeeded, and the swift method worked.