README.md

Swift Generated ObjC Header

The header file generated from Swift code, often seen as *-Swift.h, is crucial for Swift Objective-C interoperability. We use -emit-objc-header to instruct swift compiler to generate the header, which can be used to import Swift code into Objective-C. It's probably not obvious that the access modifiers, the placement of @objc annotation and the inheritance of NSObject all affect how the header file is generated.

I created a sample file and enumerated some variances to observe the different behaviors. Many compiler flags can affect how the header is generated, the ones I used are below. I will talk about some other flags at the end of this article.

xcrun swiftc -emit-object -parse-as-library -emit-objc-header -emit-objc-header-path FooClass-Swift.h FooClass.swift

public class, public method, NSObject subclass

// @interface FooClass01 : NSObject
// - (void)barFunction;
// - (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
// @end

@objc public class FooClass00: NSObject {
    @objc public func barFunction() { }
}

// @interface FooClass01 : NSObject
// - (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
// @end

@objc public class FooClass01: NSObject {
    public func barFunction() { }
}

// @interface FooClass02 : NSObject
// - (void)barFunction;
// - (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
// @end

public class FooClass02: NSObject {
    @objc public func barFunction() { }
}

⚠️ Even without @objc annotation, the NSObject subclass still generates its ObjC interface.

// @interface FooClass03 : NSObject
// - (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
// @end

public class FooClass03: NSObject {
    public func barFunction() { }
}

public class, public method, not NSObject subclass

⛔️ Build error: "only classes that inherit from NSObject can be declared @objc"

@objc public class FooClass04 {
    @objc public func barFunction() { }
}

⛔️ Build error: "only classes that inherit from NSObject can be declared @objc"

@objc public class FooClass05 {
    public func barFunction() { }
}

⚠️ Surprisingly this code can be built.

// No ObjC interface is generated

public class FooClass06 {
    @objc public func barFunction() { }
}

// No ObjC interface is generated

public class FooClass07 {
    public func barFunction() { }
}

public class, internal method, NSObject subclass

⚠️ Internal methods won't generate ObjC interfaces even they are annotated by @objc.

// @interface FooClass : NSObject
// - (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
// @end

@objc public class FooClass08: NSObject {
    @objc func barFunction() { }
}

// @interface FooClass : NSObject
// - (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
// @end

@objc public class FooClass09: NSObject {
    func barFunction() { }
}

// @interface FooClass : NSObject
// - (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
// @end

public class FooClass10: NSObject {
    @objc func barFunction() { }
}

// @interface FooClass : NSObject
// - (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
// @end

public class FooClass11: NSObject {
    func barFunction() { }
}

internal class, internal method, NSObject subclass

⚠️ Internal classes won't generate ObjC interfaces at all.

// No ObjC interface is generated

@objc class FooClass12: NSObject {
    @objc func barFunction() { }
}

// No ObjC interface is generated

@objc class FooClass13: NSObject {
    func barFunction() { }
}

// No ObjC interface is generated

class FooClass14: NSObject {
    @objc func barFunction() { }
}

// No ObjC interface is generated

class FooClass15: NSObject {
    func barFunction() { }
}

private class, NSObject subclass

⚠️ Since private classes are not visible outside of the file, no ObjC interface is generated.

Compiler Flags

As mentioned at the beginning, many compiler flags can affect the generation behavior, especially about internal classes or methods.

• -parse-as-library is necessary to build a library. With it, no internal class or method will generate the ObjC interface.
• Even with -parse-as-library, -import-objc-header can make internal classes and methods generate the ObjC interfaces. You don't need to provide a real bridging header. -import-objc-header /dev/null can do the trick. Please note that this flag cannot be used with -import-underlying-module.
• Weirdly, -application-extension can also make internal class and method generate the ObjC interface. I don't know if this is an intention or a bug.