Objective-C calling into Swift in Turbo Module library

[Zach-Dean-Attractions-io]

Hi,

I have created a library template using: npx create-react-native-library objc-swift-example, running version 0.48.6 of create-react-native-library and naming the package react-native-objc-swift-example.

I've selected Turbo Module and then Kotlin & Objective-C. Then I run the following commands

cd objc-swift-example
yarn
cd example/ios
bundle install
pod install && yarn ios

This successfully builds and shows a screen saying 'Result: 21'. At this point the implementation of the multiply function is in Objective-C++

ios/ObjcSwiftExample.h

#import "generated/RNObjcSwiftExampleSpec/RNObjcSwiftExampleSpec.h"

@interface ObjcSwiftExample : NSObject <NativeObjcSwiftExampleSpec>

@end

ios/ObjcSwiftExample.mm

#import "ObjcSwiftExample.h"

@implementation ObjcSwiftExample
RCT_EXPORT_MODULE()

- (NSNumber *)multiply:(double)a b:(double)b {
    NSNumber *result = @(a * b);

    return result;
}

- (std::shared_ptr<facebook::react::TurboModule>)getTurboModule:
    (const facebook::react::ObjCTurboModule::InitParams &)params
{
    return std::make_shared<facebook::react::NativeObjcSwiftExampleSpecJSI>(params);
}

@end

I've then tried to implement the multiply function in Swift that is currently implemented by ios/ObjcSwiftExample.mm. To do this I've created

ios/SwiftImpl.swift

import Foundation

class SwiftImpl : NSObject {
  
  @objc
  func multiply(a: Double, b: Double) -> NSNumber {
    return NSNumber(value: a * b)
  }
}

ios/ObjcSwiftExample-Bridging-Header.h (an empty file)

-//
-//  Use this file to import your target's public headers that you would like to expose to Swift.
-//

And then modified

ios/ObjcSwiftExample.mm

#import "ObjcSwiftExample.h"
+ #import "ObjcSwiftExample-Swift.h"

@implementation ObjcSwiftExample
RCT_EXPORT_MODULE()

 - (NSNumber *)multiply:(double)a b:(double)b {
- NSNumber *result = @(a * b); 
+ NSNumber *result = [[SwiftImpl new] multiplyWithA:a b:b];
  return result;
}

 - (std::shared_ptr<facebook::react::TurboModule>)getTurboModule:
    (const facebook::react::ObjCTurboModule::InitParams &)params
{
    return std::make_shared<facebook::react::NativeObjcSwiftExampleSpecJSI>(params);
}

@end

Running pod install && yarn ios returns the error error 'ObjcSwiftExample-Swift.h' file not found.

I then tried modifying the file ObjcSwiftExample.podspec to include swift source files

-s.source_files = "ios/**/*.{h,m,mm,cpp}"
+s.source_files = "ios/**/*.{h,m,mm,cpp,swift}"

Running pod install && yarn ios then returns the error error 'generated/RNObjcSwiftExampleSpec/RNObjcSwiftExampleSpec.h' file not found. I believe this is due to the code in the podspec file

s.private_header_files = "ios/generated/**/*.h"

But I think this was required to solve another issue: #690.

If I comment this code out then I run into another error, so I don't think commenting out this code is a path I want to go down.

error This file must be compiled as Obj-C++. If you are importing it, you must change your file extension to .mm.
error 'utility' file not found
error 'optional' file not found
error 'cassert' file not found
error unknown type name 'namespace'
error expected ';' after top level declarator
error too many errors emitted, stopping now
error could not build Objective-C module 'ObjcSwiftExample'

Reverting that commenting out change, I have also tried adding other options into the podspec file such as DEFINES_MODULES = YES, adding into the HEADER_SEARCH_PATHS the path for the ios/generated directory. I am unsure if I should use the SWIFT_OJBC_BRIDGING_HEADER also as Xcode seems to populate this when creating a sample Objective-C project and adding a Swift file into it, but I've not had joy with that either.

Is anyone able to help with what I am missing here?

[Zach-Dean-Attractions-io]

I figured out what I was missing here.

Firstly, the Swift file needs to be a public class and have public functions

ios/SwiftImpl.swift

import Foundation

- class SwiftImpl : NSObject {
+ @objc
+ public class SwiftImpl : NSObject {
  
  @objc
- func multiply(a: Double, b: Double) -> NSNumber {
+ public func multiply(a: Double, b: Double) -> NSNumber {
    return NSNumber(value: a * b)
  }
}

The bridging header file isn't required in this case but it doesn't seem to hurt including it.

The .podspec file needs updating as follows:

- s.source_files = "ios/**/*.{h,m,mm,cpp}"
+ s.source_files = "ios/**/*.{m,mm,cpp,swift}"

Note the removal of the h from source_files. If the h is included you will see the error, which I mentioned above:

error 'generated/RNObjcSwiftExampleSpec/RNObjcSwiftExampleSpec.h' file not found

Which comes from this issue: CocoaPods/CocoaPods#12105 (comment)

[Zach-Dean-Attractions-io]

I did also notice that when I was just adding Swift support into a standalone app that used a turbo native module (as opposed to creating a library project) for version 0.78.0 of React Native I did need the bridging header and also needed to add

#import <RCTAppDelegate.h>

into it so that the iOS app would build. This wasn't required on version 0.76.0 of React Native. I think this is because in 0.76.0 of React Native the iOS project is an Objective-C based project (i.e. AppDelegate.mm and AppDelegate.h) whereas in 0.78.0 of React Native the iOS project is a Swift based project (i.e. AppDelegate.swift`).