How do you build a custom native module in iOS (Swift/Objective-C)?

Short answer

Create a Swift class with the desired methods, use an Objective-C bridging file, and register it in the React Native module tree with the RCT_EXTERN_MODULE macro.

Explanation

Since the old React Native core relies on Objective-C, connecting Swift code requires a small Objective-C file to export the class and methods using RCT_EXTERN_MODULE and RCT_EXTERN_METHOD, and to specify whether the module needs to run on the Main Queue or a separate thread. This approach belongs to the old architecture; new projects should prefer Turbo Native Modules with Codegen.

Code example

// Swift file:
@objc(BatteryModule)
class BatteryModule: NSObject {
  @objc(getBatteryLevel:rejecter:)
  func getBatteryLevel(resolve: RCTPromiseResolveBlock, reject: RCTPromiseRejectBlock) {
    resolve(UIDevice.current.batteryLevel * 100)
  }
}

// Objective-C file that exports it (BatteryModule.m):
#import <React/RCTBridgeModule.h>
@interface RCT_EXTERN_MODULE(BatteryModule, NSObject)
RCT_EXTERN_METHOD(getBatteryLevel:(RCTPromiseResolveBlock)resolve rejecter:(RCTPromiseRejectBlock)reject)
@end

Common mistakes

• Forgetting to add @objc before Swift classes and methods, preventing the Objective-C runtime from seeing them.

Follow-up questions

• How do you build a custom native module in Android (Kotlin) and connect it to React Native?

Sources

• React Native docs — Turbo Native Modules: iOS