How can I fix the concurrency safety error for FlutterMethodNotImplemented in Swift?

Below is one way to resolve the error. The error occurs because Flutter’s built-in value for "not implemented" (FlutterMethodNotImplemented) is defined as a mutable global variable. When you reference it directly in a concurrent context (for example, if the Flutter method channel may be invoked concurrently), Swift’s concurrency rules complain about potential data races.

Two common workarounds are:

1. Wrap the call to result in a dispatch to the main thread: This ensures that the reference to FlutterMethodNotImplemented is made from the main thread where you expect it to be safe.

2. Capture a local copy of FlutterMethodNotImplemented before passing it to result: By doing so you avoid directly referencing the mutable global.

Below are code examples for both approaches.

---

Option 1: Dispatch on the Main Thread

Dispatch the call to result on the main thread. This is helpful because Flutter plugins typically work on the main thread and ensures thread safety when referencing globals.

SWIFTCopied
public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
    switch call.method {
    case "jailbroken":
        let check = IOSSecuritySuite.amIJailbroken()
        DispatchQueue.main.async {
            result(check)
        }
    case "developerMode":
        let mode = IOSSecuritySuite.amIRunInEmulator()
        DispatchQueue.main.async {
            result(mode)
        }
    default:
        DispatchQueue.main.async {
            result(FlutterMethodNotImplemented)
        }
    }
}

---

Option 2: Capture a Local Copy

Create a local constant that stores the value of FlutterMethodNotImplemented and pass that to result. This avoids capturing the mutable global variable directly.

SWIFTCopied
public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
    switch call.method {
    case "jailbroken":
        let check = IOSSecuritySuite.amIJailbroken()
        result(check)
    case "developerMode":
        result(IOSSecuritySuite.amIRunInEmulator())
    default:
        let notImplemented = FlutterMethodNotImplemented
        result(notImplemented)
    }
}

---

Either approach should resolve the concurrency error. Often, wrapping calls on the main thread is recommended in Flutter plugins unless you have a specific reason to run on another queue.

Choose the option that best fits your plugin’s architecture.