FlutterSwift

FlutterSwift is a native Swift Flutter client wrapper and platform channel implementation.

The end-goal is to allow Flutter to be used for the UI, and Swift the business logic, in a cross-platform manner. Currently this is achieved for macOS, iOS and embedded Linux, but Android is an obvious future target. It also provides a more idiomatic structured concurrency API which would not be possible in Objective-C.

Architecture

The FlutterDesktopMessenger actor wraps the API in flutter_messenger.h. To permit development on Darwin platforms, FlutterSwift also provides FlutterPlatformMessenger which wraps the existing platform binary messenger. Thus, you can develop on macOS and deploy on embedded Linux from a single codebase.

This repository will build the Sony eLinux Wayland engine as a submodule, but it doesn't at this time build the Flutter engine itself: this is assumed to be in /opt/flutter-elinux/lib or a build artifact downloaded by download-engine.sh.

Some examples follow.

Counter Demo

Assuming the Flutter SDK is installed in /opt/flutter-elinux/flutter, you can just run ./build-counter-linux.sh in the top-level directory, followed by ./run-counter-linux.sh. This will build the Flutter AOT object followed by the Swift runner.

Initialization

macOS:

import FlutterMacOS.FlutterBinaryMessenger
import FlutterSwift

override func awakeFromNib() {
    let flutterViewController = FlutterViewController() // from ObjC implementation
    let binaryMessenger = FlutterSwift
        .FlutterPlatformMessenger(wrapping: flutterViewController.engine.binaryMessenger)
    ...
}

Linux, using the native Swift client wrapper:

@main
enum SomeApp {
    static func main() {
        guard CommandLine.arguments.count > 1 else {
            print("usage: SomeApp [flutter_path]")
            exit(1)
        }
        let dartProject = DartProject(path: CommandLine.arguments[1])
        let viewProperties = FlutterViewController.ViewProperties(
            width: 640,
            height: 480,
            title: "SomeApp",
            appId: "com.example.some-app"
        )
        let window = FlutterWindow(properties: viewProperties, project: dartProject)
        guard let window else {
            debugPrint("failed to initialize window!")
            exit(2)
        }
        let binaryMessenger = viewController.engine.binaryMessenger
        ...
        window.run()
    }
}

Message channel

This shows a basic message channel handler using the JSON message codec. Note that because the binary messenger is an actors, setMessageHandler() needs to be called in an asynchronous context. On eLinux, instead of registering the channels in awakeFromNib(), call this from the main() function (perhaps indirected by a manager class).

private func messageHandler(_ arguments: String?) async -> Int? {
    debugPrint("Received message \(arguments)")
    return 12345
}       

override func awakeFromNib() {
...
    flutterBasicMessageChannel = FlutterBasicMessageChannel(
        name: "com.padl.example",
        binaryMessenger: binaryMessenger,
        codec: FlutterJSONMessageCodec.shared
    )

    task = Task {
        try! await flutterBasicMessageChannel.setMessageHandler(messageHandler)
        ...
    }
}

Method channel

var isRunning = true

private func methodCallHandler(
    call: FlutterSwift
        .FlutterMethodCall<Bool>
) async throws -> Bool {
    isRunning.toggle()
    return isRunning
}

override func awakeFromNib() {
...

    let flutterMethodChannel = FlutterMethodChannel(
        name: "com.padl.toggleCounter",
        binaryMessenger: binaryMessenger
    )
    task = Task {
        try! await flutterMethodChannel.setMethodCallHandler(methodCallHandler)
    }
}

Event channel

import AsyncAlgorithms
import AsyncExtensions
import FlutterSwift
...

/// this should go inside MainFlutterWindow
typealias Arguments = FlutterNull
typealias Event = Int32
typealias Stream = AsyncThrowingChannel<Event?, FlutterError>

var flutterEventStream = Stream()
var task: Task<(), Error>?
var counter: Event = 0

private func onListen(_ arguments: Arguments?) throws -> FlutterEventStream<Event> {
    // a FlutterEventStream is an AsyncSequence
    flutterEventStream.eraseToAnyAsyncSequence()
}

private func onCancel(_ arguments: Arguments?) throws {
    task?.cancel()
    task = nil
}

override func awakeFromNib() {
...
    let flutterEventChannel = FlutterEventChannel(
        name: "com.padl.counter",
        binaryMessenger: binaryMessenger
    )
    task = Task {
        try! await flutterEventChannel.setStreamHandler(onListen: onListen, onCancel: onCancel)
        repeat {
            await flutterEventStream.send(counter)
            count += 1
            try await Task.sleep(nanoseconds: NSEC_PER_SEC)
        } while !Task.isCancelled
    }
}

PADL / FlutterSwift