Writing custom platform-specific code

This guide describes how to write custom platform-specific code. Some platform-specific functionality is available through existing packages; see using packages.

Flutter uses a flexible system that allows you to call platform-specific APIs in a language that works directly with those APIs:

• Kotlin or Java on Android
• Swift or Objective-C on iOS
• C++ on Windows
• Objective-C on macOS
• C on Linux

Flutter’s builtin platform-specific API support doesn’t rely on code generation, but rather on a flexible message passing style. Alternatively, you can use the Pigeon package for sending structured typesafe messages with code generation:

• The Flutter portion of the app sends messages to its host, the non-Dart portion of the app, over a platform channel.

• The host listens on the platform channel, and receives the message. It then calls into any number of platform-specific APIs—using the native programming language—and sends a response back to the client, the Flutter portion of the app.

Architectural overview: platform channels

Messages are passed between the client (UI) and host (platform) using platform channels as illustrated in this diagram:

Messages and responses are passed asynchronously, to ensure the user interface remains responsive.

On the client side, MethodChannel enables sending messages that correspond to method calls. On the platform side, MethodChannel on Android (MethodChannelAndroid) and FlutterMethodChannel on iOS (MethodChanneliOS) enable receiving method calls and sending back a result. These classes allow you to develop a platform plugin with very little ‘boilerplate’ code.

Platform channel data types support and codecs

The standard platform channels use a standard message codec that supports efficient binary serialization of simple JSON-like values, such as booleans, numbers, Strings, byte buffers, and Lists and Maps of these (see StandardMessageCodec for details). The serialization and deserialization of these values to and from messages happens automatically when you send and receive values.

The following table shows how Dart values are received on the platform side and vice versa:

• Java
• Kotlin
• Obj-C
• Swift
• C++
• C

Example: Calling platform-specific code using platform channels

The following code demonstrates how to call a platform-specific API to retrieve and display the current battery level. It uses the Android BatteryManager API, the iOS device.batteryLevel API, the Windows GetSystemPowerStatus API, and the Linux UPower API with a single platform message, getBatteryLevel().

The example adds the platform-specific code inside the main app itself. If you want to reuse the platform-specific code for multiple apps, the project creation step is slightly different (see developing packages), but the platform channel code is still written in the same way.

Step 1: Create a new app project

Start by creating a new app:

• In a terminal run: flutter create batterylevel

By default, our template supports writing Android code using Kotlin, or iOS code using Swift. To use Java or Objective-C, use the -i and/or -a flags:

• In a terminal run: flutter create -i objc -a java batterylevel

Step 2: Create the Flutter platform client

The app’s State class holds the current app state. Extend that to hold the current battery state.

First, construct the channel. Use a MethodChannel with a single platform method that returns the battery level.

The client and host sides of a channel are connected through a channel name passed in the channel constructor. All channel names used in a single app must be unique; prefix the channel name with a unique ‘domain prefix’, for example: samples.flutter.dev/battery.

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

class _MyHomePageState extends State<MyHomePage> {
  static const platform = MethodChannel('samples.flutter.dev/battery');
  // Get battery level.

Next, invoke a method on the method channel, specifying the concrete method to call using the String identifier getBatteryLevel. The call might fail—for example, if the platform doesn’t support the platform API (such as when running in a simulator), so wrap the invokeMethod call in a try-catch statement.

Use the returned result to update the user interface state in _batteryLevel inside setState.

// Get battery level.
String _batteryLevel = 'Unknown battery level.';

Future<void> _getBatteryLevel() async {
  String batteryLevel;
  try {
    final int result = await platform.invokeMethod('getBatteryLevel');
    batteryLevel = 'Battery level at $result % .';
  } on PlatformException catch (e) {
    batteryLevel = "Failed to get battery level: '${e.message}'.";
  }

  setState(() {
    _batteryLevel = batteryLevel;
  });
}

Finally, replace the build method from the template to contain a small user interface that displays the battery state in a string, and a button for refreshing the value.

@override
Widget build(BuildContext context) {
  return Material(
    child: Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        children: [
          ElevatedButton(
            onPressed: _getBatteryLevel,
            child: const Text('Get Battery Level'),
          ),
          Text(_batteryLevel),
        ],
      ),
    ),
  );
}

Step 3: Add an Android platform-specific implementation

• Kotlin
• Java

import androidx.annotation.NonNull
import io.flutter.embedding.android.FlutterActivity
import io.flutter.embedding.engine.FlutterEngine
import io.flutter.plugin.common.MethodChannel

class MainActivity: FlutterActivity() {
  private val CHANNEL = "samples.flutter.dev/battery"

  override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
    super.configureFlutterEngine(flutterEngine)
    MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler {
      call, result ->
      // This method is invoked on the main thread.
      // TODO
    }
  }
}

import android.content.Context
import android.content.ContextWrapper
import android.content.Intent
import android.content.IntentFilter
import android.os.BatteryManager
import android.os.Build.VERSION
import android.os.Build.VERSION_CODES

  private fun getBatteryLevel(): Int {
    val batteryLevel: Int
    if (VERSION.SDK_INT >= VERSION_CODES.LOLLIPOP) {
      val batteryManager = getSystemService(Context.BATTERY_SERVICE) as BatteryManager
      batteryLevel = batteryManager.getIntProperty(BatteryManager.BATTERY_PROPERTY_CAPACITY)
    } else {
      val intent = ContextWrapper(applicationContext).registerReceiver(null, IntentFilter(Intent.ACTION_BATTERY_CHANGED))
      batteryLevel = intent!!.getIntExtra(BatteryManager.EXTRA_LEVEL, -1) * 100 / intent.getIntExtra(BatteryManager.EXTRA_SCALE, -1)
    }

    return batteryLevel
  }

    MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler {
      call, result ->
      // This method is invoked on the main thread.
      // TODO
    }

    MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler {
      // This method is invoked on the main thread.
      call, result ->
      if (call.method == "getBatteryLevel") {
        val batteryLevel = getBatteryLevel()

        if (batteryLevel != -1) {
          result.success(batteryLevel)
        } else {
          result.error("UNAVAILABLE", "Battery level not available.", null)
        }
      } else {
        result.notImplemented()
      }
    }

import androidx.annotation.NonNull;
import io.flutter.embedding.android.FlutterActivity;
import io.flutter.embedding.engine.FlutterEngine;
import io.flutter.plugin.common.MethodChannel;

public class MainActivity extends FlutterActivity {
  private static final String CHANNEL = "samples.flutter.dev/battery";

  @Override
  public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
  super.configureFlutterEngine(flutterEngine);
    new MethodChannel(flutterEngine.getDartExecutor().getBinaryMessenger(), CHANNEL)
        .setMethodCallHandler(
          (call, result) -> {
            // This method is invoked on the main thread.
            // TODO
          }
        );
  }
}

import android.content.ContextWrapper;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.BatteryManager;
import android.os.Build.VERSION;
import android.os.Build.VERSION_CODES;
import android.os.Bundle;

  private int getBatteryLevel() {
    int batteryLevel = -1;
    if (VERSION.SDK_INT >= VERSION_CODES.LOLLIPOP) {
      BatteryManager batteryManager = (BatteryManager) getSystemService(BATTERY_SERVICE);
      batteryLevel = batteryManager.getIntProperty(BatteryManager.BATTERY_PROPERTY_CAPACITY);
    } else {
      Intent intent = new ContextWrapper(getApplicationContext()).
          registerReceiver(null, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
      batteryLevel = (intent.getIntExtra(BatteryManager.EXTRA_LEVEL, -1) * 100) /
          intent.getIntExtra(BatteryManager.EXTRA_SCALE, -1);
    }

    return batteryLevel;
  }

          (call, result) -> {
            // This method is invoked on the main thread.
            // TODO
          }

          (call, result) -> {
            // This method is invoked on the main thread.
            if (call.method.equals("getBatteryLevel")) {
              int batteryLevel = getBatteryLevel();

              if (batteryLevel != -1) {
                result.success(batteryLevel);
              } else {
                result.error("UNAVAILABLE", "Battery level not available.", null);
              }
            } else {
              result.notImplemented();
            }
          }

You should now be able to run the app on Android. If using the Android Emulator, set the battery level in the Extended Controls panel accessible from the … button in the toolbar.

Step 4: Add an iOS platform-specific implementation

• Swift
• Objective-C

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
    let batteryChannel = FlutterMethodChannel(name: "samples.flutter.dev/battery",
                                              binaryMessenger: controller.binaryMessenger)
    batteryChannel.setMethodCallHandler({
      (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in
      // This method is invoked on the UI thread.
      // Handle battery messages.
    })

    GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

private func receiveBatteryLevel(result: FlutterResult) {
  let device = UIDevice.current
  device.isBatteryMonitoringEnabled = true
  if device.batteryState == UIDevice.BatteryState.unknown {
    result(FlutterError(code: "UNAVAILABLE",
                        message: "Battery level not available.",
                        details: nil))
  } else {
    result(Int(device.batteryLevel * 100))
  }
}

batteryChannel.setMethodCallHandler({
  [weak self] (call: FlutterMethodCall, result: FlutterResult) -> Void in
  // This method is invoked on the UI thread.
  guard call.method == "getBatteryLevel" else {
    result(FlutterMethodNotImplemented)
    return
  }
  self?.receiveBatteryLevel(result: result)
})

#import <Flutter/Flutter.h>
#import "GeneratedPluginRegistrant.h"

@implementation AppDelegate
- (BOOL)application:(UIApplication*)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions {
  FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;

  FlutterMethodChannel* batteryChannel = [FlutterMethodChannel
                                          methodChannelWithName:@"samples.flutter.dev/battery"
                                          binaryMessenger:controller.binaryMessenger];

  [batteryChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
    // This method is invoked on the UI thread.
    // TODO
  }];

  [GeneratedPluginRegistrant registerWithRegistry:self];
  return [super application:application didFinishLaunchingWithOptions:launchOptions];
}

- (int)getBatteryLevel {
  UIDevice* device = UIDevice.currentDevice;
  device.batteryMonitoringEnabled = YES;
  if (device.batteryState == UIDeviceBatteryStateUnknown) {
    return -1;
  } else {
    return (int)(device.batteryLevel * 100);
  }
}

__weak typeof(self) weakSelf = self;
[batteryChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
  // This method is invoked on the UI thread.
  if ([@"getBatteryLevel" isEqualToString:call.method]) {
    int batteryLevel = [weakSelf getBatteryLevel];

    if (batteryLevel == -1) {
      result([FlutterError errorWithCode:@"UNAVAILABLE"
                                 message:@"Battery level not available."
                                 details:nil]);
    } else {
      result(@(batteryLevel));
    }
  } else {
    result(FlutterMethodNotImplemented);
  }
}];

You should now be able to run the app on iOS. If using the iOS Simulator, note that it doesn’t support battery APIs, and the app displays ‘Battery level not available’.

Step 5: Add a Windows platform-specific implementation

Start by opening the Windows host portion of your Flutter app in Visual Studio:

1. Run flutter build windows in your project directory once to generate the Visual Studio solution file.

2. Start Visual Studio.

3. Select Open a project or solution.

4. Navigate to the directory holding your Flutter app, then into the build folder, then the windows folder, then select the batterylevel.sln file. Click Open.

Add the C++ implementation of the platform channel method:

1. Expand batterylevel > Source Files in the Solution Explorer.

2. Open the file flutter_window.cpp.

First, add the necessary includes to the top of the file, just after #include "flutter_window.h":

#include <flutter/event_channel.h>
#include <flutter/event_sink.h>
#include <flutter/event_stream_handler_functions.h>
#include <flutter/method_channel.h>
#include <flutter/standard_method_codec.h>
#include <windows.h>

#include <memory>

Edit the FlutterWindow::OnCreate method and create a flutter::MethodChannel tied to the channel name samples.flutter.dev/battery:

bool FlutterWindow::OnCreate() {
  // ...
  RegisterPlugins(flutter_controller_->engine());

  flutter::MethodChannel<> channel(
      flutter_controller_->engine()->messenger(), "samples.flutter.dev/battery",
      &flutter::StandardMethodCodec::GetInstance());
  channel.SetMethodCallHandler(
      [](const flutter::MethodCall<>& call,
         std::unique_ptr<flutter::MethodResult<>> result) {
        // TODO
      });

  SetChildContent(flutter_controller_->view()->GetNativeWindow());
  return true;
}

Next, add the C++ code that uses the Windows battery APIs to retrieve the battery level. This code is exactly the same as you would write in a native Windows application.

Add the following as a new function at the top of flutter_window.cpp just after the #include section:

static int GetBatteryLevel() {
  SYSTEM_POWER_STATUS status;
  if (GetSystemPowerStatus(&status) == 0 || status.BatteryLifePercent == 255) {
    return -1;
  }
  return status.BatteryLifePercent;
}

Finally, complete the setMethodCallHandler() method added earlier. You need to handle a single platform method, getBatteryLevel(), so test for that in the call argument. The implementation of this platform method calls the Windows code written in the previous step. If an unknown method is called, report that instead.

Remove the following code:

  channel.SetMethodCallHandler(
      [](const flutter::MethodCall<>& call,
         std::unique_ptr<flutter::MethodResult<>> result) {
        // TODO
      });

And replace with the following:

  channel.SetMethodCallHandler(
      [](const flutter::MethodCall<>& call,
         std::unique_ptr<flutter::MethodResult<>> result) {
        if (call.method_name() == "getBatteryLevel") {
          int battery_level = GetBatteryLevel();
          if (battery_level != -1) {
            result->Success(battery_level);
          } else {
            result->Error("UNAVAILABLE", "Battery level not available.");
          }
        } else {
          result->NotImplemented();
        }
      });

You should now be able to run the application on Windows. If your device doesn’t have a battery, it displays ‘Battery level not available’.

Step 6: Add a macOS platform-specific implementation

Start by opening the macOS host portion of your Flutter app in Xcode:

1. Start Xcode.

2. Select the menu item File > Open….

3. Navigate to the directory holding your Flutter app, and select the macos folder inside it. Click OK.

Add the Swift implementation of the platform channel method:

1. Expand Runner > Runner in the Project navigator.

2. Open the file MainFlutterWindow.swift located under Runner > Runner in the Project navigator.

First, add the necessary import to the top of the file, just after import FlutterMacOS:

import IOKit.ps

Create a FlutterMethodChannel tied to the channel name samples.flutter.dev/battery in the awakeFromNib method:

  override func awakeFromNib() {
    // ...
    self.setFrame(windowFrame, display: true)
  
    let batteryChannel = FlutterMethodChannel(
      name: "samples.flutter.dev/battery",
      binaryMessenger: flutterViewController.engine.binaryMessenger)
    batteryChannel.setMethodCallHandler { (call, result) in
      // This method is invoked on the UI thread.
      // Handle battery messages.
    }

    RegisterGeneratedPlugins(registry: flutterViewController)

    super.awakeFromNib()
  }
}

Next, add the macOS Swift code that uses the IOKit battery APIs to retrieve the battery level. This code is exactly the same as you would write in a native macOS app.

Add the following as a new method at the bottom of MainFlutterWindow.swift:

private func getBatteryLevel() -> Int? {
  let info = IOPSCopyPowerSourcesInfo().takeRetainedValue()
  let sources: Array<CFTypeRef> = IOPSCopyPowerSourcesList(info).takeRetainedValue() as Array
  if let source = sources.first {
    let description =
      IOPSGetPowerSourceDescription(info, source).takeUnretainedValue() as! [String: AnyObject]
    if let level = description[kIOPSCurrentCapacityKey] as? Int {
      return level
    }
  }
  return nil
}

Finally, complete the setMethodCallHandler method added earlier. You need to handle a single platform method, getBatteryLevel(), so test for that in the call argument. The implementation of this platform method calls the macOS code written in the previous step. If an unknown method is called, report that instead.

batteryChannel.setMethodCallHandler { (call, result) in
  switch call.method {
  case "getBatteryLevel":
    guard let level = getBatteryLevel() else {
      result(
        FlutterError(
          code: "UNAVAILABLE",
          message: "Battery level not available",
          details: nil))
     return
    }
    result(level)
  default:
    result(FlutterMethodNotImplemented)
  }
}

You should now be able to run the application on macOS. If your device doesn’t have a battery, it displays ‘Battery level not available’.

Step 7: Add a Linux platform-specific implementation

For this example you need to install the upower developer headers. This is likely available from your distribution, for example with:

sudo apt install libupower-glib-dev

Start by opening the Linux host portion of your Flutter app in the editor of your choice. The instructions below are for Visual Studio Code with the “C/C++” and “CMake” extensions installed, but can be adjusted for other IDEs.

1. Launch Visual Studio Code.

2. Open the linux directory inside your project.

3. Choose Yes in the prompt asking: Would you like to configure project "linux"?. This enables C++ autocomplete.

4. Open the file my_application.cc.

First, add the necessary includes to the top of the file, just after #include <flutter_linux/flutter_linux.h:

#include <math.h>
#include <upower.h>

Add an FlMethodChannel to the _MyApplication struct:

struct _MyApplication {
  GtkApplication parent_instance;
  char** dart_entrypoint_arguments;
  FlMethodChannel* battery_channel;
};

Make sure to clean it up in my_application_dispose:

static void my_application_dispose(GObject* object) {
  MyApplication* self = MY_APPLICATION(object);
  g_clear_pointer(&self->dart_entrypoint_arguments, g_strfreev);
  g_clear_object(&self->battery_channel);
  G_OBJECT_CLASS(my_application_parent_class)->dispose(object);
}

Edit the my_application_activate method and initialize battery_channel using the channel name samples.flutter.dev/battery, just after the call to fl_register_plugins:

static void my_application_activate(GApplication* application) {
  // ...
  fl_register_plugins(FL_PLUGIN_REGISTRY(self->view));

  g_autoptr(FlStandardMethodCodec) codec = fl_standard_method_codec_new();
  self->battery_channel = fl_method_channel_new(
      fl_engine_get_binary_messenger(fl_view_get_engine(view)),
      "samples.flutter.dev/battery", FL_METHOD_CODEC(codec));
  fl_method_channel_set_method_call_handler(
      self->battery_channel, battery_method_call_handler, self, nullptr);

  gtk_widget_grab_focus(GTK_WIDGET(self->view));
}

Next, add the C code that uses the Linux battery APIs to retrieve the battery level. This code is exactly the same as you would write in a native Linux application.

Add the following as a new function at the top of my_application.cc just after the G_DEFINE_TYPE line:

static FlMethodResponse* get_battery_level() {
  // Find the first available battery and report that.
  g_autoptr(UpClient) up_client = up_client_new();
  g_autoptr(GPtrArray) devices = up_client_get_devices2(up_client);
  if (devices->len == 0) {
    return FL_METHOD_RESPONSE(fl_method_error_response_new(
        "UNAVAILABLE", "Device does not have a battery.", nullptr));
  }

  UpDevice* device = (UpDevice*)(g_ptr_array_index(devices, 0));
  double percentage = 0;
  g_object_get(device, "percentage", &percentage, nullptr);

  g_autoptr(FlValue) result =
      fl_value_new_int(static_cast<int64_t>(round(percentage)));
  return FL_METHOD_RESPONSE(fl_method_success_response_new(result));
}

Finally, add the battery_method_call_handler function referenced in the earlier call to fl_method_channel_set_method_call_handler. You need to handle a single platform method, getBatteryLevel, so test for that in the method_call argument. The implementation of this function calls the Linux code written in the previous step. If an unknown method is called, report that instead.

Add the following code after the get_battery_level function:

static void battery_method_call_handler(FlMethodChannel* channel,
                                        FlMethodCall* method_call,
                                        gpointer user_data) {
  g_autoptr(FlMethodResponse) response = nullptr;
  if (strcmp(fl_method_call_get_name(method_call), "getBatteryLevel") == 0) {
    response = get_battery_level();
  } else {
    response = FL_METHOD_RESPONSE(fl_method_not_implemented_response_new());
  }

  g_autoptr(GError) error = nullptr;
  if (!fl_method_call_respond(method_call, response, &error)) {
    g_warning("Failed to send response: %s", error->message);
  }
}

You should now be able to run the application on Linux. If your device doesn’t have a battery, it displays ‘Battery level not available’.

Typesafe platform channels using Pigeon

The previous example uses MethodChannel to communicate between the host and client, which isn’t typesafe. Calling and receiving messages depends on the host and client declaring the same arguments and datatypes in order for messages to work. You can use the Pigeon package as an alternative to MethodChannel to generate code that sends messages in a structured, typesafe manner.

With Pigeon, the messaging protocol is defined in a subset of Dart that then generates messaging code for Android or iOS. You can find a more complete example and more information on the pigeon page on pub.dev.

Using Pigeon eliminates the need to match strings between host and client for the names and datatypes of messages. It supports: nested classes, grouping messages into APIs, generation of asynchronous wrapper code and sending messages in either direction. The generated code is readable and guarantees there are no conflicts between multiple clients of different versions. Supported languages are Objective-C, Java, Kotlin, and Swift (with Objective-C interop).

Pigeon example

Pigeon file:

import 'package:pigeon/pigeon.dart';

class SearchRequest {
  final String query;

  SearchRequest({required this.query});
}

class SearchReply {
  final String result;

  SearchReply({required this.result});
}

@HostApi()
abstract class Api {
  @async
  SearchReply search(SearchRequest request);
}

Dart usage:

import 'generated_pigeon.dart';

Future<void> onClick() async {
  SearchRequest request = SearchRequest(query: 'test');
  Api api = SomeApi();
  SearchReply reply = await api.search(request);
  print('reply: ${reply.result}');
}

Separate platform-specific code from UI code

If you expect to use your platform-specific code in multiple Flutter apps, you might consider separating the code into a platform plugin located in a directory outside your main application. See developing packages for details.

Publish platform-specific code as a package

To share your platform-specific code with other developers in the Flutter ecosystem, see publishing packages.

Custom channels and codecs

Besides the above mentioned MethodChannel, you can also use the more basic BasicMessageChannel, which supports basic, asynchronous message passing using a custom message codec. You can also use the specialized BinaryCodec, StringCodec, and JSONMessageCodec classes, or create your own codec.

You might also check out an example of a custom codec in the cloud_firestore plugin, which is able to serialize and deserialize many more types than the default types.

Channels and platform threading

When invoking channels on the platform side destined for Flutter, invoke them on the platform’s main thread. When invoking channels in Flutter destined for the platform side, either invoke them from any Isolate that is the root Isolate, or that is registered as a background Isolate. The handlers for the platform side can execute on the platform’s main thread or they can execute on a background thread if using a Task Queue. You can invoke the platform side handlers asynchronously and on any thread.

Using plugins and channels from background isolates

Plugins and channels can be used by any Isolate, but that Isolate has to be a root Isolate (the one created by Flutter) or registered as a background Isolate for a root Isolate.

The following example shows how to register a background Isolate in order to use a plugin from a background Isolate.

import 'package:flutter/services.dart';
import 'package:shared_preferences/shared_preferences.dart';

void _isolateMain(RootIsolateToken rootIsolateToken) async {
  BackgroundIsolateBinaryMessenger.ensureInitialized(rootIsolateToken);
  SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
  print(sharedPreferences.getBool('isDebug'));
}

void main() {
  RootIsolateToken rootIsolateToken = RootIsolateToken.instance!;
  Isolate.spawn(_isolateMain, rootIsolateToken);
}

Executing channel handlers on background threads

In order for a channel’s platform side handler to execute on a background thread, you must use the Task Queue API. Currently this feature is only supported on iOS and Android.

In Java:

@Override
public void onAttachedToEngine(@NonNull FlutterPluginBinding binding) {
  BinaryMessenger messenger = binding.getBinaryMessenger();
  BinaryMessenger.TaskQueue taskQueue =
      messenger.makeBackgroundTaskQueue();
  channel =
      new MethodChannel(
          messenger,
          "com.example.foo",
          StandardMethodCodec.INSTANCE,
          taskQueue);
  channel.setMethodCallHandler(this);
}

In Kotlin:

override fun onAttachedToEngine(@NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
  val taskQueue =
      flutterPluginBinding.binaryMessenger.makeBackgroundTaskQueue()
  channel = MethodChannel(flutterPluginBinding.binaryMessenger,
                          "com.example.foo",
                          StandardMethodCodec.INSTANCE,
                          taskQueue)
  channel.setMethodCallHandler(this)
}

In Swift:

public static func register(with registrar: FlutterPluginRegistrar) {
  let taskQueue = registrar.messenger.makeBackgroundTaskQueue()
  let channel = FlutterMethodChannel(name: "com.example.foo",
                                     binaryMessenger: registrar.messenger(),
                                     codec: FlutterStandardMethodCodec.sharedInstance,
                                     taskQueue: taskQueue)
  let instance = MyPlugin()
  registrar.addMethodCallDelegate(instance, channel: channel)
}

In Objective-C:

+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
  NSObject<FlutterTaskQueue>* taskQueue =
      [[registrar messenger] makeBackgroundTaskQueue];
  FlutterMethodChannel* channel =
      [FlutterMethodChannel methodChannelWithName:@"com.example.foo"
                                  binaryMessenger:[registrar messenger]
                                            codec:[FlutterStandardMethodCodec sharedInstance]
                                        taskQueue:taskQueue];
  MyPlugin* instance = [[MyPlugin alloc] init];
  [registrar addMethodCallDelegate:instance channel:channel];
}

Jumping to the UI thread in Android

To comply with channels’ UI thread requirement, you might need to jump from a background thread to Android’s UI thread to execute a channel method. In Android, you can accomplish this by post()ing a Runnable to Android’s UI thread Looper, which causes the Runnable to execute on the main thread at the next opportunity.

In Java:

new Handler(Looper.getMainLooper()).post(new Runnable() {
  @Override
  public void run() {
    // Call the desired channel message here.
  }
});

In Kotlin:

Handler(Looper.getMainLooper()).post {
  // Call the desired channel message here.
}

Jumping to the main thread in iOS

To comply with channel’s main thread requirement, you might need to jump from a background thread to iOS’s main thread to execute a channel method. You can accomplish this in iOS by executing a block on the main dispatch queue:

In Objective-C:

dispatch_async(dispatch_get_main_queue(), ^{
  // Call the desired channel message here.
});

In Swift:

DispatchQueue.main.async {
  // Call the desired channel message here.
}