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 whether available in Kotlin or Java code on Android, or in Swift or Objective-C code on iOS.

Flutter’s builtin platform-specific API support does not rely on code generation, but rather on a flexible message passing style. Alternatively, the package Pigeon can be used for sending structured typesafe messages via code generation:

  • The Flutter portion of the app sends messages to its host, the iOS or Android 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:

Platform channels architecture

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.

Note: If desired, method calls can also be sent in the reverse direction, with the platform acting as client to methods implemented in Dart. A concrete example of this is the quick_actions plugin.

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:

Dart Java Kotlin Obj-C Swift
null null null nil (NSNull when nested) nil
bool java.lang.Boolean Boolean NSNumber numberWithBool: NSNumber(value: Bool)
int java.lang.Integer Int NSNumber numberWithInt: NSNumber(value: Int32)
int, if 32 bits not enough java.lang.Long Long NSNumber numberWithLong: NSNumber(value: Int)
double java.lang.Double Double NSNumber numberWithDouble: NSNumber(value: Double)
String java.lang.String String NSString String
Uint8List byte[] ByteArray FlutterStandardTypedData typedDataWithBytes: FlutterStandardTypedData(bytes: Data)
Int32List int[] IntArray FlutterStandardTypedData typedDataWithInt32: FlutterStandardTypedData(int32: Data)
Int64List long[] LongArray FlutterStandardTypedData typedDataWithInt64: FlutterStandardTypedData(int64: Data)
Float64List double[] DoubleArray FlutterStandardTypedData typedDataWithFloat64: FlutterStandardTypedData(float64: Data)
List java.util.ArrayList List NSArray Array
Map java.util.HashMap HashMap NSDictionary Dictionary

Example: Calling platform-specific iOS and Android 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, and the iOS device.batteryLevel API, via 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.

Note: The full, runnable source-code for this example is available in /examples/platform_channel/ for Android with Java and iOS with Objective-C. For iOS with Swift, see /examples/platform_channel_swift/.

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 = const 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 does not 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: [
            RaisedButton(
              child: Text('Get Battery Level'),
              onPressed: _getBatteryLevel,
            ),
            Text(_batteryLevel),
          ],
        ),
      ),
    );
  }

Step 3: Add an Android platform-specific implementation

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

  1. Start Android Studio

  2. Select the menu item File > Open…

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

  4. Open the file MainActivity.kt located in the kotlin folder in the Project view. (Note: If editing with Android Studio 2.3, note that the kotlin folder is shown as if named java.)

Inside the configureFlutterEngine() method, create a MethodChannel and call setMethodCallHandler(). Make sure to use the same channel name as was used on the Flutter client side.

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 ->
      // Note: this method is invoked on the main thread.
      // TODO
    }
  }
}

Add the Android Kotlin code that uses the Android battery APIs to retrieve the battery level. This code is exactly the same as you would write in a native Android app.

First, add the needed imports at the top of the file:

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

Next, add the following method in the MainActivity class, below the configureFlutterEngine() method:

  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
  }

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 Android code written in the previous step, and returns a response for both the success and error cases using the result argument. If an unknown method is called, report that instead.

Remove the following code:

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

And replace with the following:

    MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler {
      // Note: 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()
      }
    }

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

  1. Start Android Studio

  2. Select the menu item File > Open…

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

  4. Open the MainActivity.java file located in the java folder in the Project view.

Next, create a MethodChannel and set a MethodCallHandler inside the configureFlutterEngine() method. Make sure to use the same channel name as was used on the Flutter client side.

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) -> {
            // Note: this method is invoked on the main thread.
            // TODO
          }
        );
  }
}

Add the Android Java code that uses the Android battery APIs to retrieve the battery level. This code is exactly the same as you would write in a native Android app.

First, add the needed imports at the top of the file:

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;

Then add the following as a new method in the activity class, below the configureFlutterEngine() method:

  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;
  }

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 Android code written in the previous step, and returns a response for both the success and error cases using the result argument. If an unknown method is called, report that instead.

Remove the following code:

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

And replace with the following:

          (call, result) -> {
            // Note: 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

Start by opening the iOS 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 ios folder inside it. Click OK.

Add support for Swift in the standard template setup that uses Objective-C:

  1. Expand Runner > Runner in the Project navigator.

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

Override the application:didFinishLaunchingWithOptions: function and create a FlutterMethodChannel tied to the channel name samples.flutter.dev/battery:

@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
      // Note: this method is invoked on the UI thread.
      // Handle battery messages.
    })

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

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

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

private func receiveBatteryLevel(result: FlutterResult) {
  let device = UIDevice.current
  device.isBatteryMonitoringEnabled = true
  if device.batteryState == UIDevice.BatteryState.unknown {
    result(FlutterError(code: "UNAVAILABLE",
                        message: "Battery info unavailable",
                        details: nil))
  } else {
    result(Int(device.batteryLevel * 100))
  }
}

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 iOS code written in the previous step. If an unknown method is called, report that instead.

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

Start by opening the iOS host portion of the 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 ios folder inside it. Click OK.

  4. Make sure the Xcode projects builds without errors.

  5. Open the file AppDelegate.m, located under Runner > Runner in the Project navigator.

Create a FlutterMethodChannel and add a handler inside the application didFinishLaunchingWithOptions: method. Make sure to use the same channel name as was used on the Flutter client side.

#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) {
    // Note: this method is invoked on the UI thread.
    // TODO
  }];

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

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

Add the following method in the AppDelegate class, just before @end:

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

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 iOS code written in the previous step, and returns a response for both the success and error cases using the result argument. If an unknown method is called, report that instead.

__weak typeof(self) weakSelf = self;
[batteryChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
  // Note: 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 info unavailable"
                                 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 does not support battery APIs, and the app displays ‘battery info unavailable’.

Typesafe platform channels via 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. The Pigeon package can be used 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 which then generates messaging code for Android or iOS. A more complete example and more information can be found on the Pigeon pub.dev page;

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 guarentees there will be no conflicts between multiple clients of different versions. Supported languages are Objective-C, Java, Kotlin and Swift (via Objective-C interop).

Pigeon example

Pigeon file:

import 'package:pigeon/pigeon.dart';

class SearchRequest {
  String query;
}

class SearchReply {
  String result;
}

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

Dart usage:

import 'generated_pigeon.dart'

void onClick() async {
  SearchRequest request = SearchRequest()..query = 'test';
  Api api = Api();
  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, it can be useful to separate 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

Invoke all channel methods on the platform’s main thread when writing code on the platform side. On Android, this thread is sometimes called the “main thread”, but it is technically defined as the UI thread. Annotate methods that need to be run on the UI thread with @UiThread. On iOS, this thread is officially referred to as the main thread.

Jumping to the UI thread in Android

To comply with channels’ UI thread requirement, you may need to jump from a background thread to Android’s UI thread to execute a channel method. In Android this is accomplished 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 may need to jump from a background thread to iOS’s main thread to execute a channel method. In iOS this is accomplished 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.
}