Boosting Flutter's Flexibility: A Deep Dive into Flutter MethodChannel

As Flutter continues to gain momentum for building high-performance applications across both iOS and Android, a key element in bridging the gap between the Flutter framework and native platform features is understanding the intricacies of the MethodChannel.

This powerful feature allows Flutter apps to tap into platform-specific code, unlocking the ability to leverage native APIs written in Kotlin, Java, Swift, or Objective-C. Grasping how MethodChannel functions will give you the edge in crafting apps that are not only efficient but also fully integrated with the unique capabilities of each platform.

Why is Flutter MethodChannel Important?

For achieving advanced functionality, there are instances where Flutter might not offer a Flutter-dedicated package. This is where Flutter MethodChannel shines. You can leverage platform-specific APIs by writing native code and connecting it with your Flutter code via MethodChannel.

Throughout this blog post, we will explore different aspects of the Flutter MethodChannel, from setting it up to sending asynchronous method calls, handling errors, and much more. Prepare to navigate the intricate bidirectional communication between Dart and the host platform using the MethodChannel.

Note: We expect you to be familiar with basic concepts such as creating a new app project, utilizing Android Studio or Visual Studio Code, and have some basic know-how of native code, i.e., Kotlin Code for Android and Objective C or Swift Code for iOS.

Importance of Flutter MethodChannel

Technically, we have platform channels that allow Flutter apps to execute platform-specific code, such as using device sensors, storage, battery information, file I/O, and acquiring data from the internet. The platform channels comprise two parts:

1. The MethodChannel in Dart code, used for sending method calls.
2. The MethodCallHandler on the host (Android or iOS) side receives these method calls.

It's vital to understand that method calls are passed across the channel, and the data return happens asynchronously. Here, the MethodChannel in Flutter (our Dart side) sends upgraded method calls to the MethodCallHandler at the native side (The particular android folder inside our project acting as the host).

But why the importance?

Our user interface remains responsive because the channel uses asynchronous method calls to communicate. This means it can send messages, such as method calls, to ask the host (native code) to perform a specific computation and then receive the result for further tasks in the Flutter app. This enhances your Flutter app's efficiency as it supports efficient binary serialization, which requires less memory and results in faster execution.

The MethodChannel in Flutter specifically lets you execute methods on the native side. It is powerful as it lets you access multiple Flutter apps' functionalities to the full extent while writing minimal amounts of native code.

Remember, the MethodChannel is named, and it must share its channel name with its hosting counterpart. This is important because the MethodChannel merely sends messages to the channel name, and the receiver on the host side, the MethodCallHandler, is responsible for checking and executing those method calls.

Primary Components of Flutter MethodChannel

The Flutter MethodChannel mainly involves three central components. Understanding these constituents will clarify how MethodChannel aids high-level communication between Dart and the native side.

1. Dart Side (Flutter app): The MethodChannel instance resides here. Our Flutter code uses this instance to invoke specific method calls. Remember, since MethodChannel is named, it uses this name to identify where to send its method calls. Example:

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  /* Dart Side (Flutter Code) */
  import 'package:flutter/services.dart';

  class _MyHomePageState extends State<MyHomePage> {
    static const platform = const MethodChannel('com.example.app/channel');

    // Rest of the Code
  }

2. Host Side (Platform-specific APIs): Here, MethodChannel's hosting counterpart, the MethodCallHandler, resides. It receives the method calls that the Dart side sends after checking the correct channel name. For instance, our Android Code (Kotlin code) and iOS Code (Objective C / Swift code) will have something like the following:

• For Android,

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  /* Kotlin Side (Native Android Code in MainActivity.kt) */
  import android.content.Context
  import android.content.Intent
  import io.flutter.embedding.android.FlutterActivity
  import io.flutter.embedding.engine.FlutterEngine
  import io.flutter.plugin.common.MethodChannel

  class MainActivity: FlutterActivity() {
      override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
          MethodChannel(flutterEngine.dartExecutor.binaryMessenger, "com.example.app/channel").apply {
              setMethodCallHandler { call, result ->
                  // Handle Method Calls from Flutter
              }
          }
      }
  }

• For iOS,

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/* Swift Side (Native iOS Code in AppDelegate.swift) */
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
    override func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
    ) -> Bool {
        if let controller = window?.rootViewController as? FlutterViewController {
            let channel = FlutterMethodChannel(
                name: "com.example.app/channel",
                binaryMessenger: controller.binaryMessenger)

            channel.setMethodCallHandler { [weak self]
                (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in
                // Handle Method Calls from Flutter
            }
        }
        return super.application(application, didFinishLaunchingWithOptions: launchOptions)
    }
}

3. The Method Calls: These calls are what your Flutter app sends and what your host side awaits. They involve transporting a method name (as String) and optional arguments.

Having understood the primary components, understanding how to set up the Flutte MethodChannel will be simpler and more efficient.

Setup Flutter MethodChannel

Setting up the Flutter MethodChannel involves configuring both the native and Dart sides and ensuring they agree on the channel name. Today, we'll create a simple Flutter app that invokes method calls to the native side, and we'll handle those calls in our native code.

Let’s begin with creating a new Flutter app.

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void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'MethodChannel Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter MethodChannel Demo'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  static const platform = const MethodChannel('com.example.app/channel');

  // Rest of the code
}

Proceeding with Android setup: Navigate to the Android folder inside your Flutter project and find MainActivity.kt.

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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 = "com.example.app/channel"

  override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
    super.configureFlutterEngine(flutterEngine)
    MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).apply {
      setMethodCallHandler {
        // incoming method call handler
      }
    }
  }
}

Moving to iOS setup: Open AppDelegate.swift.

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import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  private var flutterResult: FlutterResult?

  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    GeneratedPluginRegistrant.register(with: self)
    let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
    let methodChannel = FlutterMethodChannel(name: CHANNEL, binaryMessenger: controller.binaryMessenger)

    methodChannel.setMethodCallHandler({
      (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in
      // Handle Flutter method calls
    })

    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

This forms the basis for a setup communicating seamlessly between Dart and platform-specific code via MethodChannel. In the subsequent sections, we'll be delving into the configurations of these code snippets.

Communication With MethodChannel in Flutter

Now that we have a basic setup let's proceed with how communication happens via Flutter MethodChannel. Here, you'll learn about sending method calls from the Dart side to native side and receiving method calls in reverse.

Dart to Native

To invoke a method call to the native side, we use the MethodChannel instance in the Dart code. Usually, it'll look something like this:

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try {
  final String result = await platform.invokeMethod('methodName', arguments);
  // use result
} on PlatformException catch (e) {
  // handle the exception
}

Here, methodName is the name of the method you are calling on the native side, and arguments (optional) is the information you're sending over along with it.

Native to Dart

On the host side, setMethodCallHandler is used to receive and handle method calls.

On Android, it would look like this:

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MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).apply {
  setMethodCallHandler { call, result ->
    // Handle Method Calls from Flutter
    if (call.method == "methodName") {
      // Code in response to the received method call
      result.success("Received on Android side!")
    } else {
        result.notImplemented()
    }
  }
}

On iOS:

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methodChannel.setMethodCallHandler({
  (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in

  // Handle Flutter method calls
  if (call.method == "methodName") {
    result("Received on iOS side!")
  } else {
    result(FlutterMethodNotImplemented)
  }
})

The method call handler checks if the method name matches the one we're looking for, acts and sends back the results.

With this knowledge, let's understand how data is transmitted across the MethodChannel.

Sending Data Across MethodChannel

When working with Dart and Native code together, inevitably, data transmission across them becomes crucial. Fortunately, MethodChannel facilitates sending custom data along with method calls, thereby providing flexibility to developers.

Flutter uses the Standard Message Codec in the background while sending data across MethodChannel. Standard Message Codec encodes and decodes simple JSON-like values, such as booleans, numbers, Strings, byte buffers, and Lists and Maps of these.

There are some guidelines you need to follow:

• The argument type should be one that Standard Message Codec supports.
• Use equivalent data types in Dart and native code to correctly interpret the sent data.

Here is an example of sending a Map<String, dynamic> data:

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void sendUserData() {
   Map<String, dynamic> userData = {
      'name': 'John Doe',
      'age': 30,
   };

   MethodChannel('com.example.app/channel').invokeMethod('sendUserData', userData);
}

And on the native side:

For Android (Kotlin):

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MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).apply {
    setMethodCallHandler { call, result ->
        if (call.method == "sendUserData") {
            val user = call.arguments as? Map<String, Any>
            val name = user?.get("name")
            val age = user?.get("age")
            // Use name and age
            result.success("Received on Android side!")
        } else {
            result.notImplemented()
        }
    }
}

For iOS (Swift):

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methodChannel.setMethodCallHandler({
  (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in

  if (call.method == "sendUserData") {
    let user = call.arguments as! [String: Any]
    let name = user["name"]
    let age = user["age"]
    // Use name and age
    result("Received on iOS side!")
  } else {
    result(FlutterMethodNotImplemented)
  }
})

Transmitting data across the MethodChannel enables Dart values to interact with platform-specific APIs. In our next section, let's delve into data encoding and decoding, which promotes efficient binary serialization.

Data Encoding and Decoding in Flutter MethodChannel

As mentioned previously, Flutter utilizes the Standard Message Codec for encoding and decoding data across MethodChannel.

The advantage of using Standard Message Codec is that it supports sending simple JSON-like values such as:

• null
• Booleans
• Numbers (integer and floating point)
• Strings (UTF-8 encoding)
• List of these
• Maps with string keys
• Binary data (Byte arrays)

The Standard Message Codec ensures efficient binary serialization - using the least amount of memory and computation resources and providing quick execution.

This setup helps keep our user interface responsive during sending and receiving method calls.

These binary codecs ensure the received data is translated correctly on the native side and utilized effectively in native implementations. Flutter's MethodChannel uses these codecs to encode and decode the provided data.

Remember, Flutter's MethodChannel supports sending and receiving a single message and does not support streaming large amounts of data. For streaming data, event channels or stream channels are used.

Handling Errors in Flutter MethodChannel

Error handling is a quintessential part of developing robust applications. In the context of MethodChannel, there are two types of errors you need to handle - Dart side errors and native side errors.

Dart Side Error Handling

On the Dart side, invoking a method call can lead to exceptions. Hence, while making a method call, it's advisable to wrap it inside a try-catch block to handle any exceptions that might occur:

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try {
  final String result = await platform.invokeMethod('methodName', arguments);
} on PlatformException catch (e) {
  // Handle exception
}

The PlatformException contains the error code and error message that can provide information about the error occurred.

Native Side Error Handling

On the native side, while handling a method call, we can utilize the result argument of setMethodCallHandler() to pass back the error if it occurs. It has a function called error(), which can be used to send an error response:

For Android (Kotlin):

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MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).apply {
    setMethodCallHandler { call, result ->
        if (call.method == "methodName") {
            try {
                // execute function
                result.success("Successfully executed!")
            } catch (e: Exception) {
                result.error("ERROR", "Method call failed", null)
            }
        } else {
            result.notImplemented()
        }
    }
}

For iOS (Swift):

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methodChannel.setMethodCallHandler({
  (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in

  if (call.method == "methodName") {
    do {
        // execute function
        result("Successfully executed!")
    } catch let error {
        result(FlutterError(code: "ERROR", message: error.localizedDescription, details: nil))
    }
  } else {
    result(FlutterMethodNotImplemented)
  }
})

By this, you have handled any potential errors which might occur while using Flutter MethodChannel.

Best Practices for Using Flutter MethodChannel

MethodChannel is a powerful tool in Flutter, but it also demands careful usage. Here are a few best practices to ensure efficient usage of MethodChannel.

Define Unique Channel Names

Ensure your channel names are unique. The channel name is essential in routing messages to the correct MethodCallHandler. Maintain the format 'domain_name/path', which makes it unique and identifiable.

Use Correct Data Types

When passing arguments with a method call, ensure that their data type aligns with the ones the StandardMessageCodec supports. Also, make sure to use identical data types on both the Dart side and the native side.

Error Handling

Always implement procedures to handle errors on both Dart and native side. This can save a lot of debugging time and prevent unnecessary app crashes.

Code Organization

Place all your method calls in a single location in your app. This aids in clear communication between Dart and native code and makes maintenance easier.

Remember, an efficiently written app displays impeccable response time, interacting with the user's device smoothly while approaching Platform specific APIs.

Limitations and Solutions of Flutter MethodChannel

While MethodChannel is highly beneficial, it's vital to know its limitations and ways to overcome them.

1. Limited Data Types: The Standard Message Codec can only encode and decode certain data types. MethodChannel thus supports a limited set of data. To overcome this, you can serialize your custom data to a format that MethodChannel understands, like JSON, and then deserialize it on the other side.
2. Error Handling: If your native code throws an error, Flutter can't capture it. You need to handle the errors and exceptions on native side.
3. Single Message: MethodChannel supports sending and receiving a single message. It doesn’t support streaming data. In such cases, you can use EventChannel or StreamChannel.
4. Parallel Calls: If you send two method calls in parallel, one must wait for the other to finish as Dart is single threaded.
5. Platform-Specific Code: MethodChannel requires platform-specific code for iOS and Android. This can increase the amount of code you need to write and maintain.

As far as possible, using Flutter plugins that already handle Platform Channel can be more efficient and reduce your workload.

Flutter MethodChannel for Different Platforms

One key thing about Flutter MethodChannel is that it enables communication with platform-specific code. However, the actual implementational details can significantly vary across Android and iOS platforms due to the differences in their APIs and programming languages.

Android:

In Android, MethodChannel can be set inside the MainActivity.kt file that typically resides inside the 'android/app/src/main/kotlin' directory of your Flutter project. You can use the Java programming language, but the Kotlin-language is preferable due to its null-safety features.

You use MethodChannel with the same key you used on the Dart side and define a MethodCallHandler. In the handler, you write a when-clause for every Flutter method call, execute the platform-specific code, and send back a result.

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override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
  super.configureFlutterEngine(flutterEngine)
  MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).apply {
    setMethodCallHandler {
        call, result ->
            // Handle Flutter method calls
    }
  }
}

iOS:

The iOS implementation in the AppDelegate.swift file is pretty similar. Here, you define a MethodChannel with the same key and provide a MethodCall handler. Swift is preferable due to its popularity and extensive support on iOS devices.

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override func application(
  _ application: UIApplication,
  didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

  let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
  let platformChannel = FlutterMethodChannel(name: CHANNEL,
                                           binaryMessenger: controller.binaryMessenger)

  platformChannel.setMethodCallHandler({
    (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in
    // Handle Flutter method calls
  })

  return super.application(application, didFinishLaunchingWithOptions: launchOptions)
}

In both cases, remember to use the same MethodChannel name that you specified on the Dart side.

Comparison Between Flutter MethodChannel and Other Flutter Channels

Flutter provides three different types of channels to communicate between Dart code and native platforms: the MethodChannel, which we've extensively covered, along with the BasicMessageChannel and EventChannel.

All these channels serve different purposes and are used based on the requirements of the application:

• MethodChannel: This is the most frequently used channel that provides a simple, high-level API to send method calls to the native side and receive the result back in Dart. It's suitable when working with platform APIs that respond to a request with a single result.
• BasicMessageChannel: This channel is used for sending high-frequency messages. This channel is perfect for transferring serialized simple data structures as a standalone message with an average data rate.
• EventChannel: An EventChannel is suitable for one-way communication from the native side to Dart, which allows Dart to receive a stream of events, such as geolocation updates or sensor data.

Compared to BasicMessageChannel and EventChannel, the MethodChannel is more versatile because it supports sending different data types and can respond with a result.

Advancements in Flutter MethodChannel

Looking at the evolution and the current scope of the Flutter ecosystem, the MethodChannel is one aspect that continues to receive upgrades and improvements.

One major advancement has been in terms of support for null safety. With Flutter 2.0 bringing in sound null safety, the MethodChannel has been updated to fully support this, making your Flutter applications robust and less prone to null reference exceptions.

Efforts are being made to simplify working with MethodChannel, especially on the native side. For instance, some libraries allow you to annotate a native class's methods, automatically generating the required boilerplate code for MethodChannel.

Although not directly impacting MethodChannel, the overall improvements in Dart, like efficient memory management and better concurrency support, will benefit how you work with MethodChannel.

Moreover, the Flutter team has been actively working to provide many packages that utilize MethodChannel to provide easy access to platform-specific APIs, reducing the need for writing native code.

Use Flutter MethodChannel: Bring Native Capabilities to Flutter App

In this guide, we’ve dived deep into one of Flutter's essential tools: the MethodChannel. By now, you should have a clear understanding of how MethodChannel bridges your Flutter app with platform-specific features, allowing for powerful cross-platform functionality. From setting up the MethodChannel on both Dart and native sides, to handling data transmission, error management, and more, this approach opens a new level of adaptability in your app's design.

Embracing MethodChannel effectively means you’re not just building apps but crafting experiences that seamlessly utilize native device capabilities. With each iteration of Flutter, tools like MethodChannel evolve to provide even more flexibility, paving the way for creating robust, user-centered applications. Keep exploring, testing, and fine-tuning your MethodChannel skills to unlock the full potential of Flutter in your development journey.

References

To dive deeper into MethodChannel and Flutter's cross-platform capabilities, you might find the following resources helpful:

1. Flutter's method channel documentation
2. Official Flutter Platform Channels Guide

I hope you found this exploration of Flutter's MethodChannel helpful. Happy Fluttering!