撰写双端平台代码（插件编写实现）

本指南介绍了如何编写自定义的平台相关代码，某些平台相关功能可通过已有的软件包获得，具体细节可查看： 在 Flutter 里使用 Packages。

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

Flutter 使用了灵活的系统，它允许你调用相关平台的 API，无论是 Android 中的 Java 或 Kotlin 代码，还是 iOS 中的 Objective-C 或 Swift 代码。

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 的平台相关 API 支持不依赖于代码生成，而是依赖于灵活的消息传递：

Flutter’s platform-specific API support does not rely on code generation, but rather on a flexible message passing style:

• 应用程序中的 Flutter 部分通过平台通道向其宿主（应用程序中的 iOS 或 Android 部分）发送消息。

  The Flutter portion of the app sends messages to its host, the iOS or Android portion of the app, over a platform channel.

• 宿主监听平台通道并接收消息。然后，它使用原生编程语言来调用任意数量的相关平台 API，并将响应发送回客户端（即应用程序中的 Flutter 部分）。

  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.

架构概述：平台通道

Architectural overview: platform channels

消息使用平台通道在客户端（UI）和宿主（平台）之间传递，如下图所示：

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

消息和响应以异步的形式进行传递，以确保用户界面能够保持响应。

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

客户端 MethodChannel（[API][MethodChannel]）允许发送与方法调用相对应的消息。平台方面，在 Android 的 MethodChannel（[API][MethodChannelAndroid]）及 iOS 的 FlutterMethodChannel（[API][MethodChanneliOS]）上接收方法调用并返回结果。这些类允许你使用非常少的 样板 代码来开发平台插件。

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.

注意：如果需要，方法调用也可以反向发送，由平台充当客户端来调用 Dart 实现的方法。一个具体的例子是 quick_actions 插件。

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.

平台通道数据类型及编解码器

Platform channel data types support and codecs

标准平台通道使用标准消息编解码器，它支持简单的类似 JSON 值的高效二进制序列化，例如布尔值、数字、字符串、字节缓冲区及这些类型的列表和映射（详情请参阅 StandardMessageCodec）。当你发送和接收值时，它会自动对这些值进行序列化和反序列化。

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.

下表展示了如何在平台端接收 Dart 值，反之亦然：

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

示例: 通过平台通道调用平台的 iOS 和 Android 代码

Example: Calling platform-specific iOS and Android code using platform channels

以下代码演示了如何调用平台相关 API 来检索并显示当前的电池电量。它通过平台消息 getBatteryLevel() 来调用 Android 的 BatteryManager API 及 iOS 的 device.batteryLevel API。

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().

该示例在主应用程序中添加平台相关代码。如果想要将该代码重用于多个应用程序，那么项目的创建步骤将略有差异（查看 Flutter Packages 的开发和提交），但平台通道代码仍以相同方式编写。

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.

注意：可在 /examples/platform_channel/ 中获得使用 Java 实现的 Android 及使用 Objective-C 实现的 iOS 的该示例完整可运行的代码。对于用 Swift 实现的 iOS 代码，请参阅 /examples/platform_channel_swift/。

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/.

第一步：创建一个新的应用项目

Step 1: Create a new app project

首先创建一个新的应用：

Start by creating a new app:

• 在终端中运行：flutter create batterylevel

  In a terminal run: flutter create batterylevel

默认情况下，我们的模板使用 Kotlin 编写 Android 或使用 Swift 编写 iOS 代码。要使用 Java 或 Objective-C，请使用 -i 和/或 -a 标志：

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:

• 在终端中运行：flutter create -i swift -a kotlin batterylevel

  In a terminal run: flutter create -i swift -a kotlin batterylevel

第二步：创建 Flutter 平台客户端

Step 2: Create the Flutter platform client

应用程序的 State 类保持当前应用的状态。扩展它以保持当前的电池状态。

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

首先，构建通道。在返回电池电量的单一平台方法中使用 MethodChannel。

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

通道的客户端和宿主端通过传递给通道构造函数的通道名称进行连接。一个应用中所使用的所有通道名称必须是唯一的；使用唯一的 域前缀 为通道名称添加前缀，比如：samples.flutter.dev/battery。

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.
}

接下来，在方法通道上调用方法（指定通过 String 标识符 getBatteryLevel 调用的具体方法）。调用可能会失败—比如，如果平台不支持此平台 API（比如在模拟器中运行），所以将 invokeMethod 调用包裹在 try-catch 语句中。

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.

在 setState 中使用返回结果来更新 _batteryLevel 内的用户界面状态。

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;
    });
  }

最后，将模板中的 build 方法替换为包含以字符串形式显示电池状态、并包含一个用于刷新该值的按钮的小型用户界面。

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

步骤 3a: 使用 Java 添加 Android 平台的实现

Step 3a: Add an Android platform-specific implementation using Java

注意：以下步骤使用 Java。如果你更喜欢 Kotlin，请跳至步骤 3b。

Note: The following steps use Java. If you prefer Kotlin, skip to step 3b.

• Java
• Kotlin

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

          (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();
            }
          }

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

    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()
      }
    }

现在你应该可以在 Android 中运行该应用。如果使用了 Android 模拟器，请在扩展控件面板中设置电池电量，可从工具栏中的 … 按钮访问。

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.

步骤 4a：添加 iOS 平台的实现

Step 4a: Add an iOS platform-specific implementation

• Objective-C
• Swift

#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];
}

- (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) {
  // 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);
  }
}];

@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)
  }
}

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))
  }
}

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)
})

现在你应该可以在 iOS 中运行该应用。如果使用了 iOS 模拟器，注意它并不支持 battery API，并且应用会显示 ‘battery info unavailable’。

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’.

从 UI 代码中分离平台相关代码

Separate platform-specific code from UI code

如果你想要在多个 Flutter 应用中使用你的平台相关代码，则将代码分离为位于主应用目录之外的平台插件会很有用。相关细节查看 Flutter Packages 的开发和提交。

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.

将平台相关代码作为 Package 进行提交

Publish platform-specific code as a package

与 Flutter 生态中的其他开发者共享你的平台相关代码，可查看 提交 package。

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

自定义通道和编解码器

Custom channels and codecs

除了上面提到的 MethodChannel，你还可以使用更基础的 [BasicMessageChannel][BasicMessageChannel]，它支持使用自定义的消息编解码器进行基本的异步消息传递。你还可以使用专门的 [BinaryCodec][BinaryCodec]，[StringCodec][StringCodec] 和 [JSONMessageCodec][JSONMessageCodec] 类，或创建自己的编解码器。

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.
}