Platform Channel：Flutter 与原生通信

为什么需要 Platform Channel？

Flutter 运行在自己的引擎上，无法直接调用平台 API（如蓝牙、相机 native SDK、企业微信 SDK）。Platform Channel 是 Flutter 与原生（Android/iOS）通信的桥梁。

三种 Channel 类型

MethodChannel —— 最常用

Flutter 侧

class BatteryPlugin {
  static const _channel = MethodChannel('com.example/battery');

  static Future<int> getBatteryLevel() async {
    try {
      final int level = await _channel.invokeMethod('getBatteryLevel');
      return level;
    } on PlatformException catch (e) {
      throw Exception('Failed to get battery level: ${e.message}');
    }
  }

  // 带参数
  static Future<String> convertText(String text, String locale) async {
    return await _channel.invokeMethod('convertText', {
      'text': text,
      'locale': locale,
    });
  }
}

Android 侧（Kotlin）

// MainActivity.kt
class MainActivity: FlutterActivity() {
  private val CHANNEL = "com.example/battery"

  override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
    super.configureFlutterEngine(flutterEngine)
    MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL)
      .setMethodCallHandler { call, result ->
        when (call.method) {
          "getBatteryLevel" -> {
            val level = getBatteryLevel()
            if (level != -1) result.success(level)
            else result.error("UNAVAILABLE", "Battery level not available", null)
          }
          "convertText" -> {
            val text = call.argument<String>("text") ?: ""
            val locale = call.argument<String>("locale") ?: "en"
            result.success(text.uppercase(Locale(locale)))
          }
          else -> result.notImplemented()
        }
      }
  }

  private fun getBatteryLevel(): Int {
    val batteryManager = getSystemService(BATTERY_SERVICE) as BatteryManager
    return batteryManager.getIntProperty(BatteryManager.BATTERY_PROPERTY_CAPACITY)
  }
}

iOS 侧（Swift）

// AppDelegate.swift
@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(_ application: UIApplication,
      didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    let controller = window?.rootViewController as! FlutterViewController
    let channel = FlutterMethodChannel(name: "com.example/battery",
                                       binaryMessenger: controller.binaryMessenger)
    channel.setMethodCallHandler { [weak self] call, result in
      switch call.method {
      case "getBatteryLevel":
        self?.receiveBatteryLevel(result: result)
      default:
        result(FlutterMethodNotImplemented)
      }
    }
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }

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

EventChannel —— 持续事件流

// Flutter 侧：订阅传感器数据流
class GyroscopePlugin {
  static const _channel = EventChannel('com.example/gyroscope');

  static Stream<Map<String, double>> get gyroscopeStream {
    return _channel.receiveBroadcastStream().map((event) {
      final map = Map<String, double>.from(event as Map);
      return map;
    });
  }
}

// 使用
StreamBuilder<Map<String, double>>(
  stream: GyroscopePlugin.gyroscopeStream,
  builder: (_, snapshot) {
    if (!snapshot.hasData) return const SizedBox();
    return Text('x: ${snapshot.data!['x']?.toStringAsFixed(2)}');
  },
)

Pigeon —— 类型安全的代码生成

手写 Channel 容易出现字符串 typo、类型不匹配。Pigeon 从 Dart 定义生成类型安全的 Kotlin/Swift 代码：

flutter pub add --dev pigeon

// pigeons/messages.dart（定义 API）
import 'package:pigeon/pigeon.dart';

@ConfigurePigeon(PigeonOptions(
  dartOut: 'lib/src/messages.g.dart',
  kotlinOut: 'android/app/src/main/kotlin/Messages.g.kt',
  swiftOut: 'ios/Runner/Messages.g.swift',
))

class BatteryInfo {
  late int level;
  late String status;  // 'charging' | 'discharging' | 'full'
}

@HostApi()
abstract class BatteryHostApi {
  BatteryInfo getBatteryInfo();
  @async
  BatteryInfo getBatteryInfoAsync();
}

@FlutterApi()
abstract class BatteryFlutterApi {
  void onBatteryLow();  // 原生主动通知 Flutter
}

生成后，Flutter 和原生都有强类型调用，彻底告别字符串 typo。

FFI —— 直接调用 C/C++ 库

对于性能敏感的 native 库（如图像处理、算法库），可以用 Dart FFI 直接绑定：

import 'dart:ffi';
import 'dart:io';

final dylib = Platform.isAndroid
    ? DynamicLibrary.open('libmylib.so')
    : DynamicLibrary.process();

typedef NativeAdd = Int32 Function(Int32 a, Int32 b);
typedef DartAdd = int Function(int a, int b);

final add = dylib.lookupFunction<NativeAdd, DartAdd>('add');
print(add(1, 2));  // 3

FFI 不经过 Dart VM 编解码，性能接近原生。

核心考点

Q：Platform Channel 的通信原理？

Channel 通过 BinaryMessenger 传递二进制消息。默认使用 StandardMessageCodec（支持 int/double/String/List/Map），在 Dart 和原生之间序列化/反序列化。通信是异步的，底层通过 PostMessage 机制实现。

Q：三种 Channel 如何选择？

• 一次性调用：MethodChannel
• 连续数据推送：EventChannel（本质是 StreamController 的单向 Stream）
• 需要自定义编解码器或频繁双向消息：BasicMessageChannel