前言

这篇，想要介绍一下 Flutter 如何把某些功能打包给原生的 Android、Ios 写。将会介绍官方的 Sample。
https://flutter.dev/docs/development/platform-integration/platform-channels?tab=android-channel-java-tab 文件
这篇将会透过取得电池电量为范例。

架构

图片撷取来自官网 => https://flutter.dev/assets/images/docs/PlatformChannels.png

可以看到 Flutter 和 IOS、Android，都是透过 MethodChannel 来交流、交换资料。MethodChannel 是一个沟通的桥梁。invokeMethod 则是可以让 android、ios 原生的程序码知道 flutter 要呼叫的方法是哪一个。

Flutter

可以看到这边 Flutter 在 state 里面宣告了一个 MethodChannel。

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

  // Get battery level.
}

可以看到，这边我们定义了一个 result 用来接收电量资讯，我们透过 platform(MethodChannel)，去取得 invokeMethod == getBatteryLevel 的 android 原生方法。

  // 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;
    });
  }

UI

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

Android

这边建议撰写时，另外开一个 Android Sutdio Project，这样 Android Studio 会比较好编辑。
这边我们可以宣告一个 Channel 会等於 Flutter 的 Channel。
接下来我们在初始化 MethodChannel（这边要和 flutter 写的一样），并在 configureFlutterEngine 里面设置一个 MethodCallHandler 的 interface。

这边，如果你有用另外开一个 Android 的 project，就可以很快速 import 了。

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
        }
      );
}
}

写一个读取店员资运的方法

  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;
  }

接着我们可以把程序码写在 (call, result) ->{} 里面，这个 call back 会有一个 call 的参数，
这个 call 的参数对应的就是 Flutter 里面的 MethodChannel.invokeMethod('call 的参数')。
result 则会有一个 success 和 error。success 的话就会回传给 flutter 正确资料。如果是 error 的话就会回传给 flutter PlatformException。

          (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();
          }
        }

IOS

Ios 则是透过 FlutterViewController 和 FlutterMethodChannel，并一样会有一个 setMethodCallHandler 的 interface，一样会有 call 和 result 的参数。

附上完整程序码

import UIKit
import Flutter

@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({
   [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)
 })
 
 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))
   }
 }

}