Kotlin/Native 用KMM写Flutter插件

Kotlin/Native 用KMM写Flutter插件

一、用KMM写Flutter插件

Google官方有一个写Flutter例子How to write a Flutter plugin，这里把Google plugin_codelab 例子改成用KMM写Flutter插件。

二、如何运行

Android: run shared/plugin_codelab/example/android

iOS:

1、build shared.framework

use ./gradlew releaseIOSFramework
or use new version Android Studio sync

2、run shared/plugin_codelab/example/ios

Tips: before run，shared/build/cocoapods/framework/shared.framework should be generated. The shared.h header file shared/build/cocoapods/framework/shared.framework/Headers/shared.h is generated.

三、设计思路

Android/iOS插件PluginCodelabPlugin只需要实现KMM Module的接口，不写任何逻辑，把逻辑通过接口放在KMM Module中。

1、定义接口中间层用于转发数据

如参考Flutter插件的MethodCall、MethodChannel，定义CommonMethodCall数据类、CommonMethodChannel.Result接口。

data class CommonMethodCall(
    val method: String,
    val arguments: Any?,
)

class CommonMethodChannel {
    interface Result {
        fun success(result: Any?)

        fun error(errorCode: String?, errorMessage: String?, errorDetails: Any?)

        fun notImplemented()
    }
}

2、在KMM中的commonMain实现CommonCodelabPlugin插件的公共逻辑

CommonCodelabPlugin需要初始化并启动synth?.start()，处理getPlatformVersion、onKeyDown、onKeyUp逻辑。

class CommonCodelabPlugin {

    private val synth = Synth()

    init {
        synth?.start()
    }

    fun onMethodCall(call: CommonMethodCall, result: CommonMethodChannel.Result) {
        when (call.method) {
            "getPlatformVersion" -> {
                result.success(Platform().platform)
            }
            "onKeyDown" -> {
                try {
                    val arguments = call.arguments as List<*>
                    val numKeysDown = synth?.keyDown((arguments[0] as Int))
                    result.success(numKeysDown)
                } catch (ex: Exception) {
                    result.error("1", ex.message, ex.cause)
                }
            }
            "onKeyUp" -> {
                try {
                    val arguments = call.arguments as List<*>
                    val numKeysDown = synth?.keyUp((arguments[0] as Int))
                    result.success(numKeysDown)
                } catch (ex: Exception) {
                    result.error("1", ex.message, ex.cause)
                }
            }
            else -> {
                result.notImplemented()
            }
        }
    }
}

还有包括插件名称也属于公共逻辑

// 插件Channel名称
const val PLUGIN_CODE_LAB_CHANNEL = "plugin_codelab"

3、实现平台差异特性

这里只列出expect接口，具体实现平台差异特性类请查看源码

expect class Synth() {
    fun start()

    fun keyDown(key: Int): Int

    fun keyUp(key: Int): Int
}

expect class Platform() {
    val platform: String
}

4、Android Flutter实现插件KMM接口

Android Flutter实现插件KMM接口，注意这里只实现接口用于中转Flutter与Android/iOS 数据，不能有任何业务逻辑

class PluginCodelabPlugin : FlutterPlugin, MethodCallHandler {
    private var channel: MethodChannel? = null
    private var commonCodelabPlugin: CommonCodelabPlugin? = null

    override fun onAttachedToEngine(flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
        setup(this, flutterPluginBinding.binaryMessenger)
    }

    override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
        commonCodelabPlugin?.onMethodCall(
            call = CommonMethodCall(call.method, call.arguments),
            result = object : CommonMethodChannel.Result {
                override fun success(successResult: Any?) {
                    result.success(successResult)
                }

                override fun error(errorCode: String?, errorMessage: String?, errorDetails: Any?) {
                    result.error(errorCode, errorMessage, errorDetails)
                }

                override fun notImplemented() {
                    result.notImplemented()
                }
            })
    }

    override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {
        channel?.setMethodCallHandler(null)
    }

    companion object {
        private fun setup(plugin: PluginCodelabPlugin, binaryMessenger: BinaryMessenger) {
            plugin.channel = MethodChannel(binaryMessenger, PLUGIN_CODE_LAB_CHANNEL)
            plugin.channel?.setMethodCallHandler(plugin)
            plugin.commonCodelabPlugin = CommonCodelabPlugin()
        }
    }
}

5、iOS Flutter实现插件KMM接口

Android Flutter实现插件KMM接口，注意这里只实现接口用于中转Flutter与Android/iOS 数据，不能有任何业务逻辑

#import "PluginCodelabPlugin.h"

@implementation PluginCodelabPlugin{
  int _numKeysDown;
  FlutterResult _flutterResult;
  SharedCommonCodelabPlugin* _codelabPlugin;
}

- (instancetype)init {
  self = [super init];
  if (self) {
    // create music
    _codelabPlugin = [[SharedCommonCodelabPlugin alloc] init];
  }
  return self;
}

- (void)dealloc {
    // destroy music
}

+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
  FlutterMethodChannel* channel = [FlutterMethodChannel
      methodChannelWithName: SharedPluginCodeLabKt.PLUGIN_CODE_LAB_CHANNEL
            binaryMessenger:[registrar messenger]];
  PluginCodelabPlugin* instance = [[PluginCodelabPlugin alloc] init];
  [registrar addMethodCallDelegate:instance channel:channel];
}

- (void)handleMethodCall:(FlutterMethodCall *)call
                  result:(FlutterResult)result {
    SharedCommonMethodCall *methodCall = [[SharedCommonMethodCall alloc] initWithMethod:call.method arguments:call.arguments];
    _flutterResult = result;
    [_codelabPlugin onMethodCallCall:methodCall result:self ];
}

- (void)errorErrorCode:(NSString * _Nullable)errorCode errorMessage:(NSString * _Nullable)errorMessage errorDetails:(id _Nullable)errorDetails {
    NSError *error = [NSError errorWithDomain:NSCocoaErrorDomain code:errorCode.intValue userInfo:@{@"errorMessage":errorMessage, @"errorDetails":errorDetails}];
    if (_flutterResult) {
        _flutterResult(error);
    }
}

- (void)notImplemented {
    if (_flutterResult) {
        _flutterResult(FlutterMethodNotImplemented);
    }
}

- (void)successResult:(id _Nullable)result {
    if (_flutterResult) {
        _flutterResult(result);
    }
}

@end

到这里，已经完成了使用KMM开发一个Flutter插件。使用KMM开发插件的好处是公共逻辑都使用kotlin写，一般公共逻辑比较简单适合使用kotlin写，便于维护。而且，实现了KMM写插件，Flutter写UI。

四、参考链接

Github项目地址：kmm-flutter-plugin