在处理Flutter应用中集成MPesa移动支付插件时，通常需要使用MPesa提供的SDK或者API接口。由于MPesa的官方SDK可能并不直接支持Flutter，你可能需要通过原生平台通道（Platform Channels）来与MPesa的Android和iOS SDK进行交互。

以下是一个简化的代码示例，展示了如何在Flutter中通过平台通道调用MPesa支付功能。注意，这只是一个概念性的示例，实际实现可能需要根据MPesa的具体SDK和API文档进行调整。

1. 设置Flutter项目

首先，确保你的Flutter项目已经创建并设置好。

2. 创建MPesa原生插件

iOS部分

在ios/Runner目录下创建一个新的Objective-C/Swift类来封装MPesa的SDK调用。

MPesaPaymentHandler.swift

import Foundation
import UIKit
// 假设MPesa有一个名为MPesaSDK的框架
// import MPesaSDK

@objc(MPesaPaymentHandler)
class MPesaPaymentHandler: NSObject, FlutterPlatformView {
    
    private let methodChannel: FlutterMethodChannel
    
    init(withFrame frame: CGRect, viewIdentifier viewId: Int64, arguments args: Any?, binaryMessenger messenger: FlutterBinaryMessenger) {
        methodChannel = FlutterMethodChannel(name: "mpesa_payment", binaryMessenger: messenger)
        super.init()
        
        methodChannel.setMethodCallHandler({ [weak self] (call: FlutterMethodCall, result: @escaping FlutterResult) in
            guard let strongSelf = self else {
                result(FlutterError(code: "channel_destroyed", message: "The MPesa payment channel was destroyed", details: nil))
                return
            }
            
            if call.method == "initiatePayment" {
                guard let args = call.arguments as? [String: Any],
                      let phoneNumber = args["phoneNumber"] as? String,
                      let amount = args["amount"] as? Double,
                      let callbackUrl = args["callbackUrl"] as? String else {
                    result(FlutterError(code: "invalid_arguments", message: "Invalid arguments passed to initiatePayment", details: nil))
                    return
                }
                
                // 调用MPesa SDK的支付方法
                // MPesaSDK.initiatePayment(phoneNumber: phoneNumber, amount: amount, callbackUrl: callbackUrl) { success, error in
                //     if success {
                //         result(success)
                //     } else {
                //         result(FlutterError(code: "payment_failed", message: error?.localizedDescription ?? "Payment failed", details: nil))
                //     }
                // }
                
                // 由于MPesa SDK的具体实现未知，这里用占位代码表示
                let success = true // 假设支付成功
                if success {
                    result(success)
                } else {
                    result(FlutterError(code: "payment_failed", message: "Payment failed", details: nil))
                }
                
            } else {
                result(FlutterMethodNotImplemented)
            }
        })
    }
    
    // FlutterPlatformView methods
    func view() -> UIView? {
        return nil // 因为我们不需要显示任何UI组件
    }
}

注意：上面的代码中MPesaSDK的调用部分被注释掉了，因为具体的MPesa SDK调用方式未知。你需要根据MPesa的官方文档来替换这部分代码。

Android部分

在android/app/src/main/java/com/yourapp/package/目录下创建一个新的Kotlin/Java类来封装MPesa的SDK调用。

MPesaPaymentHandler.kt

package com.yourapp.package

import android.content.Context
import io.flutter.embedding.engine.plugins.FlutterPlugin
import io.flutter.embedding.engine.plugins.activity.ActivityAware
import io.flutter.embedding.engine.plugins.activity.ActivityPluginBinding
import io.flutter.plugin.common.MethodCall
import io.flutter.plugin.common.MethodChannel
import io.flutter.plugin.common.MethodChannel.MethodCallHandler
import io.flutter.plugin.common.MethodChannel.Result

// 假设MPesa有一个名为MPesaSDK的库
// import com.mpesa.sdk.MPesaSDK

class MPesaPaymentHandler(private val context: Context) : FlutterPlugin, MethodCallHandler, ActivityAware {

    private var channel: MethodChannel? = null
    private var activity: android.app.Activity? = null

    override fun onAttachedToEngine(binding: FlutterPluginBinding) {
        channel = MethodChannel(binding.binaryMessenger, "mpesa_payment")
        channel?.setMethodCallHandler(this)
    }

    override fun onMethodCall(call: MethodCall, result: Result) {
        if (call.method == "initiatePayment") {
            val arguments = call.arguments as? Map<String, Any>
            val phoneNumber = arguments?.get("phoneNumber") as? String
            val amount = arguments?.get("amount") as? Double
            val callbackUrl = arguments?.get("callbackUrl") as? String

            arguments?.let {
                // 调用MPesa SDK的支付方法
                // MPesaSDK.initiatePayment(context, phoneNumber, amount, callbackUrl) { success, error ->
                //     if (success) {
                //         result.success(null)
                //     } else {
                //         result.error("payment_failed", error?.message ?: "Payment failed", null)
                //     }
                // }

                // 由于MPesa SDK的具体实现未知，这里用占位代码表示
                val success = true // 假设支付成功
                if (success) {
                    result.success(null)
                } else {
                    result.error("payment_failed", "Payment failed", null)
                }
            } ?: result.error("invalid_arguments", "Invalid arguments passed to initiatePayment", null)
        } else {
            result.notImplemented()
        }
    }

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

    override fun onAttachedToActivity(binding: ActivityPluginBinding) {
        activity = binding.activity
    }

    override fun onDetachedFromActivityForConfigChanges() {
        activity = null
    }

    override fun onReattachedToActivityForConfigChanges(binding: ActivityPluginBinding) {
        activity = binding.activity
    }

    override fun onDetachedFromActivity() {
        activity = null
    }
}

同样，上面的代码中MPesaSDK的调用部分被注释掉了，你需要根据MPesa的官方文档来替换这部分代码。

3. 在Flutter中调用MPesa支付功能

在你的Flutter项目中，你可以通过MethodChannel来调用上面定义的MPesa支付功能。

main.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

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

class MyApp extends StatelessWidget {
  static const platform = MethodChannel('mpesa_payment');

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('MPesa Payment Example'),
        ),
        body: Center(
          child: ElevatedButton(
            onPressed: _initiatePayment,
            child: Text('Initiate Payment'),
          ),
        ),
      ),
    );
  }

  Future<void> _initiatePayment() async {
    try {
      final Map<String, dynamic> result = await platform.invokeMethod('initiatePayment', <String, dynamic>{
        'phoneNumber': '254712345678',
        'amount': 100.0,
        'callbackUrl': 'https://yourcallbackurl.com',
      });
      
      // 处理支付结果
      print('Payment result: $result');
    } on PlatformException catch (e) {
      print("Failed to initiate payment: '${e.message}'.");
    }
  }
}

总结

上面的代码示例展示了如何在Flutter项目中通过平台通道调用MPesa支付功能。由于MPesa的具体SDK和API调用方式未知，代码中的MPesa SDK调用部分被注释掉了，你需要根据MPesa的官方文档来替换这些部分。同时，确保你已经按照MPesa的要求配置了必要的权限和API密钥。