面试题：Flutter嵌入层中平台通道的原理及使用

平台通道（Platform Channel）原理

跨平台通信需求：Flutter是跨平台框架，在不同原生平台（如Android、iOS）运行时，有时需访问原生系统功能，而这些功能Flutter本身无直接访问能力，平台通道就是为解决此问题而生。
架构组成
- MethodChannel：用于在Flutter与原生平台间传递方法调用和返回结果，适用于大多数通信场景，比如调用原生方法获取数据或执行原生操作。
- EventChannel：主要用于从原生平台向Flutter发送数据流，如设备传感器数据的实时更新等。
- BasicMessageChannel：用于在Flutter和原生平台间传递序列化消息，适用于传递较复杂的数据结构等。
通信机制：Flutter端和原生端通过约定好的通道名称建立连接，基于特定的二进制消息编码（如JSON）在两端进行数据的序列化与反序列化，从而实现数据的准确传输。

实现Flutter与原生平台通信方式

Flutter端：创建相应类型的通道（如MethodChannel），指定通道名称。通过通道对象调用方法（如invokeMethod）向原生端发起请求，同时注册接收原生端响应的回调。
原生端：以Android为例，在原生代码中创建相同名称的通道实例，注册方法处理程序（MethodCallHandler）来接收Flutter端的方法调用，并根据调用执行相应逻辑，将结果返回给Flutter端。iOS端类似，通过注册处理程序来处理Flutter的调用请求并返回结果。

示例：在Flutter中通过平台通道获取设备电量信息

Flutter端代码

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

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('获取设备电量'),
        ),
        body: Center(
          child: FutureBuilder<double>(
            future: _getBatteryLevel(),
            builder: (context, snapshot) {
              if (snapshot.hasData) {
                return Text('电量: ${snapshot.data}%');
              } else if (snapshot.hasError) {
                return Text('错误: ${snapshot.error}');
              }
              return CircularProgressIndicator();
            },
          ),
        ),
      ),
    );
  }
}

Future<double> _getBatteryLevel() async {
  const platform = MethodChannel('samples.flutter.dev/battery');
  try {
    final double result = await platform.invokeMethod('getBatteryLevel');
    return result;
  } on PlatformException catch (e) {
    throw '获取电量失败: ${e.message}';
  }
}

Android原生端代码（Kotlin）

package com.example.flutter_battery

import android.content.Context
import android.content.IntentFilter
import android.os.BatteryManager
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(flutterEngine: FlutterEngine) {
    super.configureFlutterEngine(flutterEngine)
    MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler { call, result ->
      if (call.method == "getBatteryLevel") {
        val batteryLevel = getBatteryLevel()
        if (batteryLevel != -1) {
          result.success(batteryLevel)
        } else {
          result.error("UNAVAILABLE", "电量信息不可用", null)
        }
      } else {
        result.notImplemented()
      }
    }
  }

  private fun getBatteryLevel(): Int {
    val batteryLevel: Int
    val batteryIntent = applicationContext.registerReceiver(null, IntentFilter(Intent.ACTION_BATTERY_CHANGED))
    batteryLevel = batteryIntent?.getIntExtra(BatteryManager.EXTRA_LEVEL, -1) ?: -1
    val batteryScale = batteryIntent?.getIntExtra(BatteryManager.EXTRA_SCALE, -1) ?: -1
    return if (batteryLevel != -1 && batteryScale != -1) {
      (batteryLevel / batteryScale.toFloat() * 100).toInt()
    } else -1
  }
}

iOS原生端代码（Swift）

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({
      (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in
      if call.method == "getBatteryLevel" {
        let batteryLevel = self.getBatteryLevel()
        if let level = batteryLevel {
          result(level)
        } else {
          result(FlutterError(code: "UNAVAILABLE", message: "电量信息不可用", details: nil))
        }
      } else {
        result(FlutterMethodNotImplemented)
      }
    })
    GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }

  func getBatteryLevel() -> NSNumber? {
    UIDevice.current.isBatteryMonitoringEnabled = true
    guard UIDevice.current.batteryState != UIDevice.BatteryState.unknown else {
      return nil
    }
    return NSNumber(value: UIDevice.current.batteryLevel * 100)
  }
}

以上代码展示了在Flutter中通过平台通道获取设备电量信息的完整流程，Flutter端发起获取电量的方法调用，Android和iOS原生端分别实现获取电量逻辑并返回结果给Flutter端显示。

星途面试题库

面试题：Flutter嵌入层中平台通道的原理及使用

知识考点

面试题答案

平台通道（Platform Channel）原理

实现Flutter与原生平台通信方式

示例：在Flutter中通过平台通道获取设备电量信息