平台通道

Flutter APP同iOS&Android native之间的数据通信如下图所示：

消息和响应是异步传递，以确保用户界面保持响应。
在客户端，MethodChannel（API）允许发送对应于方法调用的消息。在平台端，Android上的MethodChannel（API）和iOS上的FlutterMethodChannel（API）支持接收方法调用并返回结果。

平台信道数据类型支持和编解码器

Dart	Android	iOS
null	null	nil (NSNull when nested)
bool	java.lang.Boolean	NSNumber numberWithBool:
int	java.lang.Integer	NSNumber numberWithInt:
int, if 32 bits not enough	java.lang.Long	NSNumber numberWithLong:
double	java.lang.Double	NSNumber numberWithDouble:
String	java.lang.String	NSString
Uint8List	byte[]	FlutterStandardTypedData typedDataWithBytes:
Int32List	int[]	FlutterStandardTypedData typedDataWithInt32:
Int64List	long[]	FlutterStandardTypedData typedDataWithInt64:
Float64List	double[]	FlutterStandardTypedData typedDataWithFloat64:
List	java.util.ArrayList	NSArray
Map	java.util.HashMap	NSDictionary

示例：使用平台通道调用平台特定的iOS和Android代码

1.创建Flutter 平台

首先，我们构建通道。我们使用平台的MethodChannel方法返回电池电量。

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
...
class _MyHomePageState extends State<MyHomePage> {
  static const platform = const MethodChannel('samples.flutter.io/battery');

  // Get battery level.
}

接下来，使用invokeMethod方法通过字符串的形式调用getBatteryLevel，使用invokeMethod方法时注意try-catch.

// 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;
    });
  }

最后，用下面的代码替换build方法

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

2.Android平台使用java实现

首先，在onCreate方法中创建MethodChannel并且设置MethodCallHandler回调，确保channel名称与Flutter侧的一致.

import io.flutter.app.FlutterActivity;
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;

public class MainActivity extends FlutterActivity {
    private static final String CHANNEL = "samples.flutter.io/battery";

    @Override
    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        GeneratedPluginRegistrant.registerWith(this);

        new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(
                new MethodCallHandler() {
                    @Override
                    public void onMethodCall(MethodCall call, Result result) {
                        // TODO
                    }
                });
    }
}

接下来，我们使用Android的电量API获取电量创建Android端java代码。
在onCreate方法下面创建一个新方法，如下：

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;
}

最后完成onMethodCall的回调

@Override
public void onMethodCall(MethodCall call, Result result) {
    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();
    }
}

3.iOS平台使用Object-C实现

首先，在application didFinishLaunchingWithOptions:方法中创建FlutterMethodChannel并且设置回调，确保channel名称与Flutter侧的一致.

#import <Flutter/Flutter.h>
#import "GeneratedPluginRegistrant.h"

@implementation AppDelegate
- (BOOL)application:(UIApplication*)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions {
  FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;

  FlutterMethodChannel* batteryChannel = [FlutterMethodChannel
                                          methodChannelWithName:@"samples.flutter.io/battery"
                                          binaryMessenger:controller];

  [batteryChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
    // TODO
  }];

  [GeneratedPluginRegistrant registerWithRegistry:self];
  return [super application:application didFinishLaunchingWithOptions:launchOptions];
}

接下来实现获取电量方法

- (int)getBatteryLevel {
  UIDevice* device = UIDevice.currentDevice;
  device.batteryMonitoringEnabled = YES;
  if (device.batteryState == UIDeviceBatteryStateUnknown) {
    return -1;
  } else {
    return (int)(device.batteryLevel * 100);
  }
}

__weak typeof(self) weakSelf = self
[batteryChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
  if ([@"getBatteryLevel" isEqualToString:call.method]) {
    int batteryLevel = [weakSelf getBatteryLevel];

    if (batteryLevel == -1) {
      result([FlutterError errorWithCode:@"UNAVAILABLE"
                                 message:@"Battery info unavailable"
                                 details:nil]);
    } else {
      result(@(batteryLevel));
    }
  } else {
    result(FlutterMethodNotImplemented);
  }
}];