插件与Native通信

1. Flutter向native发送通信(分析实例： SharedPreferences)

Flutter通过Dart与Native发送数据通信请求是通过MethodChannel 调用invokeMethod 来实现

const MethodChannel _kChannel =
    const MethodChannel('plugins.flutter.io/shared_preferences');

使用Dart语言调用,获取数据

获取preferences数据：
final Map<Object, Object> fromSystem =
          await _kChannel.invokeMethod('getAll');
          

通过Dart向Native发送数据，并获得返回结果

  Future<bool> _setValue(String valueType, String key, Object value) {
    final Map<String, dynamic> params = <String, dynamic>{
      'key': '$_prefix$key',
    };
    if (value == null) {
      _preferenceCache.remove(key);
      return _kChannel
          .invokeMethod('remove', params)
          .then<bool>((dynamic result) => result);
    } else {
      _preferenceCache[key] = value;
      params['value'] = value;
      return _kChannel
          .invokeMethod('set$valueType', params)
          .then<bool>((dynamic result) => result);
    }
  }

数据通信在Flutter中定义是Plugin，因此iOS需要遵循Plugin协议FlutterPlugin Android实现MethodCallHandler
，通信过程中是根据Dart中设置的ChannelName来区分对应的Plugin
对于跨平台来说流程是统一的，我们通过iOS来说通信流程，然后在此基础上稍微梳理一下Android的流程

通信基本流程

1. 注册Plugin
2. Channel与messager进行匹配
3. 通信

iOS：

声明出一个Plugin

#import <Flutter/Flutter.h>

@interface FLTSharedPreferencesPlugin : NSObject<FlutterPlugin>
@end

1. 注册
   对于每一个遵循FlutterPlugin的类都会在系统创建plugin时实现注册方法+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar> *)registrar
2. 匹配
   在系统的注册方法中需要将通信的Channel与regster所携带来的messager进行匹配绑定

  FlutterMethodChannel *channel =
      [FlutterMethodChannel methodChannelWithName:CHANNEL_NAME binaryMessenger:registrar.messenger];
  
      

3. 通信
   通信是相互的，flutter向native发送数据通信之后，native需要通过回调向flutter做出反馈

Channel通过handler来处理平台之间的数据沟通
  [channel setMethodCallHandler:^(FlutterMethodCall *call, FlutterResult result) {}
  
拿 getAll 作说明：
 NSString *method = [call method];
    NSDictionary *arguments = [call arguments];

    if ([method isEqualToString:@"getAll"]) {
      result(getAllPrefs());
      }
      1. 对于通信方法的区分是采用字符串匹配的方式来达到平台的兼容
      2. arguments中携带的是flutter传递给native的数据
      3. 采用回调的当时进行数据结果的反馈

下面是iOS具体的通信过程

 [channel setMethodCallHandler:^(FlutterMethodCall *call, FlutterResult result) {
    NSString *method = [call method];
    NSDictionary *arguments = [call arguments];

    if ([method isEqualToString:@"getAll"]) {
      result(getAllPrefs());
    } else if ([method isEqualToString:@"setBool"]) {
      NSString *key = arguments[@"key"];
      NSNumber *value = arguments[@"value"];
      [[NSUserDefaults standardUserDefaults] setBool:value.boolValue forKey:key];
      result(@YES);
    } else if ([method isEqualToString:@"setInt"]) {
      NSString *key = arguments[@"key"];
      NSNumber *value = arguments[@"value"];
      // int type in Dart can come to native side in a variety of forms
      // It is best to store it as is and send it back when needed.
      // Platform channel will handle the conversion.
      [[NSUserDefaults standardUserDefaults] setValue:value forKey:key];
      result(@YES);
    } else if ([method isEqualToString:@"setDouble"]) {
      NSString *key = arguments[@"key"];
      NSNumber *value = arguments[@"value"];
      [[NSUserDefaults standardUserDefaults] setDouble:value.doubleValue forKey:key];
      result(@YES);
    } else if ([method isEqualToString:@"setString"]) {
      NSString *key = arguments[@"key"];
      NSString *value = arguments[@"value"];
      [[NSUserDefaults standardUserDefaults] setValue:value forKey:key];
      result(@YES);
    } else if ([method isEqualToString:@"setStringList"]) {
      NSString *key = arguments[@"key"];
      NSArray *value = arguments[@"value"];
      [[NSUserDefaults standardUserDefaults] setValue:value forKey:key];
      result(@YES);
    } else if ([method isEqualToString:@"commit"]) {
      // synchronize is deprecated.
      // "this method is unnecessary and shouldn't be used."
      result(@YES);
    } else if ([method isEqualToString:@"remove"]) {
      [[NSUserDefaults standardUserDefaults] removeObjectForKey:arguments[@"key"]];
      result(@YES);
    } else if ([method isEqualToString:@"clear"]) {
      NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
      for (NSString *key in getAllPrefs()) {
        [defaults removeObjectForKey:key];
      }
      result(@YES);
    } else {
      result(FlutterMethodNotImplemented);
    }
  }];

Android待后续梳理