目录结构如下：

grpc_golang_flutter (根目录)
       |-- golang_server（go语言服务目录）
       |---- server.go
       |---- client.go

vscode配置文件settings.json内容如下：

{
    "go.gopath": "H:/grpc_golang_flutter/modules",
    "terminal.integrated.env.windows": {
        "GOPATH": "H:/grpc_golang_flutter/modules",
        "PATH": "${env:PATH};H:/grpc_golang_flutter/modules/bin"
    },
}

以上环境变量也可使用命令行手工设置。

go语言服务端

在grpc_golang_flutter/golang_server目录下初始化go module：

go mod init grpc_golang_flutter/golang_server

安装 gRPC
使用如下命令进行gRPC安装：

go get -u google.golang.org/protobuf/cmd/protoc-gen-go
go get -u google.golang.org/grpc/cmd/protoc-gen-go-grpc

安装 Protocol Buffers v3
安装该协议编译器用来生成 gRPC 服务代码。最简便的方法是在https://github.com/google/protobuf/releases下载一个二进制压缩包。解压后加入环境变量，如下：

export PATH=$PATH:执行路径 (linux下)
set PATH=%PATH%;执行路径   (windows下)

执行以下命令安装该协议插件，执行后会生成protoc-gen-go.exe用来生成go语言proto接口文件：

go get github.com/golang/protobuf/protoc-gen-go

在golang_server目录下创建目录“helloworld”并新建文件“helloworld.proto”，填写如下内容：

syntax = "proto3";

package helloworld;
// 服务端定义
service Greeter {
// 服务端返馈信息方法
rpc SayHello (HelloRequest) returns (HelloReply) {}
}
// 包含用户名的请求信息
message HelloRequest {
string name = 1;
}
// 服务端响应信息
message HelloReply {
string message = 1;
}

然后使用protoc命令，在grpc_golang_flutter根目录执行以下命令生成helloworld.pb.go

protoc --proto_path=golang_server/helloworld --go_out=plugins=grpc:golang_server/helloworld golang_server/helloworld/helloworld.proto

服务端server.go代码如下：

package main

import (
    "context"
    "log"
    "fmt"
    "net"
    "google.golang.org/grpc"
    pb "grpc_golang_flutter/golang_server/helloworld"
)

const (
    port = ":50051"
)

// server is used to implement helloworld.GreeterServer.
type server struct{}
// SayHello implements helloworld.GreeterServer
func (s *server) SayHello(ctx context.Context, in *pb.HelloRequest) (*pb.HelloReply, error) {
    log.Printf("Received: %v", in.Name)
    return &pb.HelloReply{Message: "Hello " + in.Name}, nil
}

func main() {
    lis, err := net.Listen("tcp", port)
    if err != nil {
        log.Fatalf("failed to listen: %v", err)
    }
    s := grpc.NewServer()
    pb.RegisterGreeterServer(s, &server{})
    fmt.Println("启动服务",port)
    if err := s.Serve(lis); err != nil {
        log.Fatalf("failed to serve: %v", err)
    }
}

客户端client.go代码如下：

package main

import (
    "log"
    "os"
    "golang.org/x/net/context"
    "google.golang.org/grpc"
    pb "grpc_golang_flutter/golang_server/helloworld"
)

const (
    address     = "localhost:50051"
    defaultName = "world"
)

func main() {
    // Set up a connection to the server.
    conn, err := grpc.Dial(address, grpc.WithInsecure())
    if err != nil {
        log.Fatalf("did not connect: %v", err)
    }
    defer conn.Close()
    c := pb.NewGreeterClient(conn)
    // Contact the server and print out its response.
    name := defaultName
    if len(os.Args) > 1 {
        name = os.Args[1]
    }
    r, err := c.SayHello(context.Background(), &pb.HelloRequest{Name: name})
    if err != nil {
        log.Fatalf("could not greet: %v", err)
    }
    log.Printf("Greeting: %s", r.Message)
}

flutter客户端

在grpc_golang_flutter目录下执行以下命令：

flutter create flutter_client

安装Dart的protoc插件：

pub global activate protoc_plugin

在pubspec.yaml文件中引入protobuf和grpc库(注意位置)，如下图：

image.png

grpc_golang_flutter/lib下创建helloworld文件夹，
grpc_golang_flutter目录下执行以下命令生成dart的proto接口文件：

protoc --proto_path=golang_server/helloworld --dart_out=grpc:flutter_client/lib/helloworld golang_server/helloworld/helloworld.proto

main.dart文件使用以下代码覆盖(初步控制台实现打印输出，待完善)：

import 'package:flutter/material.dart';
import 'package:flutter_client/helloworld/helloworld.pbgrpc.dart';
import 'package:grpc/grpc.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

getRemoteData() async {
  final name = "hello";
  final channel = ClientChannel(
    '192.168.3.4',
    port: 50051,
    options: const ChannelOptions(credentials: ChannelCredentials.insecure()),
  );
  final stub = GreeterClient(channel);
  try {
    var response = await stub.sayHello(HelloRequest()..name = name);
    print('服务端返回信息: ${response.message}');
  } catch (e) {
    print('Caught error: $e');
  }
  await channel.shutdown();
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      floatingActionButton: FloatingActionButton(
        onPressed: getRemoteData,
        tooltip: '获取grpc数据',
        child: Icon(Icons.add),
      ),
    );
  }
}

运行结果如下：

image.png

在系统发布时，android手机需要在android/app/src/main目录下的AndroidManifest.xml文件中<manifest标签下加入以下内容：

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />