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A library that implements the JSON-RPC 2.0 spec.


A JSON-RPC 2.0 server exposes a set of methods that can be called by clients. These methods can be registered using Server.registerMethod:

import "package:json_rpc_2/json_rpc_2.dart" as json_rpc;
import "package:stream_channel/stream_channel.dart";
import "package:web_socket_channel/io.dart";

main() async {
  var socket = IOWebSocketChannel.connect('ws://localhost:4321');
  var server = new json_rpc.Server(socket);

  // Any string may be used as a method name. JSON-RPC 2.0 methods are
  // case-sensitive.
  var i = 0;
  server.registerMethod("count", () {
    // Just return the value to be sent as a response to the client. This can
    // be anything JSON-serializable, or a Future that completes to something
    // JSON-serializable.
    return i++;

  // Methods can take parameters. They're presented as a [Parameters] object
  // which makes it easy to validate that the expected parameters exist.
  server.registerMethod("echo", (params) {
    // If the request doesn't have a "message" parameter, this will
    // automatically send a response notifying the client that the request
    // was invalid.
    return params.getNamed("message");

  // [Parameters] has methods for verifying argument types.
  server.registerMethod("subtract", (params) {
    // If "minuend" or "subtrahend" aren't numbers, this will reject the
    // request.
    return params.getNum("minuend") - params.getNum("subtrahend");

  // [Parameters] also supports optional arguments.
  server.registerMethod("sort", (params) {
    var list = params.getList("list");
    if (params.getBool("descending", orElse: () => false)) {
      return params.list.reversed;
    } else {
      return params.list;

  // A method can send an error response by throwing a
  // `json_rpc.RpcException`. Any positive number may be used as an
  // application- defined error code.
  const DIVIDE_BY_ZERO = 1;
  server.registerMethod("divide", (params) {
    var divisor = params.getNum("divisor");
    if (divisor == 0) {
      throw new json_rpc.RpcException(
          DIVIDE_BY_ZERO, "Cannot divide by zero.");

    return params.getNum("dividend") / divisor;

  // To give you time to register all your methods, the server won't actually
  // start listening for requests until you call `listen`.


A JSON-RPC 2.0 client calls methods on a server and handles the server's responses to those method calls. These methods can be called using Client.sendRequest:

import "package:json_rpc_2/json_rpc_2.dart" as json_rpc;
import "package:stream_channel/stream_channel.dart";
import "package:web_socket_channel/html.dart";

main() async {
  var socket = HtmlWebSocketChannel.connect('ws://localhost:4321');
  var client = new json_rpc.Client(socket);

  // This calls the "count" method on the server. A Future is returned that
  // will complete to the value contained in the server's response.
  client.sendRequest("count").then((result) => print("Count is $result."));

  // Parameters are passed as a simple Map or, for positional parameters, an
  // Iterable. Make sure they're JSON-serializable!
  client.sendRequest("echo", {"message": "hello"})
      .then((echo) => print('Echo says "$echo"!'));

  // A notification is a way to call a method that tells the server that no
  // result is expected. Its return type is `void`; even if it causes an
  // error, you won't hear back.

  // If the server sends an error response, the returned Future will complete
  // with an RpcException. You can catch this error and inspect its error
  // code, message, and any data that the server sent along with it.
  client.sendRequest("divide", {"dividend": 2, "divisor": 0})
      .catchError((error) {
    print("RPC error ${error.code}: ${error.message}");

  // The client won't subscribe to the input stream until you call `listen`.


Although JSON-RPC 2.0 only explicitly describes clients and servers, it also mentions that two-way communication can be supported by making each endpoint both a client and a server. This package supports this directly using the Peer class, which implements both Client and Server. It supports the same methods as those classes, and automatically makes sure that every message from the other endpoint is routed and handled correctly.