一、什么是webSocket

webSocket是HTML5开始提供的一种浏览器与服务器进行全双工通讯的网络技术，属于应用层协议。它基于TCP传输协议，并复用HTTP的握手通道。对大部分web开发者来说，上面这段描述有点枯燥，其实只要记住几点：

1. WebSocket可以在浏览器里使用
2. 支持双向通信
3. 使用很简单

有哪些优点

说到优点，这里的对比参照物是HTTP协议，概括地说就是：支持双向通信，更灵活，更高效，可扩展性更好。

1. 支持双向通信，实时性更强。
2. 更好的二进制支持。
3. 较少的控制开销。连接创建后，ws客户端、服务端进行数据交换时，协议控制的数据包头部较小。在不包含头部的情况下，服务端到客户端的包头只有2~10字节（取决于数据包长度），客户端到服务端的的话，需要加上额外的4字节的掩码。而HTTP协议每次通信都需要携带完整的头部。
4. 支持扩展。ws协议定义了扩展，用户可以扩展协议，或者实现自定义的子协议。（比如支持自定义压缩算法等）

二、iOS中如何使用webSocket

     webSocket 实现了服务端推机制（主动向客户端发送消息）。新的 web 浏览器全都支持 WebSocket，这使得它的使用超级简单。通过 WebSocket 能够打开持久连接，大部分网络都能轻松处理 WebSocket 连接。在 iOS 中使用 WebSocket 比较麻烦，你必须进行大量的设置，而且内置的 API 根本帮不上忙。这时 Starscream 出现了——这个小巧、易于使用的库让你所有的烦恼不翼而飞。 

1、根据url创建socket

            var request = URLRequest(url: URL(string: "url")!)request.timeoutInterval = 5//超时时间socket = WebSocket(request: request)socket.delegate = self//接收到消息走代理方法

2、发送消息

socket.write(string: sendStr)

3、接收消息，在代理方法中

func websocketDidReceiveMessage(socket: WebSocketClient, text: String) {//接收到字符串消息}func websocketDidReceiveData(socket: WebSocketClient, data: Data) {printLog(“\(data)”)//接收到data消息}

三、常见问题

1、如何确保client向特定的client发送消息

发送消息时带着要发送给哪些client（唯一标识性数组）发送给cloud，cloud根据要发送给的client数组向相应的client发送消息

 func sendTextTo(deviceIDs: [String], text: String) {if socket == nil  {return}if socket.isConnected == false {return}let cmdMessage = DQMessage(Type: 1, MsgGID: UUID().uuidString, Receivers: deviceIDs, Content: text, Time: nil, Publisher: nil, axOrderIDs: nil)if let sendStr = DQMessage.toJsonString(messages: [cmdMessage]) {socket.write(string: sendStr)}}

2.如何保持链接

十分钟发送一次心跳包，app进入前台时，app断网重连时，app失去web连接时，重新连接

NotificationCenter.default.addObserver(self, selector: #selector(appDidBecomeActive), name: UIApplication.didBecomeActiveNotification, object: nil)do {try reachability.startNotifier()} catch {print("Unable to start notifier")}reachability.whenReachable = { [weak self] reachability inself?.reconnectTimes = 10if self?.socket == nil {return}self?.socket.connect()}if #available(iOS 10.0, *) {timer = Timer.scheduledTimer(withTimeInterval: 30, repeats: true) { timer inlet now = Date().timeIntervalSince1970let s = now - self.lastReceivedMessageTimeif s >= 600 && s <= 660  {self.sendHeartBeat()} else if s > 660 {self.reconnect()}}} else {timer = Timer.scheduledTimer(timeInterval: 30, target: self, selector: #selector(handleTimer), userInfo: nil, repeats: true)}@objc func handleTimer(timer: Timer) {let now = Date().timeIntervalSince1970let s = now - self.lastReceivedMessageTimeif s >= 600 && s <= 660  {self.sendHeartBeat()} else if s > 660 {self.reconnect()}}@objc func appDidBecomeActive(_ application: UIApplication) {if self.socket == nil {return}if self.socket.isConnected == false && self.reachability.connection != .none {self.socket.connect()}}

3.如何保证消息送达

      client到cloud：client中维护一个message数据表（包括字段是否发送成功sent）cloud收到消息之后向client返回ack，client收到ack后将该条message标记为sent=1已发送，60秒client未收到ack，视为发送失败，重新发送，  cloud端message表中已经存在该条消息，则忽略，但是向客户端client发送ack

       cloud到clinet：client收到消息后向cloud返回ack，cloud收到ack标记消息为已发送成功， 60秒cloud未收到ack，视为发送失败，从新发送，client端message表中已经存在该条消息，则忽略，但是向客户端client发送ack

func websocketDidReceiveMessage(socket: WebSocketClient, text: String) {printLog("Web Socket receive \(text)")lastReceivedMessageTime = Date().timeIntervalSince1970if text == "$" {printLog("Received Heart Beat!!!!")return}guard let messageArray = DQMessage.from(jsonData: text.data(using: .utf8)!) else { return }for message in messageArray {if message.Type == 99 { //ACKDBPool.write { db intry? db.execute("Update MessageRecord Set sent = 1 Where messageGID = '\(message.MsgGID)'")}continue}//收到消息后回复ACK,这样服务器会标记这条消息发送成功sendACK(message: message)//从收到的消息列表中对比msgid, 如果已经收到过,则忽略这条消息, 去重处理var shouldReturn = falseDBQueue.inDatabase { db indo {if let count = try Int.fetchOne(db, "Select count(*) from MessageRecord where messageGID = '\(message.MsgGID)'"), count > 0 {//数据库里有这条消息,说明已经收到过,忽略掉shouldReturn = true}} catch {Log.shareInstance.log(message: "读取数据库错误")printLog("读取数据库错误")self.createTable()}}if shouldReturn == true {return}//发完ACK将message存到数据库let aMessage = essageRecord()aMessage.messageGID = message.MsgGIDaMessage.type = message.TypeaMessage.time = message.TimeaMessage.publisher = message.Publisherif message.Type == 1 { //textguard let content = message.Content else { continue }aMessage.message = message.Contentif content.hasPrefix("cmd::") {let ar = content.components(separatedBy: "::")var para: String? = nilif ar.count == 3 {para = ar[2]}let cmdString = "\(ar[0])::\(ar[1])".lowercased()let command = AldeloCommand(rawValue: cmdString) ?? AldeloCommand.unknownif command == .clinePrint {if let ar = para?.components(separatedBy: ","), ar.count == 2 {if let orderID = Int64(ar[0]), orderID > 0 {gotPrintCommandBlock?([orderID],ar[1].boolValue(),true, message)delegate?.receivedPrintCommand(axOrderIDs: [orderID], packingPrint: ar[1].boolValue(), isClientWebSocket: true)}}} else {gotCommandBlock?(command,para, message)delegate?.receivedCommand(cmd: command,parameter: para,  message: message)}} else {gotMessageBlock?(message)delegate?.receivedMessage(message: message)}} else if message.Type == 2 { //printguard let orderIDs = message.axOrderIDs else { return }aMessage.message = "\(orderIDs)"gotPrintCommandBlock?(orderIDs,true,false,message)delegate?.receivedPrintCommand(axOrderIDs: orderIDs, packingPrint: true, isClientWebSocket: false)} else if message.Type == 3 { //QR paymentguard let content = message.Content else { continue }aMessage.message = contentgotQRPaymentBlock?(content)delegate?.receivedQRPayment(content: content)} else if message.Type == 4 { //cloud 强制反激活printLog("cloud 强制反激活 .....")}DBPool.write { db intry? aMessage.insert(db)}}}

参考 Starscream 在 GitHub 上的项目主页 。