Revel示例 - 聊天室

聊天室

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聊天室应用程序示例如下：

• 使用channel来实现一个聊天室（pub-sub模式），俗称的发布-订阅模式
• 使用Comet和Websockets

应用程序的文件结构如下：

chat/app/
    chatroom           # Chat room routines
        chatroom.go

    controllers
        app.go         # The welcome screen, allowing user to pick a technology
        refresh.go     # Handlers for the "Active Refresh" chat demo
        longpolling.go # Handlers for the "Long polling" ("Comet") chat demo
        websocket.go   # Handlers for the "Websocket" chat demo

    views
        ...            # HTML and Javascript

Browse the code on Github

首先我们来看一下这个聊天室是怎么实现的，chatroom.go.

聊天室作为一个独立的go-routine运行， 如下所示：

func init() {
    go chatroom()
}

chatroom() 函数简单的在3个channel中选择并执行响应的action

var (
    // Send a channel here to get room events back.  It will send the entire
    // archive initially, and then new messages as they come in.
    subscribe = make(chan (chan<- Subscription), 10)
    // Send a channel here to unsubscribe.
    unsubscribe = make(chan (<-chan Event), 10)
    // Send events here to publish them.
    publish = make(chan Event, 10)
)

func chatroom() {
    archive := list.New()
    subscribers := list.New()

    for {
        select {
        case ch := <-subscribe:
            // Add subscriber to list and send back subscriber channel + chat log.
        case event := <-publish:
            // Send event to all subscribers and add to chat log.
        case unsub := <-unsubscribe:
            // Remove subscriber from subscriber list.
        }
    }
}

我们来分别看一下每一个都是怎么实现的。

Subscribe

case ch := <-subscribe:
        var events []Event
        for e := archive.Front(); e != nil; e = e.Next() {
            events = append(events, e.Value.(Event))
        }
        subscriber := make(chan Event, 10)
        subscribers.PushBack(subscriber)
        ch <- Subscription{events, subscriber}

一个订阅有两个属性：

• 聊天日志
• 一个订阅者能在上面监听并获得新信息的channel

Publish

    case event := <-publish:
        for ch := subscribers.Front(); ch != nil; ch = ch.Next() {
            ch.Value.(chan Event) <- event
        }
        if archive.Len() >= archiveSize {
            archive.Remove(archive.Front())
        }
        archive.PushBack(event)

发布的event一个一个发送给订阅者的channel，然后event被添加到archive，archive里面的数量大于10，前面的会被移出。

Unsubscribe

case unsub := <-unsubscribe:
        for ch := subscribers.Front(); ch != nil; ch = ch.Next() {
            if ch.Value.(chan Event) == unsub {
                subscribers.Remove(ch)
            }
        }

订阅者channel在list中被移除。

Handlers

现在你知道了聊天室是怎么运行的，我们可以看一看handler是怎么使用不同的技术的。

主动刷新

主动刷新聊天室通过javascript每隔5秒刷新页面来从服务器获取新信息：

// Scroll the messages panel to the end
  var scrollDown = function() {
    $('#thread').scrollTo('max')
  }

  // Reload the whole messages panel
  var refresh = function() {
    $('#thread').load('/refresh/room?user= #thread .message', function() {
      scrollDown()
    })
  }

  // Call refresh every 5 seconds
  setInterval(refresh, 5000)

Refresh/Room.html

以下是请求的action：

func (c Refresh) Room(user string) rev.Result {
    subscription := chatroom.Subscribe()
    defer subscription.Cancel()
    events := subscription.Archive
    for i, _ := range events {
        if events[i].User == user {
            events[i].User = "you"
        }
    }
    return c.Render(user, events)
}

refresh.go

它订阅chatroom并传递archive到template来做页面渲染。这里没有什么值得看的。

长轮询（Comet）

 长轮询javascript聊天室使用一个ajax请求server并保持这个连接一直打开知道有一个新消息到来。javascript提供了一个lastReceived时间戳来告诉server，客户端知道的最新消息是哪个。

var lastReceived = 0
  var waitMessages = '/longpolling/room/messages?lastReceived='
  var say = '/longpolling/room/messages?user='

  $('#send').click(function(e) {
    var message = $('#message').val()
    $('#message').val('')
    $.post(say, {message: message})
  });

  // Retrieve new messages
  var getMessages = function() {
    $.ajax({
      url: waitMessages + lastReceived,
      success: function(events) {
        $(events).each(function() {
          display(this)
          lastReceived = this.Timestamp
        })
        getMessages()
      },
      dataType: 'json'
    });
  }
  getMessages();

LongPolling/Room.html

对应的handler

func (c LongPolling) WaitMessages(lastReceived int) rev.Result {
    subscription := chatroom.Subscribe()
    defer subscription.Cancel()

    // See if anything is new in the archive.
    var events []chatroom.Event
    for _, event := range subscription.Archive {
        if event.Timestamp > lastReceived {
            events = append(events, event)
        }
    }

    // If we found one, grand.
    if len(events) > 0 {
        return c.RenderJson(events)
    }

    // Else, wait for something new.
    event := <-subscription.New
    return c.RenderJson([]chatroom.Event{event})
}

longpolling.go

在这种实现里面，它能简单的阻塞在订阅channel上（假设它已经发回了所有信息到archive）。

Websocket

Websocket聊天室，当用户加载了聊天室页面后，javascript打开了一个websocket连接。

 // Create a socket
  var socket = new WebSocket('ws://127.0.0.1:9000/websocket/room/socket?user=')

  // Message received on the socket
  socket.onmessage = function(event) {
    display(JSON.parse(event.data))
  }

  $('#send').click(function(e) {
    var message = $('#message').val()
    $('#message').val('')
    socket.send(message)
  });

WebSocket/Room.html

第一件事是订阅新的events并加入房间和发出archive，如下所示：

func (c WebSocket) RoomSocket(user string, ws *websocket.Conn) rev.Result {
    // Join the room.
    subscription := chatroom.Subscribe()
    defer subscription.Cancel()

    chatroom.Join(user)
    defer chatroom.Leave(user)

    // Send down the archive.
    for _, event := range subscription.Archive {
        if websocket.JSON.Send(ws, &event) != nil {
            // They disconnected
            return nil
        }
    }

websocket.go

下面我们必须从订阅监听新的event， 无论如何websocket库只提供一个阻塞call来获得一个新frame，为了在它们之间选择，我们必须包装它们。

    // In order to select between websocket messages and subscription events, we
    // need to stuff websocket events into a channel.
    newMessages := make(chan string)
    go func() {
        var msg string
        for {
            err := websocket.Message.Receive(ws, &msg)
            if err != nil {
                close(newMessages)
                return
            }
            newMessages <- msg
        }
    }()

websocket.go

现在我们能在newMessages channel上选择新的websocket消息。

最后一点就是这样做的 - 它从websocket等待一个新消息（如果用户说了什么的话）或从订阅并传播消息到其他用户。

// Now listen for new events from either the websocket or the chatroom.
    for {
        select {
        case event := <-subscription.New:
            if websocket.JSON.Send(ws, &event) != nil {
                // They disconnected.
                return nil
            }
        case msg, ok := <-newMessages:
            // If the channel is closed, they disconnected.
            if !ok {
                return nil
            }

            // Otherwise, say something.
            chatroom.Say(user, msg)
        }
    }
    return nil
}

websocket.go

如果我们发现websocket channel已经关闭，然后我们返回nil。

 

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