我們在開發Web應用時，有時候需要將Server端的的信息Push到客戶端。常見的一個場景就是微博應用，需要將一個用戶的收聽實時消息推送到Web端，也就是用戶的更新用戶的Timeline。
對此通用的解決方案就是Long Polling——支持XMLHttpRequest的瀏覽器都可以使用，使得其適用范圍廣。對此需要注意的就是Server端的處理能力，最好能用類似Node.js的Non-Block式的並發。GitHub有個項目SignalR使得的在asp.net中實現Server Side的Push變得簡單。為了使用SignalR，我們需要在NuGet中搜索SignalR並安裝。
安裝完成后我們便可以使用SignalR了，我們先看Client端的實現：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Welcome to Microblog</title>
<script src="../Scripts/jquery-1.6.4.js" type="text/javascript"></script>
<script src="../Scripts/jquery.signalR.js" type="text/javascript"></script>
<script src="../signalr/hubs"></script>
</head>
<body>
<form id="form1" runat="server">
<div>
<div>
<span>MicroBlog ID:</span><input type="text" id="uid" name="uid" value=" " />
<input type="button" name="refreshTimeline" id="refreshTimeline" value="Refresh Timeline" />
</div>
<ul id="messages"></ul>
</div>
</form>
<script type="text/javascript">
$(function () {
// Proxy created on the fly
var conn = $.connection.microBlogServer;
// Declare a function so the server can invoke it
conn.newMessage = function (msg) {
$('#messages').append('<li>' + msg + '</li>');
};
conn.notify = function(notice) {
$('#messages').append('<li>[system notice]:\t' + notice + '</li>');
}
// Start the connection
$.connection.hub.start();
//call server side refresh when all things ready
$("#refreshTimeline").click(function() {
conn.refreshTimeline($('#uid').val());
});
});
</script>
</body>
</html>
服務器端也十分簡單：
public class MicroBlogServer:Hub, IDisconnect
{
private static Dictionary<string, bool> cancelQueue = new Dictionary<string, bool>();
public void RefreshTimeline(string uid)
{
cancelQueue.Add(this.Context.ConnectionId, false);
var msgTask = new Task(new Action<object>(PushUserMessage), uid);
msgTask.Start();
}
private void PushUserMessage(object uid)
{
while (true)
{
if (cancelQueue[this.Context.ConnectionId])
{
cancelQueue.Remove(this.Context.ConnectionId);
return;
}
string msg = GetNewMessage(uid.ToString());
Caller.newMessage(msg); //push new message to client
string notice = GetSystemNotification();
if (!string.IsNullOrEmpty(notice))
Clients.notify(notice);
}
}
public System.Threading.Tasks.Task Disconnect()
{
if (cancelQueue.Keys.Contains(this.Context.ConnectionId))
{
cancelQueue[this.Context.ConnectionId] = true;
}
return null;
}
#region User Message Generator
private string GetNewMessage(string uid)
{
Thread.Sleep(3000);
return string.Format("message from xxx to {0} ({1})", uid, DateTime.Now);
}
private string GetSystemNotification()
{
if (new Random().Next(6) >= 5)
return "system will shutdown for maintenance";
else
return string.Empty;
}
#endregion
}