XE2提供了一个ChatRoomDemo的例子，REST形式的。要实现这个功能，客户端js调用服务器端的代码，实现聊天者的相互沟通。
服务端的代码
function TChatRoomServerMethods.SendMessage(const Msg: String): Boolean;
var
MesgTrimmed: String;
Session: TDSSession;
JSONMsg: TJSONObject;
begin
MesgTrimmed := Trim(Msg);
//no message to send, so just exit
if Msg = EmptyStr then
Exit(false);
//get the current session
Session := TDSSessionManager.GetThreadSession;
//if the session is invalid, don't send a message
if (Session = nil) or (not TChatRoomUsers.Instance.UserExists(Session.UserName)) then
Exit(false);
//wrap the message in a JSON object
JSONMsg := TJSONObject.Create;
JSONMsg.AddPair(TJSONPair.Create('notificationType', 'message'));
JSONMsg.AddPair(TJSONPair.Create('from', Session.UserName));
JSONMsg.AddPair(TJSONPair.Create('message', GetHTMLEscapedString(MesgTrimmed)));
//Send the message to all logged in users
Result := ServerContainerForm.ChatRoomServer.BroadcastMessage(CHAT_ROOM_ID, JSONMsg);
end;
function TChatRoomServerMethods.SendMessageToUser(const Msg, UserName: String): Boolean;
var
MesgTrimmed: String;
Session: TDSSession;
JSONMsg: TJSONObject;
Resp: TJSONValue;
begin
MesgTrimmed := Trim(Msg);
//no message to send, so just exit
if Msg = EmptyStr then
Exit(false);
//no user to send message to
if not TChatRoomUsers.Instance.UserExists(UserName) then
Exit(false);
//get the current session
Session := TDSSessionManager.GetThreadSession;
//if the session is invalid, don't send a message
if (Session = nil) or (not TChatRoomUsers.Instance.UserExists(Session.UserName)) then
Exit(false);
//don't message yourself!
if AnsiCompareText(Session.UserName, UserName) = 0 then
Exit(false);
//wrap the message in a JSON object
JSONMsg := TJSONObject.Create;
JSONMsg.AddPair(TJSONPair.Create('notificationType', 'privatemessage'));
JSONMsg.AddPair(TJSONPair.Create('from', Session.UserName));
JSONMsg.AddPair(TJSONPair.Create('message', GetHTMLEscapedString(MesgTrimmed)));
//Send the message to all logged in users
Result := ServerContainerForm.ChatRoomServer.NotifyCallback(UserName, UserName, JSONMsg, Resp);
//we don't care about the response message from the other client, only if it was successfully sent
FreeAndNil(Resp);
end;
这里用到了两个函数，DSServer.BroadcastMessage和DSServer.NotifyCallback。现在要关心的是，这两个函数如何将数据，推送到了客户端的。
js客户端是通过startChannel()函数里面，建立了一个ClientChannel和一个ClientCallback，然后ClientChannel.connect(ClientCallback)的里面，建立了一个
CallbackLoop，这个CallbackLoop就是通过XMLHTTPRequest搭上去的常链接了。
查看客户端的CallbackFramework.js代码，可以看到CallbackLoop的start函数
/*
* Starts the loop, registering the client callback on the server and the initial client callback specified
* @param firstCallback the first callback to register, as you can't register a client with the server without specifying the first callback
*/
this.start = function(firstCallback) {
if (this.stopped && (!nullOrEmptyStr(this.clientChannel) || firstCallback.serverChannelNames.length > 0))
{
this.stopped = false;
//passes empty string for the ConsumeClientChannel last parameter, since this is initiating the channel, and has no value
//passes true after the callback to say a response from the server is expected
this.executor.executeMethod("ConsumeClientChannel", "GET",
[this.clientChannel.serverChannelName, this.clientChannel.channelId,
firstCallback.callbackId, arrayToCSV(firstCallback.serverChannelNames), this.securityToken, ""],
this.callback, true);
if (isReferenceAFunction(this.clientChannel.onChannelStateChange)) {
this.clientChannel.onChannelStateChange(new ClientChannelEventItem(this.clientChannel.EVENT_CHANNEL_START,
this.clientChannel, firstCallback));
}
}
};
服务器上的代码执行Datasnap.DSPlatform单元的
function TDBXServerComponent.ConsumeClientChannel(const ChannelName, ClientManagerId,
CallbackId, ChannelNames, SecurityToken: String; ResponseData: TJSONValue): TJSONValue;
begin
Result := ConsumeClientChannelTimeout(ChannelName, ClientManagerId, CallbackId, ChannelNames,
SecurityToken, -1, ResponseData);
end;
需要留意的是，js的executeMethod函数的this.callback参数。
/*
* This function executes the given method with the specified parameters and then
* notifies the callback when a response is received.
* @param url the url to invoke
* @param contentParam the parameter to pass through the content of the request (or null)
* @param requestType must be one of: GET, POST, PUT, DELETE
* @param callback An optioanl function with three parameters, the response object, the request's status (IE: 200) and the specified 'owner'
* The object will be an array, which can contain string, numeric, JSON array or JSON object types.
* @param hasResult true if a result from the server call is expected, false to ignore any result returned.
* This is an optional parameter and defaults to 'true'
* @param accept The string value to set for the Accept header of the HTTP request, or null to set as application/json
* @return if callback in null then this function will return the result that would have
* otherwise been passed to the callback
*/
this.executeMethodURL = function(url, contentParam, requestType, callback, hasResult, accept) {
if (hasResult == null)
{
hasResult = true;
}
requestType = validateRequestType(requestType);
var request = getXmlHttpObject(); //得到XMLHTTPRequest对象
//async is only true if there is a callback that can be notified on completion
var useCallback = (callback != null);
request.open(requestType, url, useCallback);
if (useCallback)
{
request.onreadystatechange = function() { //注册回调
if (request.readyState == 4)
{
//the callback will be notified the execution finished even if there is no expected result
JSONResult = hasResult ? parseHTTPResponse(request) : null;
callback(JSONResult, request.status, owner); //执行回调，也就是executeMethod函数的this.callback了。
}
};
}
if(contentParam != null)
{
contentParam = JSON.stringify(contentParam);
}
request.setRequestHeader("Accept", (accept == null ? "application/json" : accept));
request.setRequestHeader("Content-Type", "text/plain;charset=UTF-8");
request.setRequestHeader("If-Modified-Since", "Mon, 1 Oct 1990 05:00:00 GMT");
var sessId = getSessionID();
if(sessId != null)
{
request.setRequestHeader("Pragma", "dssession=" + sessId);
}
if (this.authentication != null)
{
request.setRequestHeader("Authorization", "Basic " + this.authentication);
}
request.send(contentParam); //发送请求
//if a callback wasn't used then simply return the result.
//otherwise, return nothing because this function will finish executing before
//the server call returns, so the result text will be empty until it is passed to the callback
if (hasResult && !useCallback)
{
return parseHTTPResponse(request);
}
};
到这里，还没找到常链接，XMLHTTPRequest对象发送了请求，取得了结果，就退出了。这常链接在哪实现的？？
继续看CallbackLoop.callback的代码
/*
* The callback which will handle a value passed in from the server and then pass
* back a response to the server as long as the channel is active.
*/
this.callback = function(responseObject, requestStatus, callbackLoop) {
if (callbackLoop != null && !callbackLoop.stopped && responseObject != null)
{
//resolve the true response object
responseObject = (responseObject.result != null) ? responseObject.result : responseObject;
responseObject = isArray(responseObject) ? responseObject[0] : responseObject;
//if the session this callback was created on has sense expired then stop the callback loop,
//preventing any calls to callbackLoop.sendResponse from executing
var sessId = getSessionID();
if (sessId == null)
{
callbackLoop.stopped = true;
}
//session expired, so notify local callbacks and then stop the loop
if (responseObject.SessionExpired != null)
{
callbackLoop.stopped = true;
for(var i = 0; i < clientChannel.callbacks.length; i++)
{
clientChannel.callbacks[i].notifyCallback(responseObject);
}
//notify that the channel has been closed
if (isReferenceAFunction(clientChannel.onChannelStateChange)) {
clientChannel.onChannelStateChange(new ClientChannelEventItem(clientChannel.EVENT_SERVER_DISCONNECT,
clientChannel, null));
}
}
//broadcast to all of the callbacks listening on the given channel
else if (responseObject.broadcast != null) //广播
{
var paramArray = responseObject.broadcast;
var paramValue = paramArray[0];
//used to determine if the paramValue is (on the server) a JSONValue or a TObject
var dataType = paramArray[1];
var broadcastChannel = responseObject.channel == null ? clientChannel.serverChannelName : responseObject.channel;
var doForAll = clientChannel.serverChannelName == broadcastChannel;
for(var i = 0; i < clientChannel.callbacks.length; i++)
{
var currentCallback = clientChannel.callbacks[i];
//Broadcast to the callback if the channel being broadcast to is the one specified in the ClientChannel,
//or if it appears in the array of channels this specific callback cares about.
if (doForAll || arrayIndexOf(currentCallback.serverChannelNames, broadcastChannel) > -1) {
currentCallback.notifyCallback(paramValue, dataType);
}
}
callbackLoop.sendResponse(true, callbackLoop);
}
//Invoke the specified callback
else if (responseObject.invoke != null) //触发特别的callback
{
var paramArray = responseObject.invoke;
var callbackKey = paramArray[0];
var paramValue = paramArray[1];
//used to determine if the paramValue is (on the server) a JSONValue or a TObject
var dataType = paramArray[2];
var currCallback;
for(var i = 0; i < clientChannel.callbacks.length; i++)
{
currCallback = clientChannel.callbacks[i];
if (currCallback.callbackId == callbackKey)
{
callbackLoop.sendResponse(currCallback.notifyCallback(paramValue, dataType), callbackLoop);
break;
}
}
}
//if an error has occured notify the callbacks and stop the loop
else if (responseObject.error != null)
{
callbackLoop.stopped = true;
for(var i = 0; i < clientChannel.callbacks.length; i++)
{
clientChannel.callbacks[i].notifyCallback(responseObject, "error");
}
//notify that the channel has been closed by
if (isReferenceAFunction(clientChannel.onChannelStateChange)) {
clientChannel.onChannelStateChange(new ClientChannelEventItem(this.clientChannel.EVENT_SERVER_DISCONNECT,
this.clientChannel, null));
}
}
//If the result key is 'close' or 'closeChannel' then no response should be sent, which means
//the recursion of this loop will end. Otherwise, send a response to the server with
//a value of false so the loop will continue and the server will know the invocation failed
else if (responseObject.closeChannel == null && responseObject.close == null)
{
callbackLoop.sendResponse(false, callbackLoop); //这里了，只要channel不关，就又再次送出Request，如此往复循环，也就成了callback Loop了。sendResponse同样是call DSAmin.ConsumeClientChannel用的是POST,而不是GET了。
}
else
{
callbackLoop.stopped = true;
//notify each callback that it has been closed
for(var i = 0; i < clientChannel.callbacks.length; i++)
{
clientChannel.callbacks[i].notifyCallback(responseObject, "closed");
}
//notify that the channel has been closed
if (isReferenceAFunction(clientChannel.onChannelStateChange)) {
clientChannel.onChannelStateChange(new ClientChannelEventItem(clientChannel.EVENT_CHANNEL_STOP,
clientChannel, null));
}
}
}
else
{
if (callbackLoop != null) {
if (!callbackLoop.stopped && isReferenceAFunction(clientChannel.onChannelStateChange)) {
//notify that the channel has been closed by the server
clientChannel.onChannelStateChange(new ClientChannelEventItem(clientChannel.EVENT_SERVER_DISCONNECT,
clientChannel, null));
}
callbackLoop.stopped = true;
}
}
};
也就是说，服务器往客户端推送信息，其实是靠js客户端不断的轮询实现的，这也是没法子的事情，基于HTTP的无状态链接，也只有这么做。另外，所谓的常链接，其实不是只有一个链接，因为每次轮询，都是新起一个XMLHttpRequest发出的请求的。
在ChatRoom的例子中，上面的notifyCallback，call的是下面的代码ClientCallback的第三个回调参数
//create the messaging callback, to handle messages from the server and from other clients
var callback = new ClientCallback(channel, userName, function(jsonValue, dataType) {
if (jsonValue != null)
{
if (dataType == "closed")
{
addMessage(null, "You have been disconnected from the server. Sorry!", null, true);
}
else if (jsonValue.notificationType != null)
{
var type = jsonValue.notificationType;
//the list of users has changed, so update it on the page
if (type == "user_login" || type == "user_logout")
{
loadUsers();
}
//you received a public or private message, so add it to the message area
else if (type == "message" || type == "privatemessage")
{
var isPrivate = type == "privatemessage";
var from = jsonValue.from;
var message = jsonValue.message;
addMessage(from, message, isPrivate);
}
}
//your session has expired!
else if(jsonValue.SessionExpired != null)
{
addMessage(null, jsonValue.SessionExpired, null, true);
//NOTE: you don't need to call stopChannel here, because the session has expired and therefore
//this is the last message you will receive before the tunnel closes.
}
}
return true;
});
到这里，我们就能拿到前面的TDSServer.NotifyCallback和TDSServer.BroadcastMessage里面的const Msg: TJSONValue参数了，然后做自己的事情。
