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  • Generic callbacks

    - by bobobobo
    Extends So, I'm trying to learn template metaprogramming better and I figure this is a good exercise for it. I'm trying to write code that can callback a function with any number of arguments I like passed to it. // First function to call int add( int x, int y ) ; // Second function to call double square( double x ) ; // Third func to call void go() ; The callback creation code should look like: // Write a callback object that // will be executed after 42ms for "add" Callback<int, int, int> c1 ; c1.func = add ; c1.args.push_back( 2 ); // these are the 2 args c1.args.push_back( 5 ); // to pass to the "add" function // when it is called Callback<double, double> c2 ; c2.func = square ; c2.args.push_back( 52.2 ) ; What I'm thinking is, using template metaprogramming I want to be able to declare callbacks like, write a struct like this (please keep in mind this is VERY PSEUDOcode) <TEMPLATING ACTION <<ANY NUMBER OF TYPES GO HERE>> > struct Callback { double execTime ; // when to execute TYPE1 (*func)( TYPE2 a, TYPE3 b ) ; void* argList ; // a stored list of arguments // to plug in when it is time to call __func__ } ; So for when called with Callback<int, int, int> c1 ; You would automatically get constructed for you by < HARDCORE TEMPLATING ACTION > a struct like struct Callback { double execTime ; // when to execute int (*func)( int a, int b ) ; void* argList ; // this would still be void*, // but I somehow need to remember // the types of the args.. } ; Any pointers in the right direction to get started on writing this?

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  • Javascript Closures, Callbacks, This and That

    - by nazbot
    I am having some trouble getting a callback function to work. Here is my code: SomeObject.prototype.refreshData = function() { var read_obj = new SomeAjaxCall("read_some_data", { }, this.readSuccess, this.readFail); } SomeObject.prototype.readSuccess = function(response) { this.data = response; this.someList = []; for (var i = 0; i < this.data.length; i++) { var systemData = this.data[i]; var system = new SomeSystem(systemData); this.someList.push(system); } this.refreshList(); } Basically SomeAjaxCall is making an ajax request for data. If it works we use the callback 'this.readSuccess' and if it fails 'this.readFail'. I have figured out that 'this' in the SomeObject.readSuccess is the global this (aka the window object) because my callbacks are being called as functions and not member methods. My understanding is that I need to use closures to keep the 'this' around, however, I have not been able to get this to work. If someone is able show me what I should be doing I would appreciate it greatly. I am still wrapping my head around how closures work and specifically how they would work in this situation. Thanks!

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  • WCF Service with callbacks coming from background thread?

    - by Mark Struzinski
    Here is my situation. I have written a WCF service which calls into one of our vendor's code bases to perform operations, such as Login, Logout, etc. A requirement of this operation is that we have a background thread to receive events as a result of that action. For example, the Login action is sent on the main thread. Then, several events are received back from the vendor service as a result of the login. There can be 1, 2, or several events received. The background thread, which runs on a timer, receives these events and fires an event in the wcf service to notify that a new event has arrived. I have implemented the WCF service in Duplex mode, and planned to use callbacks to notify the UI that events have arrived. Here is my question: How do I send new events from the background thread to the thread which is executing the service? Right now, when I call OperationContext.Current.GetCallbackChannel<IMyCallback>(), the OperationContext is null. Is there a standard pattern to get around this? I am using PerSession as my SessionMode on the ServiceContract. UPDATE: I thought I'd make my exact scenario clearer by demonstrating how I'm receiving events from the vendor code. My library receives each event, determines what the event is, and fires off an event for that particular occurrence. I have another project which is a class library specifically for connecting to the vendor service. I'll post the entire implementation of the service to give a clearer picture: [ServiceBehavior( InstanceContextMode = InstanceContextMode.PerSession )] public class VendorServer:IVendorServer { private IVendorService _vendorService; // This is the reference to my class library public VendorServer() { _vendorServer = new VendorServer(); _vendorServer.AgentManager.AgentLoggedIn += AgentManager_AgentLoggedIn; // This is the eventhandler for the event which arrives from a background thread } public void Login(string userName, string password, string stationId) { _vendorService.Login(userName, password, stationId); // This is a direct call from the main thread to the vendor service to log in } private void AgentManager_AgentLoggedIn(object sender, EventArgs e) { var agentEvent = new AgentEvent { AgentEventType = AgentEventType.Login, EventArgs = e }; } } The AgentEvent object contains the callback as one of its properties, and I was thinking I'd perform the callback like this: agentEvent.Callback = OperationContext.Current.GetCallbackChannel<ICallback>(); How would I pass the OperationContext.Current instance from the main thread into the background thread?

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  • Cannot set textbox value in Callback handler in asp.net

    - by user102533
    I am using ASP.NET Callbacks (that implements the ICallbackEventHandler) and in the handler, I try to set a value of the text box: txtName.text = "Test"; but this value is not set. Is this a limitation with callback? It appears I cannot do much in a callback handler other than sending back a string to the client side (ofcourse I can access the Session etc)

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  • Disabling model's after_find only when called from certain controllers

    - by Lynn C
    I have an after_find callback in a model, but I need to disable it in a particular controller action e.g. def index @people = People.find(:all) # do something here to disable after_find()? end def show @people = People.find(:all) # after_find() should still be called here! end What is the best way to do it? Can I pass something in to .find to disable all/particular callbacks? Can I somehow get the controller name in the model and not execute the callback based on the controller name (I don't like this)..? Help!

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  • Practise Questions for Templates,Functors,CallBack functions in c++?

    - by Eternal Learner
    Hi, I have been reading templates,functors,callback function for the past week and have referred some good books and articles. I however feel that, unless I can get good practice - programming in templates and use functors-callbacks there is no way I can really understand all the concepts or fluently use them while coding. Could anyone suggest some articles or books or websites where , there is a definition of the problem and also a solution to the same. I could just write code for the problem and check later on if my solution is good enough.. I am also aware that some of our stack-overflow members are experts in templates and callback functions. It would be great if they could design a problem and also post a solution , where a lot of template beginners like me could benefit.

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  • WCF Publish/Subscribe and using callbacks to send data to specific users

    - by manemawanna
    Hello thanks for looking, I'm working on a project at the moment and have become a little stuck. I'm creating a client server app, which allows a client to subscribe to the server to have messages forwarded to it. The issue I'm having is that when the client subscribes I wish for them to only recieve updates that relate to them. The system basically passes messages from a SQL server DB which the server monitors. When a new message is recieved the server should only forward the message to the clients that it applys to, based on whos logged on the client machine. I've had a look and found code samples which sign up for messages to be broadcast across all clients who have subscribed, but not any that show how to identify individual clients and if messages apply to them. If anyone could help or point me in the right direction it would be appreciated. You can now find some of my code below: namespace AnnouncementServiceLibrary { [ServiceContract(CallbackContract = typeof(IMessageCallback))] public interface IMessageCheck { [OperationContract] void MessageCheck(); } } namespace AnnouncementServiceLibrary { public interface IMessageCallback { [OperationContract(IsOneWay = true)] void OnNewMessage(Mess message); } } public bool Subscribe() { try { IMessageCallback callback = OperationContext.Current.GetCallbackChannel<IMessageCallback>(); //If they dont already exist in the subscribers list, adds them to it if (!subscribers.Contains(callback)) subscribers.Add(callback); return true; } catch { //Otherwise if an error occurs returns false return false; } } Subscribe/Unsubscribe: private static readonly List<IMessageCallback> subscribers = new List<IMessageCallback>(); /// <summary> /// Unsubscribes the user from recieving new messages when they become avaliable /// </summary> /// <returns>Returns a bool that indicates whether the operation worked or not</returns> public bool Unsubscribe() { try { IMessageCallback callback = OperationContext.Current.GetCallbackChannel<IMessageCallback>(); //If they exist in the list of subscribers they are then removed if (subscribers.Contains(callback)) subscribers.Remove(callback); return true; } catch { //Otherwise if an error occurs returns false return false; } } Finally this at the moment isnt't working as basically when a user subscribes as it loops through I want it to filter based on the users userID: #region IMessageCheck Members /// <summary> /// This method checks for new messages recieved based on those who have subscribed for the service /// </summary> public void MessageCheck() { //A continuous loop to keep the method going while(true) { //Changes the thread to a sleep state for 2 mins? Thread.Sleep(200000); //Go through each subscriber based on there callback information subscribers.ForEach(delegate(IMessageCallback callback) { //Checks if the person who wanted the callback can still be communicated with if (((ICommunicationObject)callback).State == CommunicationState.Opened) { //Creates a link to the database and gets the required information List<Mess> mess = new List<Mess>(); List<Message> me; List<MessageLink> messLink; AnnouncementDBDataContext aDb = new AnnouncementDBDataContext(); me = aDb.Messages.ToList(); messLink = aDb.MessageLinks.ToList(); //Query to retrieve any messages which are newer than the time when the last cycle finished var result = (from a in messLink join b in me on a.UniqueID equals b.UniqueID where b.TimeRecieved > _time select new { b.UniqueID, b.Author, b.Title, b.Body, b.Priority, a.Read, b.TimeRecieved }); //Foreach result a new message is created and returned to the PC that subscribed foreach (var a in result) { Mess message = new Mess(a.UniqueID, a.Author, a.Title, a.Body, a.Priority, (bool)a.Read, a.TimeRecieved); callback.OnNewMessage(message); } } //If the requesting PC can't be contacted they are removed from the subscribers list else { subscribers.Remove(callback); } }); //Sets the datetime so the next cycle can measure against to see if new messages have been recieved _time = DateTime.Now; } } #endregion

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  • Programming pattern to flatten deeply nested ajax callbacks?

    - by chiborg
    I've inherited JavaScript code where the success callback of an Ajax handler initiates another Ajax call where the success callback may or may not initiate another Ajax call. This leads to deeply nested anonymous functions. Maybe there is a clever programming pattern that avoids the deep-nesting and is more DRY. jQuery.extend(Application.Model.prototype, { process: function() { jQuery.ajax({ url:myurl1, dataType:'json', success:function(data) { // process data, then send it back jQuery.ajax({ url:myurl2, dataType:'json', success:function(data) { if(!data.ok) { jQuery.ajax({ url:myurl2, dataType:'json', success:mycallback }); } else { mycallback(data); } } }); } }); } });

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  • Asynchronuos callback saves value but prints FAILED

    - by sprasad12
    Hi, I am using nested Asynchronous callbacks to save my front-end data to the back-end database. The data is being save into the tables the way i want them to, but it is printing that it failed. Here is the code: oksave.addClickHandler(new ClickHandler(){ public void onClick(ClickEvent event) { if(erasync == null) erasync = GWT.create(EntityRelationService.class); AsyncCallback<Void> callback = new AsyncCallback<Void>(){ public void onFailure(Throwable caught) { String msg = caught.getLocalizedMessage(); if (caught instanceof NotFoundException) { msg = ((NotFoundException) caught).getType() + ((NotFoundException) caught).getMessage(); } System.out.println("Failed" + msg); } public void onSuccess(Void result) { Label success = new Label("Name : " + pname.getText() + " was successfully saved"); Button close = new Button("close"); VerticalPanel sp = new VerticalPanel(); d1 = new DialogBox(); sp.add(success); sp.add(close); close.addClickHandler(new ClickHandler(){ @Override public void onClick(ClickEvent event) { if(erasync == null) erasync = GWT.create(EntityRelationService.class); AsyncCallback<Void> callbackOthers = new AsyncCallback<Void>(){ @Override public void onFailure(Throwable caught) { String msg = caught.getLocalizedMessage(); if (caught instanceof NotFoundException) { msg = ((NotFoundException) caught).getType() + ((NotFoundException) caught).getMessage(); } System.out.println("Failed" + msg); } @Override public void onSuccess(Void result) { System.out.println("Success"); } }; erasync.setEntityType(name, top, left, pname, callbackOthers); }); }; erasync.setProject(name, callback); }); Here it prints successful for the first callback, but for the nested one it says failed though it saves the value. Am i missing something? Any input will be of great help. Thank you.

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  • JQuery live event binding prevents additional callbacks

    - by Alex Ciminian
    Hey! I was building an AJAX listing of elements in my site, with the ability to delete them (also via AJAX). The following piece of code handles the deletion: $('ul.action-menu a.delete').live('click', function () { $.post($(this).attr('href'), function (data) { var recvData = eval( '(' + data + ')' ); if ((recvData.status == 1) && (recvData.delId)) { $('#alert-' + recvData.delId).fadeOut(); } else { alert(recvData.message); } }); return false; }); This works just fine. The problem is that, for elements that were not there when the page was loaded (i.e. that were added dynamically), the post callback does not get executed and it doesn't fade out after being deleted (the AJAX call is being made, it just doesn't execute the callback). Do you have any idea why this is happening? Thanks, Alex

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  • HTML5 <video> callbacks?

    - by Andrew
    I'm working on a site for a client and they're insistent on using HTML5's video tag as the delivery method for some of their video content. I currently have it up and running with a little help from http://videojs.com/ to handle the Internet Explorer Flash fallback. One thing they've asked me to do is, after the videos finish playing (they're all a different length), fade them out and then fade a picture in place of the video --- think of it like a poster frame after the video. Is this even possible? Can you get the timecode of a currently playing movie via Javascript or some other method? I know Flowplayer (http://flowplayer.org/demos/scripting/grow.html) has an onFinish function, is that the route I should take in lieu of the HTML5 video method? Does the fact that IE users will be getting a Flash player require two separate solutions? Any input would be greatly appreciated. I'm currently using jQuery on the site, so I'd like to keep the solution in that realm if at all possible. Thanks!

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  • C++: Dependency injection, circular dependency and callbacks

    - by Jonathan
    Consider the (highly simplified) following case: class Dispatcher { public: receive() {/*implementation*/}; // callback } class CommInterface { public: send() = 0; // call } class CommA : public CommInterface { public: send() {/*implementation*/}; } Various classes in the system send messages via the dispatcher. The dispatcher uses a comm to send. Once an answer is returned, the comm relays it back to the dispatcher which dispatches it back to the appropriate original sender. Comm is polymorphic and which implementation to choose can be read from a settings file. Dispatcher has a dependency on the comm in order to send. Comm has a dependency on dispatcher in order to callback. Therefor there's a circular dependency here and I can't seem to implement the dependency injection principle (even after encountering this nice blog post).

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  • Passing VB Callback function to C dll - noob is stuck.

    - by WaveyDavey
    Callbacks in VB (from C dll). I need to pass a vb function as a callback to a c function in a dll. I know I need to use addressof for the function but I'm getting more and more confused as to how to do it. Details: The function in the dll that I'm passing the address of a callback to is defined in C as : PaError Pa_OpenStream( PaStream** stream, const PaStreamParameters *inputParameters, const PaStreamParameters *outputParameters, double sampleRate, unsigned long framesPerBuffer, PaStreamFlags streamFlags, PaStreamCallback *streamCallback, void *userData ); where the function is parameter 7, *streamCallback. The type PaStreamCallback is defines thusly: typedef int PaStreamCallback( const void *input, void *output, unsigned long frameCount, const PaStreamCallbackTimeInfo* timeInfo, PaStreamCallbackFlags statusFlags, void *userData ); In my vb project I have: Private Declare Function Pa_OpenStream Lib "portaudio_x86.dll" _ ( ByVal stream As IntPtr _ , ByVal inputParameters As IntPtr _ , ByVal outputParameters As PaStreamParameters _ , ByVal samprate As Double _ , ByVal fpb As Double _ , ByVal paClipoff As Long _ , ByVal patestCallBack As IntPtr _ , ByVal data As IntPtr) As Integer (don't worry if I've mistyped some of the other parameters, I'll get to them later! Let's concentrate on the callback for now.) In module1.vb I have defined the callback function: Function MyCallback( ByVal inp As Byte, _ ByVal outp As Byte, _ ByVal framecount As Long, _ ByVal pastreamcallbacktimeinfo As Byte, _ ByVal pastreamcallbackflags As Byte, _ ByVal userdata As Byte) As Integer ' do clever things here End Function The external function in the dll is called with err = Pa_OpenStream( ptr, _ nulthing, _ outputParameters, _ SAMPLE_RATE, _ FRAMES_PER_BUFFER, _ clipoff, _ AddressOf MyCallback, _ dataptr) This is broken in the declaration of the external function - it doesn't like the type IntPtr as a function pointer for AddressOf. Can anyone show me how to implement passing this callback function please ? Many thanks David

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  • JQuery UI: Accordion callbacks

    - by user177215
    I need my javascript to only do the callback when I OPEN a section on the accordion, as of right now it does a callback when I open OR close a section because I'm only using a click function. Is there a way I can modify my existing click function to only run when the given section is activated? My current click function: $("a#mimetypes").click(function() { $("span#mimetypesthrobber").loading(true, { max: 1500 }) $.getJSON("../mimetypes", function(data) { //callback }); }); Thanks! EDIT: I already tried this with another part of the accordion and it wasn't working properly: $('.ui-accordion').bind('accordionchange', function(event, ui) { if (ui.newHeader == "Encoders") { EncodersGet(); } });

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  • D callbacks in C functions

    - by Caspin
    I am writing D2 bindings for Lua. This is in one of the Lua header files. typedef int (*lua_CFunction) (lua_State *L); I assume the equivalent D2 statement would be: extern(C) alias int function( lua_State* L ) lua_CFunction; Lua also provides an api function: void lua_pushcfunction( lua_State* L, string name, lua_CFunction func ); If I want to push a D2 function does it have to be extern(C) or can I just use the function? int dfunc( lua_State* L ) { std.stdio.writeln("dfunc"); } extern(C) int cfunc( lua_State* L ) { std.stdio.writeln("cfunc"); } lua_State* L = lua_newstate(); lua_pushcfunction(L, "cfunc", &cfunc); //This will definitely work. lua_pushcfunction(L, "dfunc", &dfunc); //Will this work? If I can only use cfunc, why? I don't need to do anything like that in C++. I can just pass the address of a C++ function to C and everything just works.

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  • Using Moq callbacks correctly according to AAA

    - by Hadi Eskandari
    I've created a unit test that tests interactions on my ViewModel class in a Silverlight application. To be able to do this test, I'm mocking the service interface, injected to the ViewModel. I'm using Moq framework to do the mocking. to be able to verify bounded object in the ViewModel is converted properly, I've used a callback: [Test] public void SaveProposal_Will_Map_Proposal_To_WebService_Parameter() { var vm = CreateNewCampaignViewModel(); var proposal = CreateNewProposal(1, "New Proposal"); Services.Setup(x => x.SaveProposalAsync(It.IsAny<saveProposalParam>())).Callback((saveProposalParam p) => { Assert.That(p.plainProposal, Is.Not.Null); Assert.That(p.plainProposal.POrderItem.orderItemId, Is.EqualTo(1)); Assert.That(p.plainProposal.POrderItem.orderName, Is.EqualTo("New Proposal")); }); proposal.State = ObjectStates.Added; vm.CurrentProposal = proposal; vm.Save(); } It is working fine, but if you've noticed, using this mechanism the Assert and Act part of the unit test have switched their parts (Assert comes before Acting). Is there a better way to do this, while preserving correct AAA order?

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  • jQuery callbacks - short version

    - by Frankie
    I've dug the manual and tried extensivelly to drop the extra function(){...} call after the error and success to do it like mootools but to no good luck. Is there any way i can avoid the extra function in jQuery? function doFileDelete(ui) { $.ajax({ error: function() { doFileDeleteAnimation(ui, false) }, success: function() { doFileDeleteAnimation(ui, true) }, url: '/url-to-delete-file' }); } Thank you!

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  • fullcalendar : updating option function callbacks after init

    - by Paul Maneesilasan
    Ok, so I have a problem with setting options whose values are callback functions when I try to set them after plugin initialization. I think this would be a common behavior, to dynamically set event callback after init'ing the calendar. Here is a snipit of code: $(document).ready(function() { $('#calendar').fullCalendar({ editable: false ,events:[{"title":"meeting.title","start":"2010-05-21 15:58:16 UTC"},{"title":"meeting.title","start":"2010-05-24 15:58:16", "url":"http://google.com"}] /* ,eventClick: function(event) { if (event.url) { window.open(event.url); return false; } } */ }); $('#calendar').fullCalendar('options', 'eventClick', function(event) { if (event.url) { window.open(event.url); return false; } }); }); You can see that I have setting the eventClick function as an init option commented out. If I do it that way, it works fine. However if I try to set it after the init, it doesn't work :( Is the some other way to do this? Or am I stuck with having to set the behavior upfront?

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  • CMBufferQueueCreate fail, required parameters?

    - by Agustin
    Reading the documentation about iOS SDK CMBufferQueueCreate, it says that getDuration and version are required, all the others callbacks can be NULL. but running the following code: CFAllocatorRef allocator; CMBufferCallbacks *callbacks; callbacks = malloc(sizeof(CMBufferCallbacks)); callbacks->version = 0; callbacks->getDuration = timeCallback; callbacks->refcon = NULL; callbacks->getDecodeTimeStamp = NULL; callbacks->getPresentationTimeStamp = NULL; callbacks->isDataReady = NULL; callbacks->compare = NULL; callbacks->dataBecameReadyNotification = NULL; CMItemCount capacity = 4; OSStatus s = CMBufferQueueCreate(allocator, capacity, callbacks, queue); NSLog(@"QUEUE: %x", queue); NSLog(@"STATUS: %i", s); with timeCallback: CMTime timeCallback(CMBufferRef buf, void *refcon){ return CMTimeMake(1, 1); } and queue is: CMBufferQueueRef* queue; queue creations fails (queue = 0) and returns a status of: kCMBufferQueueError_RequiredParameterMissing = -12761, The callbacks variable is correctly initialized, at least the debugger says so. Somebody have played arround with CMBufferQueue? google doesn't know about that! Thanks

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  • KO 2.3.4 - Accessing validation array from callbacks in models

    - by kenny99
    Hi, Apologies if this is an oversight or sheer stupidity on my part but I can't quite figure out how to access the validation array from a callback in a model (using ORM and KO 2.3.4). I want to be able to add specific error messages to the validation array if a callback returns false. e.g This register method: public function register(array & $array, $save = FALSE) { // Initialise the validation library and setup some rules $array = Validation::factory($array) ->pre_filter('trim') ->add_rules('email', 'required', 'valid::email', array($this, 'email_available')) ->add_rules('confirm_email', 'matches[email]') ->add_rules('password', 'required', 'length[5,42]') ->add_rules('confirm_password', 'matches[password]'); return ORM::validate($array, $save); } Callback: public function email_available($value) { return ! (bool) $this->db ->where('email', $value) ->count_records($this->table_name); } I can obviously access the current model from the callback, but I was wondering what the best way to add custom error from the callback would be?

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  • XMPP4R Callbacks dont seem to work

    - by Sid
    Im using xmpp4r and trying to get the hang of a basic chat feature that I wish to implement later in my Rails app. My fundamentals on Ruby Threads is still a bit shaky so I would appreciate any help on this. Though I register the callback i dont get a response from my gmail account. I am able to send a message but my ruby program terminates. In order to prevent it from terminating I tried to stop on of the threads in the program but I cant seem to get it working. require 'rubygems' require "xmpp4r/client" require "xmpp4r/roster" include Jabber def connect client = Client.new(JID::new("[email protected]")) client.connect client.auth("test") client.send(Presence.new.set_type(:available)) client end def create_message(message, to_email) msg = Jabber::Message::new(to_email, message) msg.type = :chat msg end def subscribe(email_id) pres = Presence.new.set_type(:subscribe).set_to(email_id) pres end client = connect roster = Roster::Helper.new(client) roster.add_subscription_request_callback do |item,pres| roster.accept_subscription(pres.from) end def create_callback(client) $t4= Thread.new do client.add_message_callback do |m| puts m.body puts "................................Callback working" end end end puts "Client has connected" msg = create_message("Welcome to the winter of my discontent", "[email protected]") client.send(msg) create_callback(client) def check(client) $t3 = Thread.new do loop do puts "t3 still running........." Thread.current.stop $t4.join end end end check(client)

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  • valid json still fails on IE with jquery's ajax or getJSON callbacks

    - by lock
    everytime my page loads, im supposed to create a datatable (also a jquery plugin) but when im fetching the contents, using .ajax or .getJSON always goes straight ahead to the error function, without even telling me what went wrong inside the callback $.ajax({ cache: false, type: "POST", url: oSettings.sAjaxSource, data: {'newdate' : date}, contentType: "application/json; charset=utf-8", dataType: "json", success: function(json) { console.log('retrieving json data'); }, error: function() { console.log("An error has occurred. Please try again."); } }); that's the actual code with the callback stripped for security purposes... this works fine in firefox which actually executes what's on the callback function but IE simply fails and proceeds to writing my log i've read alot that the primary reason the JSON calls fails for IE is whenever there are trailing commas or simply malformed JS but i used JSONLint already and verified that my json object is a valid one :(

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  • How to implement generic callbacks in C++

    - by Kylotan
    Forgive my ignorance in asking this basic question but I've become so used to using Python where this sort of thing is trivial that I've completely forgotten how I would attempt this in C++. I want to be able to pass a callback to a function that performs a slow process in the background, and have it called later when the process is complete. This callback could be a free function, a static function, or a member function. I'd also like to be able to inject some arbitrary arguments in there for context. (ie. Implementing a very poor man's coroutine, in a way.) On top of that, this function will always take a std::string, which is the output of the process. I don't mind if the position of this argument in the final callback parameter list is fixed. I get the feeling that the answer will involve boost::bind and boost::function but I can't work out the precise invocations that would be necessary in order to create arbitrary callables (while currying them to just take a single string), store them in the background process, and invoke the callable correctly with the string parameter.

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  • Passing Derived Class Instances as void* to Generic Callbacks in C++

    - by Matthew Iselin
    This is a bit of an involved problem, so I'll do the best I can to explain what's going on. If I miss something, please tell me so I can clarify. We have a callback system where on one side a module or application provides a "Service" and clients can perform actions with this Service (A very rudimentary IPC, basically). For future reference let's say we have some definitions like so: typedef int (*callback)(void*); // This is NOT in our code, but makes explaining easier. installCallback(string serviceName, callback cb); // Really handled by a proper management system sendMessage(string serviceName, void* arg); // arg = value to pass to callback This works fine for basic types such as structs or builtins. We have an MI structure a bit like this: Device <- Disk <- MyDiskProvider class Disk : public virtual Device class MyDiskProvider : public Disk The provider may be anything from a hardware driver to a bit of glue that handles disk images. The point is that classes inherit Disk. We have a "service" which is to be notified of all new Disks in the system, and this is where things unravel: void diskHandler(void *p) { Disk *pDisk = reinterpret_cast<Disk*>(p); // Uh oh! // Remainder is not important } SomeDiskProvider::initialise() { // Probe hardware, whatever... // Tell the disk system we're here! sendMessage("disk-handler", reinterpret_cast<void*>(this)); // Uh oh! } The problem is, SomeDiskProvider inherits Disk, but the callback handler can't receive that type (as the callback function pointer must be generic). Could RTTI and templates help here? Any suggestions would be greatly appreciated.

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