What is a callback

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What s a callback and how is it implemented in C

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I just met you  And this is crazy  But here s my number  delegate   So if something happens  event   Call me  maybe  callback

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A callback lets you pass executable code as an argument to other code   In C and C   this is implemented as a function pointer   In  NET you would use a delegate to manage function pointers   A few uses include error signaling and controlling whether a function acts or not   Wikipedia

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In computer programming  a callback is executable code that is passed as an argument to other code     Wikipedia  Callback  computer science   C  has delegates for that purpose  They are heavily used with events  as an event can automatically invoke a number of attached delegates  event handlers

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Delegates do the same thing as interface-based callbacks in C    COM uses these   although are much simpler to use   Note that Microsoft put delegates into its Java implementation  J    but Sun doesn t like them  java sun com  so don t expect to see them in the official version of Java any time soon   I ve hacked together a preprocessor to let you use them in C    so don t feel left out if you re not programming in C  or on the  NET platform  i e  in Managed C   or Visual Basic NET    If you re used to function pointers in C  a delegate is basically a pair of pointers rolled into one    A pointer to an object  optional  A pointer to a method of that object   That means a single delegate passes all the information needed to locate a function in your program  whether it s a static method or associated with an object   You define them like this in C    public delegate void FooCallbackType  int a  int b  int c      When you want to use them  you make delegate out of the function you want to call   class CMyClass       public void FunctionToCall  int a  int b  int c                    This is the callback            public void Foo                 FooCallbackType myDelegate   new FooCallbackType              this FunctionToCall               Now you can pass that to the function            that needs to call you back            If you want to make a delegate to point to a static method  it just looks the same   class CMyClassWithStaticCallback       public static void StaticFunctionToCall  int a  int b  int c                    This is the callback            public static void Foo                 FooCallbackType myDelegate   new FooCallbackType              CMyClass StaticFunctionToCall              All in all  they do the same thing as interface-based callbacks in C    but cause a bit less trouble because you don t need to worry about naming your functions or making helper objects  and you can make delegates out of any method  They re more flexible

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A callback is a function that will be called when a process is done executing a specific task   The usage of a callback is usually in asynchronous logic   To create a callback in C   you need to store a function address inside a variable  This is achieved using a delegate or the new lambda semantic Func or Action       public delegate void WorkCompletedCallBack string result        public void DoWork WorkCompletedCallBack callback                callback  Hello world               public void Test                 WorkCompletedCallBack callback   TestCallBack     Notice that I am referencing a method without its parameter         DoWork callback              public void TestCallBack string result                Console WriteLine result           In today C   this could be done using lambda like       public void DoWork Action lt string gt  callback                callback  Hello world               public void Test                 DoWork  result    gt  Console WriteLine result

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Dedication to LightStriker  Sample Code   class CallBackExample       public delegate void MyNumber        public static void CallMeBack                 Console WriteLine  He She is calling you   Pick your phone               Console Read              public static void MetYourCrush MyNumber number                int j          Console WriteLine  is she he interested 0 1              var i   Console ReadLine            if  int TryParse i  out j                         var interested    j    0    false   true              if  interested   event                                 call his her number                 number                              else                               Console WriteLine  Nothing happened                        Console Read                                      static void Main string   args                MyNumber number   Program CallMeBack          Console WriteLine  You have just met your crush and given your number            MetYourCrush number           Console Read            Console Read                     Code Explanation   I created the code to implement the funny explanation provided by LightStriker in the above one of the replies  We are passing delegate  number  to a method  MetYourCrush   If the Interested  event  occurs in the method  MetYourCrush  then it will call the delegate  number  which was holding the reference of CallMeBack method  So  the CallMeBack method will be called  Basically  we are passing delegate to call the callback method    Please let me know if you have any questions

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Definition     A callback is executable code that   is passed as an argument to other code     Implementation     Parent can Read public class Parent       public string Read      reads here            Child need Info public class Child       private string information         declare a Delegate     delegate string GetInfo           use an instance of the declared Delegate     public GetInfo GetMeInformation       public void ObtainInfo                    Child will use the Parent capabilities via the Delegate         information   GetMeInformation              Usage  Parent Peter   new Parent    Child Johny   new Child        Tell Johny from where to obtain info Johny GetMeInformation   Peter Read   Johny ObtainInfo       here Johny  asks  Peter to read   Links   more details for C

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If you referring to ASP Net callbacks      In the default model for ASP NET Web   pages  the user interacts with a page   and clicks a button or performs some   other action that results in a   postback  The page and its controls   are re-created  the page code runs on   the server  and a new version of the   page is rendered to the browser    However  in some situations  it is   useful to run server code from the   client without performing a postback    If the client script in the page is   maintaining some state information    for example  local variable values     posting the page and getting a new   copy of it destroys that state    Additionally  page postbacks introduce   processing overhead that can decrease   performance and force the user to wait   for the page to be processed and   re-created       To avoid losing client state and not   incur the processing overhead of a   server roundtrip  you can code an   ASP NET Web page so that it can   perform client callbacks  In a client   callback  a client-script function   sends a request to an ASP NET Web   page  The Web page runs a modified   version of its normal life cycle  The   page is initiated and its controls and   other members are created  and then a   specially marked method is invoked    The method performs the processing   that you have coded and then returns a   value to the browser that can be read   by another client script function    Throughout this process  the page is   live in the browser    Source  http   msdn microsoft com en-us library ms178208 aspx  If you are referring to callbacks in code   Callbacks are often delegates to methods that are called when the specific operation has completed or performs a sub-action  You ll often find them in asynchronous operations  It is a programming principle that you can find in almost every coding language   More info here  http   msdn microsoft com en-us library ms173172 aspx

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Probably not the dictionary definition  but a callback usually refers to a function  which is external to a particular object  being stored and then called upon a specific event    An example might be when a UI button is created  it stores a reference to a function which performs an action  The action is handled by a different part of the code but when the button is pressed  the callback is called and this invokes the action to perform   C   rather than use the term  callback  uses  events  and  delegates  and you can find out more about delegates here

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callback work steps   1  we have to implement ICallbackEventHandler Interface  2  Register the client script     String cbReference   Page ClientScript GetCallbackEventReference this   arg    ReceiveServerData    context        String callbackScript    function UseCallBack arg  context            cbReference             Page ClientScript RegisterClientScriptBlock this GetType     UseCallBack   callbackScript  true     1  from UI call Onclient click call javascript function  for EX - builpopup p1 p2 p3      var finalfield  p1 p2 p3                  UseCallBack finalfield          data from the client passed to server side by using UseCallBack   2   public void RaiseCallbackEvent string eventArgument   In eventArgument we get the passed data               do some server side operation and passed to   callbackResult         3    GetCallbackResult        using this method data will be passed to client ReceiveServerData   function  side   callbackResult  4  Get the data at client side                       ReceiveServerData text      in text server response   we wil get

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A callback is a function pointer that you pass in to another function   The function you are calling will  callback   execute  the other function when it has completed   Check out this link

[c#] What is a callback?

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