What is a callback function

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Call After would be a better name than the stupid name  callback  When or if condition gets met within a function  call another function  the Call After function  the one received as argument    Rather than hard-code an inner function within a function  one writes a function to accept an already-written Call After function as argument  The Call After might get called based on state changes detected by code in the function receiving the argument

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Call back vs Callback Function  A Callback is a function that is to be executed after another function has finished executing     hence the name    call back      What is a Callback Function    In JavaScript  functions are objects  Because of this  functions can take functions as arguments  and can be returned by other functions  Functions that do this are called higher-order functions  Any function that is passed as an argument is called a callback function  a callback function is a function that is passed to another function  let s call this other function otherFunction  as a parameter  and the callback function is called  or executed  inside the otherFunction       function action x  y  callback            return callback x  y                  function multiplication x  y            return x   y                 function addition x  y            return x   y             alert action 10  10  multiplication       output  100      alert action 10  10  addition       output  20  In SOA  callback allows the Plugin Modules to access services from the container environment  Source

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The Callback page on Wikipedia explains it very well   In computer programming  a callback is a reference to executable code  or a piece of executable code  that is passed as an argument to other code  This allows a lower-level software layer to call a subroutine  or function  defined in a higher-level layer

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I believe this  callback  jargon has been mistakenly used in a lot of places  My definition would be something like      A callback function is a function that you pass to someone and let   them call it at some point of time    I think people just read the first sentence of the wiki definition      a callback is a reference to executable code  or a piece of   executable code  that is passed as an argument to other code    I ve been working with lots of APIs  see various of bad examples  Many people tend to name a function pointer  a reference to executable code  or anonymous functions a piece of executable code   callback   if they are just functions why do you need another name for this   Actually only the second sentence in wiki definition reveals the differences between a callback function and a normal function      This allows a lower-level software layer to call a subroutine  or   function  defined in a higher-level layer    so the difference is who you are going to pass the function and how your passed in function is going to be called  If you just define a function and pass it to another function and called it directly in that function body  don t call it a callback  The definition says your passed in function is gonna be called by  lower-level  function   I hope people can stop using this word in ambiguous context  it can t help people to understand better only worse

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A callback function is a function you specify to an existing function method  to be invoked when an action is completed  requires additional processing  etc   In Javascript  or more specifically jQuery  for example  you can specify a callback argument to be called when an animation has finished   In PHP  the preg replace callback   function allows you to provide a function that will be called when the regular expression is matched  passing the string s  matched as arguments

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Assume we have a function sort int  arraytobesorted void   algorithmchosen  void   where it can accept a function pointer as its argument which can be used at some point in sort   s implementation   Then   here the code that is being addressed by the function pointer algorithmchosen is called as callback function    And see the advantage is that we can choose any algorithm like     1     algorithmchosen   bubblesort   2     algorithmchosen   heapsort   3     algorithmchosen   mergesort         Which were  say have been implemented with the prototype     1     void bubblesort void     2     void heapsort void     3     void mergesort void           This is a concept used in achieving Polymorphism in Object Oriented Programming

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Opaque Definition  A callback function is a function you provide to another piece of code  allowing it to be called by that code   Contrived example  Why would you want to do this  Let s say there is a service you need to invoke  If the service returns immediately  you just    Call it Wait for the result Continue once the result comes in   For example  suppose the service were the factorial function  When you want the value of 5   you would invoke factorial 5   and the following steps would occur    Your current execution location is saved  on the stack  but that s not important  Execution is handed over to factorial When factorial completes  it puts the result somewhere you can get to it Execution comes back to where it was in  1    Now suppose factorial took a really long time  because you re giving it huge numbers and it needs to run on some supercomputing cluster somwhere  Let s say you expect it to take 5 minutes to return your result  You could    Keep your design and run your program at night when you re asleep  so that you re not staring at the screen half the time Design your program to do other things while factorial is doing its thing   If you choose the second option  then callbacks might work for you   End-to-end design  In order to exploit a callback pattern  what you want is to be able to call factorial in the following way   factorial really big number  what to do with the result    The second parameter  what to do with the result  is a function you send along to factorial  in the hope that factorial will call it on its result before returning    Yes  this means that factorial needs to have been written to support callbacks   Now suppose that you want to be able to pass a parameter to your callback  Now you can t  because you re not going to be calling it  factorial is  So factorial needs to be written to allow you to pass your parameters in  and it will just hand them over to your callback when it invokes it  It might look like this   factorial  number  callback  params        result   number       i can make up operators in my pseudocode     callback  result  params      Now that factorial allows this pattern  your callback might look like this   logIt  number  logger        logger log number      and your call to factorial would be   factorial 42  logIt  logger    What if you want to return something from logIt  Well  you can t  because factorial isn t paying attention to it   Well  why can t factorial just return what your callback returns   Making it non-blocking  Since execution is meant to be handed over to the callback when factorial is finished  it really shouldn t return anything to its caller  And ideally  it would somehow launch its work in another thread   process   machine and return immediately so that you can continue  maybe something like this   factorial param 1  param 2             new factorial worker task param 1  param 2            return      This is now an  asynchronous call   meaning that when you call it  it returns immediately but hasn t really done its job yet  So you do need mechanisms to check on it  and to obtain its result when its finished  and your program has gotten more complex in the process    And by the way  using this pattern the factorial worker task can launch your callback asynchronously and return immediately   So what do you do   The answer is to stay within the callback pattern  Whenever you want to write  a   f   g a    and f is to be called asynchronously  you will instead write  f g    where g is passed as a callback    This fundamentally changes the flow-topology of your program  and takes some getting used to    Your programming language could help you a lot by giving you a way to create functions on-the-fly  In the code immediately above  the function g might be as small as print  2 a 1   If your language requires that you define this as a separate function  with an entirely unnecessary name and signature  then your life is going to get unpleasant if you use this pattern a lot    If  on the other hand  you language allows you to create lambdas  then you are in much better shape  You will then end up writing something like  f  func a    print 2 a 1        which is so much nicer   How to pass the callback  How would you pass the callback function to factorial  Well  you could do it in a number of ways     If the called function is running in the same process  you could pass a function pointer Or maybe you want to maintain a dictionary of fn name -- gt  fn ptr in your program  in which case you could pass the name Maybe your language allows you to define the function in-place  possible as a lambda  Internally it is creating some kind of object and passing a pointer  but you don t have to worry about that  Perhaps the function you are calling is running on an entirely separate machine  and you are calling it using a network protocol like HTTP  You could expose your callback as an HTTP-callable function  and pass its URL    You get the idea   The recent rise of callbacks  In this web era we have entered  the services we invoke are often over the network  We often do not have any control over those services i e  we didn t write them  we don t maintain them  we can t ensure they re up or how they re performing   But we can t expect our programs to block while we re waiting for these services to respond  Being aware of this  the service providers often design APIs using the callback pattern   JavaScript supports callbacks very nicely e g  with lambdas and closures  And there is a lot of activity in the JavaScript world  both on the browser as well as on the server  There are even JavaScript platforms being developed for mobile   As we move forward  more and more of us will be writing asynchronous code  for which this understanding will be essential

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This makes callbacks sound like return statements at the end of methods   I m not sure that s what they are   I think Callbacks are actually a call to a function  as a consequence of another function being invoked and completing   I also think Callbacks are meant to address the originating invocation  in a kind of  hey  that thing you asked for  I ve done it - just thought I would let you know - back over to you

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Developers are often confused by what a callback is because of the name of the damned thing   A callback function is a function which is    accessible by another function  and is invoked after the first function if that first function completes   A nice way of imagining how a callback function works is that it is a function that is  called at the back  of the function it is passed into   Maybe a better name would be a  call after  function    This construct is very useful for asynchronous behaviour where we want an activity to take place whenever a previous event completes   Pseudocode      A function which accepts another function as an argument     and will automatically invoke that function when it completes - note that there is no explicit call to callbackFunction  funct printANumber int number  funct callbackFunction        printout  The number you provided is      number         a function which we will use in a driver function as a callback function funct printFinishMessage         printout  I have finished printing numbers           Driver method funct event        printANumber 6  printFinishMessage       Result if you called event     The number you provided is  6 I have finished printing numbers    The order of the output here is important  Since callback functions are called afterwards   I have finished printing numbers  is printed last  not first    Callbacks are so-called due to their usage with pointer languages  If you don t use one of those  don t labour over the name  callback   Just understand that it is just a name to describe a method that s supplied as an argument to another method  such that when the parent method is called  whatever condition  such as a button click  a timer tick etc  and its method body completes  the callback function is then invoked   Some languages support constructs where multiple callback function arguments are supported  and are called based on how the parent function completes  i e  one callback is called in the event that the parent function completes successfully  another is called in the event that the parent function throws a specific error  etc

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One important usage area is that you register one of your function as a handle  i e  a callback  and then send a message   call some function to do some work or processing  Now after the processing is done  the called function would call our registered function  i e  now call back is done   thus indicating us processing is done   This wikipedia link explains quite well graphically

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look at the image     Main program calls library function  which might be system level function also  with callback function name  This callback function might be implemented in multiple way  The main program choose one callback as per requirement    Finally  the library function calls the callback function during execution

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In computer programming  a callback is a reference to executable code  or a piece of executable code  that is passed as an argument to other code  This allows a lower-level software layer to call a subroutine  or function  defined in a higher-level layer     - Wikipedia  Callback in C using Function Pointer  In C  callback is implemented using Function Pointer  Function Pointer - as the name suggests  is a pointer to a function   For example  int   ptrFunc       Here  ptrFunc is a pointer to a function that takes no arguments and returns an integer  DO NOT forget to put in the parenthesis  otherwise the compiler will assume that ptrFunc is a normal function name  which takes nothing and returns a pointer to an integer   Here is some code to demonstrate the function pointer    include lt stdio h gt  int func int  int   int main void        int result1 result2         declaring a pointer to a function which takes        two int arguments and returns an integer as result        int   ptrFunc  int int           assigning ptrFunc to func s address                            ptrFunc func          calling func   through explicit dereference        result1     ptrFunc  10 20           calling func   through implicit dereference                result2   ptrFunc 10 20                   printf  result1    d result2    d n  result1 result2       return 0     int func int x  int y        return x y      Now let us try to understand the concept of Callback in  C using function pointer   The complete program has three files  callback c  reg callback h and  reg callback c      callback c     include lt stdio h gt   include reg callback h      callback function definition goes here    void my callback void        printf  inside my callback n       int main void           initialize function pointer to     my callback        callback ptr my callback my callback                              printf  This is a program demonstrating function callback n           register our callback function        register callback ptr my callback                                 printf  back inside main program n        return 0        reg callback h    typedef void   callback  void   void register callback callback ptr reg callback        reg callback c     include lt stdio h gt   include reg callback h      registration goes here    void register callback callback ptr reg callback        printf  inside register callback n           calling our callback function my callback          ptr reg callback                                        If we run this program  the output will be  This is a program demonstrating function callback inside register callback inside my callback back inside main program  The higher layer function calls a lower layer function as a normal call and the callback mechanism allows the lower layer function to call the higher layer function through a pointer to a callback function   Callback in Java Using Interface  Java does not have the concept of function pointer It implements Callback mechanism through its Interface mechanism Here instead of a function pointer  we declare an Interface having a method which will be called when the callee finishes its task  Let me demonstrate it through an example   The Callback Interface  public interface Callback       public void notify Result result       The Caller or the Higher  Level Class  public Class Caller implements Callback   Callee ce   new Callee this     pass self to the callee    Other functionality   Call the Asynctask ce doAsynctask     public void notify Result result     Got the result after the callee has finished the task   Can do whatever i want with the result       The Callee or the lower layer function  public Class Callee   Callback cb  Callee Callback cb   this cb   cb     doAsynctask      do the long running task   get the result cb notify result    after the task is completed  notify the caller       Callback Using EventListener pattern   List item   This pattern is used to notify 0 to n numbers of Observers Listeners that a particular task has finished   List item   The difference between Callback mechanism and EventListener Observer mechanism is that in callback  the callee notifies the single caller  whereas in Eventlisener Observer  the callee can notify anyone who is interested in that event  the notification may go to some other parts of the application which has not triggered the task   Let me explain it through an example   The Event Interface  public interface Events    public void clickEvent    public void longClickEvent        Class Widget  package com som itsolutions training java exampleeventlistener   import java util ArrayList  import java util Iterator   public class Widget implements Events       ArrayList lt OnClickEventListener gt  mClickEventListener   new ArrayList lt OnClickEventListener gt          ArrayList lt OnLongClickEventListener gt  mLongClickEventListener   new ArrayList lt OnLongClickEventListener gt           Override     public void clickEvent                TODO Auto-generated method stub         Iterator lt OnClickEventListener gt  it   mClickEventListener iterator                    while it hasNext                         OnClickEventListener li   it next                        li onClick this                                   Override     public void longClickEvent                TODO Auto-generated method stub         Iterator lt OnLongClickEventListener gt  it   mLongClickEventListener iterator            while it hasNext                 OnLongClickEventListener li   it next                li onLongClick this                         public interface OnClickEventListener               public void onClick  Widget source              public interface OnLongClickEventListener               public void onLongClick  Widget source              public void setOnClickEventListner OnClickEventListener li           mClickEventListener add li             public void setOnLongClickEventListner OnLongClickEventListener li           mLongClickEventListener add li             Class Button  public class Button extends Widget  private String mButtonText  public Button        public String getButtonText     return mButtonText    public void setButtonText String buttonText    this mButtonText   buttonText        Class Checkbox  public class CheckBox extends Widget  private boolean checked  public CheckBox     checked   false    public boolean isChecked    return  checked    true     public void setCheck boolean checked   this checked   checked        Activity Class  package com som itsolutions training java exampleeventlistener   public class Activity implements Widget OnClickEventListener       public Button mButton      public CheckBox mCheckBox      private static Activity mActivityHandler      public static Activity getActivityHandle            return mActivityHandler            public Activity                  mActivityHandler   this          mButton   new Button            mButton setOnClickEventListner this           mCheckBox   new CheckBox            mCheckBox setOnClickEventListner this                  public void onClick  Widget source                if source    mButton               mButton setButtonText  Thank you for clicking me                   System out println   Button  mButton  getButtonText                       if source    mCheckBox               if mCheckBox isChecked    false                   mCheckBox setCheck true                   System out println  The checkbox is checked                                 else                  mCheckBox setCheck false                   System out println  The checkbox is not checked                                                public void doSomeWork Widget source           source clickEvent                 Other Class  public class OtherClass implements Widget OnClickEventListener  Button mButton  public OtherClass    mButton   Activity getActivityHandle   mButton  mButton setOnClickEventListner this    interested in the click event                          of the button    Override public void onClick Widget source    if source    mButton   System out println  Other Class has also received the event notification             Main Class  public class Main   public static void main String   args       TODO Auto-generated method stub Activity a   new Activity    OtherClass o   new OtherClass    a doSomeWork a mButton   a doSomeWork a mCheckBox         As you can see from the above code  that we have an interface called events which basically lists all the events that may happen for our application  The Widget class is the base class for all the UI components like Button  Checkbox  These UI components are the objects that actually receive the events from the framework code  Widget class implements the Events interface and also it has two nested interfaces namely OnClickEventListener  amp  OnLongClickEventListener  These two interfaces are responsible for listening to events that may occur on the Widget derived UI components like Button or Checkbox  So if we compare this example with the earlier Callback example using Java Interface  these two interfaces work as the Callback interface  So the higher level code  Here Activity  implements these two interfaces  And whenever an event occurs to a widget  the higher level code  or the method of these interfaces implemented in the higher level code  which is here Activity  will be called   Now let me discuss the basic difference between Callback and Eventlistener pattern  As we have mentioned that using Callback  the Callee can notify only a single Caller  But in the case of EventListener pattern  any other part or class of the Application can register for the events that may occur on the Button or Checkbox  The example of this kind of class is the OtherClass  If you see the code of the OtherClass  you will find that it has registered itself as a listener to the ClickEvent that may occur in the Button defined in the Activity  Interesting part is that  besides the Activity   the Caller   this OtherClass will also be notified whenever the click event occurs on the Button

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Let s keep it simple  What is a call back function   Example by Parable and Analogy  I have a secretary  Everyday I ask her to   i  drop off the firm s outgoing mail at the post office  and after she s done that  to do   ii  whatever task I wrote for her on one of those sticky notes    Now  what is the task on the sticky-note  The task varies from day to day   Suppose on this particular day  I require her to print off some documents  So I write that down on the sticky note  and I pin it on her desk along with the outgoing mail she needs to post   In summary    first  she needs to drop off the mail and  immediately after that is done  she needs to print off some documents    The call back function is that second task  printing off those documents  Because it is done AFTER the mail is dropped off  and also because the sticky note telling her to print the document is given to her along with the mail she needs to post   Let s now tie this in with programming vocabulary   The method name in this case is  DropOffMail   And the call back function is  PrintOffDocuments  The PrintOffDocuments is the call back function because we want the secretary to do that  only after DropOffMail has run  So I would  pass  PrintOffDocuments as an  argument  to the DropOffMail method  This is an important point    That s all it is  Nothing more  I hope that cleared it up for you - and if not  post a comment and I ll do my best to clarify

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A callback is an idea of passing a function as a parameter to another function and have this one invoked once the process has completed   If you get the concept of callback through awesome answers above  I recommend you should learn the background of its idea    What made them Computer-Scientists  develop callback   You might learn a problem  which is blocking  especially blocking UI  And callback is not the only solution to it  There are a lot of other solutions ex  Thread  Futures  Promises

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Callbacks are most easily described in terms of the telephone system  A function call is analogous to calling someone on a telephone  asking her a question  getting an answer  and hanging up  adding a callback changes the analogy so that after asking her a question  you also give her your name and number so she can call you back with the answer    -- Paul Jakubik   Callback Implementations in C

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Callback Function A function which passed to another function as an argument   function test function            alert  Hello world          setTimeout test function  2000     Note  In above example test function used as an argument for setTimeout function

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A callback function  also known as a higher-order function  is a function that is passed to another function as a parameter  and the callback function is called  or executed  inside the parent function       button 1   click function       alert  button 1 Clicked          Here we have pass a function as a parameter to the click method  And the click method will call  or execute  the callback function we passed to it

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A layman response would be that it is a function that is not called by you but rather by the user or by the browser after a certain event has happened or after some code has been processed

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The simple answer to this  question is that a callback function is a function that is called through a function pointer  If you pass the pointer  address  of a function as an argument to another  when that pointer is used to call the function it points to it is said that a call back is made

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A callback function is one that should be called when a certain condition is met  Instead of being called immediately  the callback function is called at a certain point in the future   Typically it is used when a task is being started that will finish asynchronously  ie will finish some time after the calling function has returned    For example  a function to request a webpage might require its caller to provide a callback function that will be called when the webpage has finished downloading

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A callback function is a function you pass  as a reference or a pointer  to a certain function or object   This function or object will call this function back any time later  possibly multiple times  for any kind of purpose      notifying the end of a task requesting comparison between two item  like in c qsort    reporting progress of a process notifying events delegating the instanciation of an object delegating the painting of an area        So describing a callback as a function being called at the end of another function or task is overly simplifying  even if it s a common use case

[language-agnostic] What is a callback function?

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