How to explain callbacks in plain english How are they different from calling one function from another function

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How to explain callbacks in plain English  How are they different from calling one function from another function taking some context from the calling function  How can their power be explained to a novice programmer

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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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A callback is a method that is scheduled to be executed when a condition is met   An  real world  example is a local video game store  You are waiting for Half-Life 3  Instead of going to the store every day to see if it is in  you register your email on a list to be notified when the game is available  The email becomes your  callback  and the condition to be met is the game s availability   A  programmers  example is a web page where you want to perform an action when a button is clicked  You register a callback method for a button and continue doing other tasks  When if the user cicks on the button  the browser will look at the list of callbacks for that event and call your method   A callback is a way to handle events asynchronously  You can never know when the callback will be executed  or if it will be executed at all  The advantage is that it frees your program and CPU cycles to perform other tasks while waiting for the reply

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A callback is a function that will be called by a second function  This second function doesn t know in advance what function it will call   So the identity of the callback function is stored somewhere  or passed to the second function as a parameter   This  identity   depending on the programming language  might be the address of the callback  or some other sort of pointer  or it might be the name of the function   The principal is the same  we store or pass some information that unambiguously identifies the function   When the time comes  the second function can call the callback  supplying parameters depending on the circumstances at that moment   It might even choose the callback from a set of possible callbacks   The programming language must provide some kind of syntax to allow the second function to call the callback  knowing its  identity    This mechanism has a great many possible uses   With callbacks  the designer of a function can let it be customized by having it call whatever callbacks are provided   For example  a sorting function might take a callback as a parameter  and this callback might be a function for comparing two elements to decide which one comes first   By the way  depending on the programming language  the word  function  in the above discussion might be replaced by  block    closure    lambda   etc

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edited when we have two functions say functionA and functionB if functionA depends on functionB   then we call functionB as a callback function this is widely used in Spring framework

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Callbacks allows you to insert your own code into another block of code to be executed at another time  that modifies or adds to the behavior of that other block of code to suit your needs  You gain flexibility and customizability while being able to have more maintainable code    Less hardcode   easier to maintain and change   less time   more business value   awesomeness   For example  in javascript  using Underscore js  you could find all even elements in an array like this   var evens     filter  1  2  3  4  5  6   function num   return num   2    0        gt   2  4  6    Example courtesy of Underscore js  http   documentcloud github com underscore  filter

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Callback functions    We define a callback function named callback give it a parameter otherFunction and invoke it inside the function body   function callback otherFunction       otherFunction        When we invoke callback function it expects an argument of the type function  so we invoke it with an anonymous function  However  it produces an error if the argument is not of type function    callback function   console log  SUCCESS        callback 1      error   Baking the pizza example   oven to bake pizza base topped with ingredients  Now here  oven is the callback function   and pizza base with ingredients is the otherFunction   Point to note is that different pizza ingredients produce different types of pizzas but the oven which bakes it stays the same   That is somewhat a job of a callback function  which keeps expecting functions with different functionalities in order to produce different custom outcomes

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For teaching callbacks  you have to teach the pointer first  Once the students understand the idea of pointer to a variable  idea of callbacks will get easier  Assuming you are using C C    these steps can be followed    First show your students how to use and manipulate variables using pointers alongside using the normal variable identifiers  Then teach them there are things that can be done only with pointers like passing a variable by reference   Then tell them how executable code or functions are just like some other data or variables  in the memory  So  functions also have addresses or pointers  Then show them how functions can be called with function pointers and tell these are called callbacks  Now  the question is  why all these hassle for calling some functions  What is the benefit  Like data pointers  function pointer aka callbacks has some advantages over using normal identifiers  The first one is  function identifiers or function names cannot be used as normal data  I mean  you cannot make a data structure with functions like an array or a linked list of functions   But with callbacks  you can make an array  a linked list or use them with other data like in dictionary of key-value pairs or trees  or any other things  This is a powerful benefit  And other benefits are actually child of this one  The most common use of callbacks is seen in event driver programming  Where one or more functions are executed based on some incoming signal  With callbacks  a dictionary can be maintained to map signals with callbacks  Then the input signal resolution and execution of corresponding code become much easier  The second use of callbacks coming in my mind is higher order functions  The functions which takes other functions as input arguments  And to send functions as arguments  we need callbacks  An example can be a function which take an array and a callback  Then it performs the callback on each of the item of the array and return the results in another array  If we pass the function a doubling callback  we get a doubled valued array  If we pass a squaring callback  we get squares  For square roots  just send appropriate callback  This cannot be done with normal functions    There might many more things  Involve the students and they will discover  Hope this helps

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Usually we sent variables to functions     Suppose you have task where the variable needs to be processed before being given as an argument - you can use callback    function1 var1  var2  is the usual way    What if I want var2 to be processed and then sent as an argument  function1 var1  function2 var2    This is one type of callback - where function2 executes some code and returns a variable back to the initial function

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Without callback neither others special programming resources  like threading  and others   a program is exactly a sequence of instructions which are executed sequentially one after the other  and even with a kind of  dynamic behavior  determined by certain conditions  all possible scenarios shall be previously programmed   So  If we need to provide a real dynamic behavior to a program we can use callback  With callback you can instructs by parameters  a program to call an another program providing some previously defined parameters and can expects some results  this is the contract or operation signature   so these results can be produced processed by third-party program which wasn t previously known   This technique is the foundation of polymorphism applied to programs  functions  objects and all others unities of code ran by computers   The human world used as example to callback is nice explained when you are doing some job  lets suppose you are a painter  here you are the main program  that paints  and call your client sometimes to ask him to approve the result of your job  so  he decides if the picture is good  your client is the third-party program    In the above example you are a painter and  delegate  to others the job to approve the result  the picture is the parameter  and each new client  the called-back  function   changes the result of your work deciding what he wants about the picture  the decision made by the clients are the returned result from the  callback function     I hope this explanation can be useful

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In plain english a callback is a promise  Joe  Jane  David and Samantha share a carpool to work  Joe is driving today  Jane  David and Samantha have a couple of options    Check the window every 5 minutes to see if Joe is out Keep doing their thing until Joe rings the door bell    Option 1  This is more like a polling example where Jane would be stuck in a  loop  checking if Joe is outside  Jane can t do anything else in the mean time   Option 2  This is the callback example  Jane tells Joe to ring her doorbell when he s outside  She gives him a  function  to ring the door bell  Joe does not need to know how the door bell works or where it is  he just needs to call that function i e  ring the door bell when he s there   Callbacks are driven by  events   In this example the  event  is Joe s arrival  In Ajax for example events can be  success  or  failure  of the asynchronous request and each can have the same or different callbacks    In terms of JavaScript applications and callbacks  We also need to understand  closures  and application context  What  this  refers to can easily confuse JavaScript developers  In this example within each person s  ring the door bell    method callback there might be some other methods that each person need to do based on their morning routine ex   turn off the tv     We would want  this  to refer to the  Jane  object or the  David  object so that each can setup whatever else they need done before Joe picks them up  This is where setting up the callback with Joe requires parodying the method so that  this  refers to the right object   Hope that helps

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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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Think of a method as giving a task to a coworker  A simple task might be the following   Solve these equations  x   2   y 2   x   3   y   Your coworker diligently does the math and gives you the following result   x   -6 y   -4   But your coworker has a problem  he doesn t always understand notations  such as    but he does understand them by their description  Such as exponent  Everytime he finds one of these you get back the following   I don t understand       This requires you to rewrite your entire instruction set again after explaining what the character means to your coworker  and he doesn t always remember in between questions  And he has difficulty remembering your tips as well  such as just ask me  He always follows your written directions as best he can however   You think of a solution  you just add the following to all of your instructions   If you have any questions about symbols  call me at extension 1234 and I will tell you its name    Now whenever he has a problem he calls you and asks  rather than giving you a bad response and making the process restart

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I am going to try to keep this dead simple  A  callback  is any function that is called by another function which takes the first function as a parameter  A lot of the time  a  callback  is a function that is called when something happens  That something can be called an  event  in programmer-speak   Imagine this scenario  you are expecting a package in a couple of days  The package is a gift for your neighbor  Therefore  once you get the package  you want it brought over to the neighbors  You are out of town  and so you leave instructions for your spouse    You could tell them to get the package and bring it to the neighbors  If your spouse was as stupid as a computer  they would sit at the door and wait for the package until it came  NOT DOING ANYTHING ELSE  and then once it came they would bring it over to the neighbors  But there s a better way  Tell your spouse that ONCE they receive the package  they should bring it over the neighbors  Then  they can go about life normally UNTIL they receive the package   In our example  the receiving of the package is the  event  and the bringing it to the neighbors is the  callback   Your spouse  runs  your instructions to bring the package over only when the package arrives  Much better   This kind of thinking is obvious in daily life  but computers don t have the same kind of common sense  Consider how programmers normally write to a file   fileObject   open file    now that we have WAITED for the file to open  we can write to it fileObject write  We are writing to the file      now we can continue doing the other  totally unrelated things our program does   Here  we WAIT for the file to open  before we write to it  This  blocks  the flow of execution  and our program cannot do any of the other things it might need to do  What if we could do this instead     we pass writeToFile  A CALLBACK FUNCTION   to the open function fileObject   open file  writeToFile    execution continues flowing -- we don t wait for the file to be opened   ONCE the file is opened we write to it  but while we wait WE CAN DO OTHER THINGS    It turns out we do this with some languages and frameworks  It s pretty cool  Check out Node js to get some real practice with this kind of thinking

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What Is a Callback Function   The simple answer to this first 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   Callback function is hard to trace  but sometimes it is very useful  Especially when you are designing libraries  Callback function is like asking your user to gives you a function name  and you will call that function under certain condition   For example  you write a callback timer  It allows you to specified the duration and what function to call  and the function will be callback accordingly     Run myfunction   every 10 seconds for 5 times      Or you can create a function directory  passing a list of function name and ask the library to callback accordingly     Callback success   if success  callback fail   if failed       Lets look at a simple function pointer example  void cbfunc          printf  called        int main                             function pointer           void   callback  void                          point to your callback function           callback  void   cbfunc                         perform callback          callback          return 0       How to pass argument to callback function   Observered that function pointer to implement callback takes in void    which indicates that it can takes in any type of variable including structure  Therefore you can pass in multiple arguments by structure    typedef struct myst        int a       char b 10    myst   void cbfunc myst  mt          fprintf stdout  called  d  s   mt- gt a mt- gt b       int main                func pointer        void   callback  void             param      myst m       m a 10       strcpy m b  123                callback    void  cbfunc        point to callback function         callback  amp m                     perform callback and pass in the param         return 0

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You have some code you want to run   Normally  when you call it you are then waiting for it to be finished before you carry on  which can cause your app to go grey produce a spinning time for a cursor      An alternative method is to run this code in parallel and carry on with your own work   But what if your original code needs to do different things depending on the response from the code it called   Well  in that case you can pass in the name location of the code you want it to call when it s done   This is a  call back    Normal code  Ask for Information- Process Information- Deal with results of Processing- Continue to do other things   With callbacks  Ask for Information- Process Information- Continue to do other things  And at some later point- Deal with results of Processing

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Let s pretend you were to give me a potentially long-running task  get the names of the first five unique people you come across  This might take days if I m in a sparsely populated area  You re not really interested in sitting on your hands while I m running around so you say   When you ve got the list  call me on my cell and read it back to me  Here s the number     You ve given me a callback reference--a function that I m supposed to execute in order to hand off further processing   In JavaScript it might look something like this   var lottoNumbers       var callback   function theNames      for  var i 0  i lt theNames length  i          lottoNumbers push theNames i  length           db executeQuery  SELECT name                      FROM tblEveryOneInTheWholeWorld                      ORDER BY proximity DESC                      LIMIT 5   callback    while  lottoNumbers length  lt  5      playGolf      playLotto lottoNumbers     This could probably be improved in lots of ways  E g   you could provide a second callback  if it ends up taking longer than an hour  call the red phone and tell the person that answers that you ve timed out

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I m stunned to see so many intelligent people failing to stress the reality that the word  callback  has come to be used in two inconsistent ways   Both ways involve the customization of a function by passing additional functionality  a function definition  anonymous or named  to an existing function  ie    customizableFunc customFunctionality    If the custom functionality is simply plugged into the code block  you have customized the function  like so       customizableFucn customFunctionality          var data   doSomthing          customFunctionality data                     Though this kind of injected functionality is often called a  callback   there is nothing contingent about it  A very obvious example is the forEach method in which a custom function is supplied as an argument to be applied to each element in an array to modify the array   But this is fundamentally distinct from the use of  callback  functions for asynchronous programming  as in AJAX or node js or simply in assigning functionality to user interaction events  like mouse clicks    In this case  the whole idea is to wait for a contingent event to occur before executing the custom functionality  This is obvious in the case of user interaction  but is also important in i o  input output  processes that can take time  like reading files from disk   This is where the term  callback  makes the most obvious sense  Once an i o process is started  like asking for a file to be read from disk or a server to return data from an http request  an asynchronous program doesn t wait around for it to finish  It can go ahead with whatever tasks are scheduled next  and only respond with the custom functionality after it has been notified that the read file or http request is completed  or that it failed  and that the data is available to the custom functionality   It s like calling a business on the phone and leaving your  callback  number  so they can call you when someone is available to get back to you  That s better than hanging on the line for who knows how long and not being able to attend to other affairs   Asynchronous use inherently involves some means of listening for the desired event  e g  the completion of the i o process  so that  when it occurs  and only when it occurs  the custom  callback  functionality is executed   In the obvious AJAX example  when the data actually arrives from the server  the  callback  function is triggered to use that data to modify the DOM and therefore redraw the browser window to that extent   To recap   Some people use the word  callback  to refer to any kind of custom functionality that can be injected into an existing function as an argument   But  at least to me  the most appropriate use of the word is where the injected  callback  function is used asynchronously -- to be executed only upon the occurrence of an event that it is waiting to be notified of

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In non-programmer terms  a callback is a fill-in-the-blank in a program   A common item on many paper forms is  Person to call in case of emergency   There is a blank line there  You write in someone s name and phone number  If an emergency occurs  then that person gets called    Everyone gets the same blank form  but Everyone can write a different emergency contact number    This is key  You do not change the form  the code  usually someone else s   However you can fill in missing pieces of information  your number    Example 1   Callbacks are used as customized methods  possibly for adding to changing a program s behavior  For example  take some C code that performs a function  but does not know how to print output  All it can do is make a string  When it tries to figure out what to do with the string  it sees a blank line   But  the programmer gave you the blank to write your callback in   In this example  you do not use a pencil to fill in a blank on a sheet of paper  you use the function set print callback the callback     The blank variable in the module code is the blank line  set print callback is the pencil  and the callback is your information you are filling in    You ve now filled in this blank line in the program  Whenever it needs to print output  it will look at that blank line  and follow the instructions there  i e  call the function you put there   Practically  this allows the possibility of printing to screen  to a log file  to a printer  over a network connection  or any combination thereof  You have filled in the blank with what you want to do   Example 2   When you get told you need to call an emergency number  you go and read what is written on the paper form  and then call the number you read  If that line is blank nothing will be done    Gui programming works much the same way  When a button is clicked  the program needs to figure out what to do next  It goes and looks for the callback  This callback happens to be in a blank labeled  Here s what you do when Button1 is clicked   Most IDEs will automatically fill in the blank for you  write the basic method  when you ask it to  e g  button1 clicked   However that blank can have any method you darn well please  You could call the method run computations or butter the biscuits as long as you put that callback s name in the proper blank  You could put  555-555-1212  in the emergency number blank  It doesn t make much sense  but it s permissible     Final note  That blank line that you re filling in with the callback  It can be erased and re-written at will   whether you should or not is another question  but that is a part of their power

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I think it s an rather easy task to explain   At first callback are just ordinary functions  And the further is  that we call this function  let s call it A  from inside another function  let s call it B     The magic about this is that I decide  which function should be called by the function from outside B    At the time I write the function B I don t know which callback function should be called   At the time I call function B I also tell this function to call function A  That is all

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There s two points to explain  one is how a callback works  passing around a function that can be called without any knowledge of its context   the other what it s used for  handling events asynchronously    The analogy of waiting for a parcel to arrive that has been used by other answers is a good one to explain both  In a computer program  you would tell the computer to expect a parcel  Ordinarily  it would now sit there and wait  and do nothing else  until the parcel arrives  possibly indefinitely if it never arrives  To humans  this sounds silly  but without further measures  this is totally natural to a computer   Now the callback would be the bell at your front door  You provide the parcel service with a way to notify you of the parcel s arrival without them having to know where  even if  you are in the house  or how the bell works   For instance  some  bells  actually dispatch a phone call   Because you provided a  callback function  that can be  called  at any time  out of context  you can now stop sitting at the front porch and  handle the event   of parcel arrival  whenever it s time

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Imagine a friend is leaving your house  and you tell her  Call me when you get home so that I know you arrived safely   that is  literally  a call back  That s what a callback function is  regardless of language  You want some procedure to pass control back to you when it has completed some task  so you give it a function to use to call back to you   In Python  for example   grabDBValue   lambda x  passValueToGUIWindow x       grabDBValue could be written to only grab a value from a database and then let you specify what to actually do with the value  so it accepts a function  You don t know when or if grabDBValue will return  but if when it does  you know what you want it to do  Here  I pass in an anonymous function  or lambda  that sends the value to a GUI window  I could easily change the behavior of the program by doing this   grabDBValue   lambda x  passToLogger x       Callbacks work well in languages where functions are first class values  just like the usual integers  character strings  booleans  etc  In C  you can  pass  a function around by passing around a pointer to it and the caller can use that  in Java  the caller will ask for a static class of a certain type with a certain method name since there are no functions   methods   really  outside of classes  and in most other dynamic languages you can just pass a function with simple syntax   Protip   In languages with lexical scoping  like Scheme or Perl  you can pull a trick like this   my  var   2  my  val   someCallerBackFunction sub callback   return  var   3        Perlistas note  I know the sub doesn t need a name  this is for illustration    val in this case will be 6 because the callback has access to the variables declared in the lexical environment where it was defined  Lexical scope and anonymous callbacks are a powerful combination warranting further study for the novice programmer

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You feel ill so you go to the doctor  He examines you and determines you need some medication  He prescribes some meds and calls the prescription into your local pharmacy  You go home  Later your pharmacy calls to tell you your prescription is ready  You go and pick it up

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Plain and simple  A callback is a function that you give to another function  so that it can call it   Usually it is called when some operation is completed  Since you create the callback before giving it to the other function  you can initialize it with context information from the call site  That is why it is named a call back  - the first function calls back into the context from where it was called

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This of it in terms of downloading a webpage   Your program runs on a cellphone and is requesting the webpage http   www google com  If you write your program synchronously  the function you write to download the data will be running continuously until all the data is download  This means your UI will not refresh and will basically appear frozen  If you write your program using callbacks  you request the data and say  execute this function when you ve finished   This allows the UI to still allow user interaction while the file is downloading  Once the webpage has finished downloading  your result function  callback  is called and you can handle the data   Basically  it allows you to request something and continue executing while waiting for the result  Once the result comes back to you via a callback function  you can pick up the operation where it left off

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A callback is a self-addressed stamped envelope   When you call a function  that is like sending a letter   If you want that function to call another function you provide that information in the form of a reference or address

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Often an application needs to execute different functions based upon its context state  For this  we use a variable where we would store the information about the function to be called   According to its need the application will set this variable with the information about function to be called and will call the function using the same variable   In javascript  the example is below  Here we use method argument as a variable where we store information about function   function processArray arr  callback        var resultArr   new Array         for  var i   arr length-1  i  gt   0  i--          resultArr i    callback arr i        return resultArr     var arr    1  2  3  4   var arrReturned   processArray arr  function arg   return arg   -1        arrReturned would be  -1  -2  -3  -4

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Johny the programmer needs a stapler  so he goes down to the office supply department  and ask for one  after filling the request form he can either stand there and wait for the clerk go look around the warehouse for the stapler  like a blocking function call  or go do something else meantime   since this usually takes time  johny puts a note together with the request form asking them to call him when the stapler is ready for pickup  so meantime he can go do something else like napping on his desk

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in php it would be something like    lt  php  function string  string   callback          results   array           upper    gt  strtoupper  string            lower    gt  strtolower  string                   if is callable  callback             call user func  callback   results            string  Alex   function  name        echo  name  lower

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Always better to start with an example       Let s assume you have two modules A and B    You want module A to be notified when some event condition occurs in module B  However  module B has no idea about your module A  All it knows is an address to a particular function  of module A  through a function pointer that is provided to it by module A    So all B has to do now  is  callback  into module A when a particular event condition occurs by using the function pointer  A can do further processing inside the callback function      A clear advantage here is that you are abstracting out everything about module A from module B  Module B does not have to care who what module A is

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How to explain callbacks in plain English     In plain English  a callback function is like a Worker who  calls back  to his Manager when he has completed a Task       How are they different from calling one function from another function   taking some context from the calling function    It is true that you are calling a function from another function  but the key is that the callback is treated like an Object  so you can change which Function to call based on the state of the system  like the Strategy Design Pattern       How can their power be explained to a novice programmer    The power of callbacks can easily be seen in AJAX-style websites which need to pull data from a server  Downloading the new data may take some time  Without callbacks  your entire User Interface would  freeze up  while downloading the new data  or you would need to refresh the entire page rather than just part of it  With a callback  you can insert a  now loading  image and replace it with the new data once it is loaded   Some code without a callback   function grabAndFreeze         showNowLoading true       var jsondata   getData  http   yourserver com data messages json           User Interface  freezes  while getting data        processData jsondata       showNowLoading false       do other stuff       not called until data fully downloaded    function processData jsondata       do something with the data    var count   jsondata results   jsondata results length   0         counter messages   text   Fetched   count   new items   join               results messages   html jsondata results      no new messages         With Callback   Here is an example with a callback  using jQuery s getJSON   function processDataCB jsondata       callback  update UI with results    showNowLoading false      var count   jsondata results   jsondata results length   0         counter messages   text   Fetched   count   new items   join               results messages   html jsondata results      no new messages        function grabAndGo        and don t freeze     showNowLoading true           results messages   html now loading image         getJSON  http   yourserver com data messages json   processDataCB          Call processDataCB when data is downloaded  no frozen User Interface         do other stuff       called immediately     With Closure   Often the callback needs to access state from the calling function using a closure  which is like the Worker needing to get information from the Manager before he can complete his Task  To create the closure  you can inline the function so it sees the data in the calling context      Grab messages  chat users  etc by changing dtable  Run callback cb when done    function grab dtable  cb         if  null    dtable    dtable    messages         var uiElem         dtable      showNowLoading true  dtable           results    uiElem  html now loading image         getJSON  http   yourserver com user   dtable   json   cb    function  jsondata              Using a closure  can  see  dtable argument and uiElem variables above         var count   jsondata results   jsondata results length   0              counterMsg     Fetched   count   new   dtable  join                     no new chatters messages etc            defaultResultsMsg      no new    dtable       join              showNowLoading false  dtable              counter    uiElem  text counterMsg              results   uiElem  html jsondata results    defaultResultsMsg                  User Interface calls cb when data is downloaded         do other stuff       called immediately     Usage      update results chatters when chatters json data is downloaded  grab  chatters        update results messages when messages json data is downloaded grab  messages        call myCallback jsondata  when  history json  data is loaded  grab  history   myCallback      Closure  Finally  here is a definition of closure from Douglas Crockford   Functions can be defined inside of other functions  The inner function has access to the vars and parameters of the outer function  If a reference to an inner function survives  for example  as a callback function   the outer function s vars also survive   See also    http   javascript crockford com survey html http   api jquery com jQuery when  http   api jquery com jQuery getJSON  http   github com josher19 jQuery-Parse

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A metaphorical explanation   I have a parcel I want delivered to a friend  and I also want to know when my friend receives it   So I take the parcel to the post office and ask them to deliver it  If I want to know when my friend receives the parcel  I have two options    a  I can wait at the post office until it is delivered    b  I will get an email when it is delivered   Option  b  is analogous to a callback

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Imagine you need a function that returns 10 squared so you write a function   function tenSquared    return 10 10     Later you need 9 squared so you write another function   function nineSquared    return 9 9     Eventually you will replace all of these with a generic function   function square x   return x x     The exact same thinking applies for callbacks  You have a function that does something and when done calls doA   function computeA                doA result       Later you want the exact same function to call doB instead you could duplicate the whole function   function computeB                doB result       Or you could pass a callback function as a variable and only have to have the function once   function compute callback               callback result       Then you just have to call compute doA  and compute doB    Beyond simplifying code  it lets asynchronous code let you know it has completed by calling your arbitrary function on completion  similar to when you call someone on the phone and leave a callback number

[function] How to explain callbacks in plain english? How are they different from calling one function from another function?

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