I have a a fragment containing a googleMap where I am creating a bunch of Markers (which also is clickable). They are spiced with different information (colors, shapes and so on) from a room livedata query. In addition I have some MaterialButton buttons (which are styled as pushbuttons) where I toggle the Marker visible status on. At the moment, the "setup" of theese markers takes some time (200ms-2 secs, depends of amount of markers). To get out of that waiting, I was planning to use a viewmodelscope. Since there are some clicklisteners for theese buttons defined in there (they should do some action with the markers), will they still be alive when the viewmodelscope coroutine section ends, and If they are alive, do they still live in the correct coroutine-context, and do I need to do some housekeeping on the listeners when fragment and/or viewmodel ends?
I.E:
class MapsFragment:Fragment(){
private lateinit var mapsViewModel : MapsViewModel
private lateinit var googleMap : GoogleMap
//...
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
mapsViewModel = ViewModelProvider(requireActivity()).get(MapsViewModel::class.java)
_binding = FragmentMapsBinding.inflate(inflater, container, false)
val root:View = binding.root
//...
return root
}//onCreateView
//...
override fun onViewCreated(view: View, savedInstanceState:Bundle?){
super.onViewCreated(view, savedInstanceState)
//...
mapFragment?.getMapAsync(_googleMap->
_googleMap?.let{safeGoogleMap->
googleMap = safeGoogleMap
}?:let{
Log.e(TAG,"googleMap is null!!")
return#getMapAsync
}
//...
mapsViewModel.apply{
liveDataMapsListFromFiltered?.observe(
viewLifecycleOwner
){mapDetailList->
viewModelScope.launch{
binding.apply{
//...
siteMarkers.map{
siteMarker.remove() //removes existing markes from map on update
}
siteMarkers.clear() //empty the siteMarker array on update
//...
mapDetailList?.map{
it.apply{
//...
coordinateSiteLongitude?.let { lng->
coordinateSiteLatitude?.let { lat->
siteMarkerLatLng = LatLng(lat,lng)
siteLatLngBoundsBuilder?.include(siteMarkerLatLng)
}
}
//...
siteMarkerLatLng?.let { safeSiteMarkerLatLng ->
val siteMarkerOptions =
MarkerOptions()
.position(safeSiteMarkerLatLng)
.anchor(0.5f, 0.5f)
.visible(siteMarkerState)
.flat(true)
.title(setTicketNumber(ticketNumber?.toDouble()))
.snippet(appointmentName)//TODO: Consider build siteId instead
.icon(siteIcon[iconType])
siteMarkers.add(
googleMap.addMarker(siteMarkerOptions) //Here are the markers added
)
}//siteMarkerLatLng?.let
}//it.apply
}//mapDetailList?.map
onSiteCheckedChangeListener?.let{
fragmentMapsMapTagSelector
?.apTagSelectorMaterialButtonSite
?.removeOnCheckedChangeListener(it) //clearing listener on button before update
}
onSiteCheckedChangeListener = MaterialButton.OnCheckedChangeListener { siteButton, isChecked ->
siteMarkers.map {
it.isVisible = isChecked
}
}.also {
fragmentMapsMapTagSelector
?.mapTagSelectorMaterialButtonSite
?.addOnCheckedChangeListener(it)
}
//Will this onCheckedChangeListener still survive when this viewmodelscope runs to the end ?
}//binding.apply
}//viewModelScope.launch
}//liveDataMapsListFromFiltered.observe
}//mapsviewModel.apply
}//getMapAsync
}//onViewCreated
}//MapsFragment
I think you misunderstand what a CoroutineScope is. It determines the lifecycle of coroutines that it runs, but not of the objects created in the process of running those coroutines.
viewModelScope is a CoroutineScope that automatically cancels any coroutines it is running when the associated ViewModel is torn down. The coroutine doesn't know what you're doing with it. Cancelling a coroutine merely stops it from running to completion, like returning from a function early. In your code, you set your listeners and haven't stored references to them besides in the views they are set to, so their lives are tied to their respective view's lives.
If you were going to use a coroutine in your fragment to set up something for your UI, you would use the Fragment's lifecycleScope, not the ViewModel's viewModelScope. Like if you were fetching something to show in your UI, you would want that coroutine to be cancelled when the Fragment is destroyed, not the ViewModel which might be outliving the Fragment.
Your use of a coroutine in your example code looks pointless, because I don't see any blocking or asynchronous suspend functions being called. You mentioned setting up site markers is taking like 200ms. I'm not familiar with Google Maps since I haven't used it in the past several years, so I'm not sure which part is time-consuming. Usually, UI elements do not allow you to interact with them on background threads, so you might be out of luck. But maybe the time-consuming part is allowed to be done on background threads. You'll have to read the documentation. Using a coroutine for this won't make it take less time, but can prevent the UI from stuttering/freezing.
If you were going to do some long computation with a coroutine, you would need to switch dispatchers to do the blocking work and interact with the UI elements back on the main dispatcher. Simply putting something in a coroutine doesn't make it take less time, but it provides a convenient way to do something on another thread and then continue on the main thread after the result is ready. For example:
lifecycleScope.launchWhenStarted { // lifecycle coroutines launch on main thread by default
val result = withContext(Dispatchers.Default) { // switch to dispatcher for background work
doTimeConsumingCalculation()
}
// back on main thread:
applyResultsToMyViews(result)
}
By using launchWhenStarted instead of launch, a Fragment's lifecycleScope will pause the coroutine when the Fragment is not attached, which will prevent potential crashes from trying to update UI using requireContext() or requireActivity() when there is no Activity.
I have been dealing with this problem for days already. I am at my wits' end!
I can't seem to find a definitive answer anywhere on any of the forums, documentation, etc.
Everything looks fine at first run, or when I load a next level for the user to play. But if the user hits the ESC key to load a different level, the ENTER FRAME listener does not get removed and it duplicates all the triggers in it, showing the player going really fast, and all funky, because it builds on top of the previously instantiated ENTER FRAME listener.
I don't know if I have a problem of an anonymous function, or an unknown instance being referenced in my removeEvent... command... Bottom line, I give up and I need this working HELP!!!
Here's the code:
function initPlay():void
{
//code here determining what display object to add to the list and assign it to the currentLevel variable (a movieclip)
if(userIsLoadingOtherLevel){
removeEnterFrameListener();
addChild(currentLevel);
}
if(userIsGointToNextLevel)
addChild(currentLevel);
currentLevel.addEventListener(Event.ENTER_FRAME, onEnterFrame);
function onEnterFrame(event:Event):void
{
//collision detection, parallax scrolling, etc, etc is done here.
if(allCoinsCollected)
loadNextLevel();
if(ESCKeyPressed)
ESCKeyPressHandler();
}
function loadNextLevel():void
{
removeChild(currentLevel);
newLevelToLoad++
removeEnterFrameListener();
initPlay();
}
function ESCKeyPressHandler():void
{
removeChild(currentLevel);
initPlay();
}
function removeEnterFrameListener();
{
currentLevel.removeEventListener(Event.ENTER_FRAME,onEnterFrame)
trace("currentLevel.hasEventListener(Event.ENTER_FRAME) = "+currentLevel.hasEventListener(Event.ENTER_FRAME)); //outputs TRUE if called from loadNextLevel but FALSE if called from initPlay() !!!
}
}
I also tried to add and remove the eventListener to stage, MovieClip(Root), or nothing at all and the result is always the same.
I know that there may be other ways to design such a process, but please note I am not really flexible at the moment on doing this because the project is very long (about 4000 lines of code) and removing the ENTER FRAME this way, crazy or not should still work!!
THANK YOU in advance for anyone willing to help.
The problem appears to be the nested functions inside the initPlay() method.
Each time you call initPlay() you are defining new functions. Some of these nested functions call initPlay() themselves.
Functions are objects (memory references). So each time you call initPlay() you are making new references to new functions. So when you try to remove an event listener, you're only able to remove one of these event handlers (the one in the current scope of execution).
I'm not sure if I'm explaining this clearly, perhaps this example will help. I'll use numbers to represent the references to each function, and a simple scenario that is similar to yours:
function example():void
{
addEventListener(MouseEvent.CLICK, mouseClickHandler);
function mouseClickHandler(event:Event):void
{
if (someCondition)
{
example();
}
else
{
removeEventListener(MouseEvent.CLICK, mouseClickHandler);
}
}
}
When we run this function the first time, a new function is defined within the scope of the example() function. Lets use the number 1 to represent the reference to this nested function. someCondition is true on the first time around, and so the example() function is called again.
On the second execution of the example() function, a new reference to the mouse event handler is created (#2). We also add the event listener again. At this point, there are two event handling functions in memory, and both will be executed when the event is dispatched.
Let's say that in the second invocation of example() that someCondition is false and now we want to remove the listener. When we call:
removeEventListener(MouseEvent.CLICK, mouseClickHandler);
It's referring to event handler #2. Event handler #1 still exists, and because it's hidden in the scope of the first invocation of example() it can't be removed here.
My simple example breaks down after this... but I hope it makes it clear why your event handlers shouldn't be nested inside a function. Admittedly, this is difficult to describe and even more so in a real world example like yours. But I'm pretty confident that this is the source of most, if not all, of the issues you describe.
Here's how I was able to get around this without changing the scope of the nested functions (although I agree that would be the preferred solution) by creating a boolean variable called "loadingNewGame" and changing it to true from outside the onEnterFrame (in fact, this assignment was done from initPlay() and then from onEnterframe I called removeEnterFrameListener() function. This did the trick.
here's the code in case anybody is interested:
// package, and other code here.
var loadingNewGame:Boolean = new Boolean(false);
function initPlay():void
{
//code here determining what display object to add to the list and assign
//it to the currentLevel variable (a movieclip)
if(userIsLoadingOtherLevel)
{
loadingNewGame = true;
removeEnterFrameListener();
addChild(currentLevel);
}
if(userIsGointToNextLevel)
addChild(currentLevel);
loadingNewGame:Boolean = false;
currentLevel.addEventListener(Event.ENTER_FRAME, onEnterFrame);
function onEnterFrame(event:Event):void
{
if(loadingNewGame)
removeChild(currentLevel);
//collision detection, parallax scrolling, etc, etc is done here.
if(allCoinsCollected)
loadNextLevel();
if(ESCKeyPressed)
ESCKeyPressHandler();
}
function loadNextLevel():void
{
removeChild(currentLevel);
newLevelToLoad++
removeEnterFrameListener();
initPlay();
}
function ESCKeyPressHandler():void
{
initPlay();
}
function removeEnterFrameListener();
{
currentLevel.removeEventListener(Event.ENTER_FRAME,onEnterFrame)
trace("currentLevel.hasEventListener(Event.ENTER_FRAME) = "+currentLevel.hasEventListener(Event.ENTER_FRAME));
//outputs true
}
I have two function on my AS3 program, one fires when the width and height changes:
stage.addEventListener(Event.RESIZE, resizeListener);
function resizeListener (e:Event):void {
//some commands
}
And the second one fires one a number of milliseconds pass:
var myTimer:Timer = new Timer(clockUpdate, 0);
myTimer.addEventListener(TimerEvent.TIMER, updateData);
myTimer.start();
function updateData(e:TimerEvent):void {
trace("AUTOUPDATE");
trace(e);
}
I need to fires those function also manually, lets say when the user press a button, but i don't know what parameters i have to send them when they are called manually.
I tried just resizeListener() and updateData() but of course it fails asking me for the parameter.
You can make parameters in a function optional by providing a default value. This is an example by taking your two functions above and making the event parameters optional:
function resizeListener(e:Event = null):void {
//some commands
}
and
function updateData(e:TimerEvent = null):void {
trace("AUTOUPDATE");
trace(e);
}
Calling, for example, resizeListener() will now execute the function and the value of e will default to null.
Making the Event parameter optional, resizeListener(e:Event=null), as in walkietokyo's answer, is a perfectly valid and often convenient solution. Another alternative is to put the stuff you want to be able to do without the event being triggered in a separate function, that can be called by the event handler and from anywhere else.
So assuming for example that what you want to do on resize is to rearrange the layout, and you also want to do that same layout setup at initialization, or at the click of a button, or anytime really, you could do something like this:
stage.addEventListener(Event.RESIZE, resizeListener);
function resizeListener(e:Event):void {
rearrangeLayout();
}
function rearrangeLayout():void {
// The actual rearrangement goes here, instead of in resizeListener. This can be called from anywhere.
}
Which way to do it is probably a matter of taste or can vary from case to case, really, both works fine.
A benefit of separating things in an event handler and another function is that there will not arise a situation where you would have to check if the e:Event parameter is null or not. In other words, you would have code that is dependent on the Event, if any, in the event handler, and code that is independent of the Event in a more general function (not an event handler).
So in a more general and schematic case, the structure would be something like this:
addEventListener(Event.SOME_EVENT, eventListener);
function eventListener(e:Event):void {
// Code that needs the Event parameter goes here (if any).
// Call other function(s), for the stuff that needs to be done when the event happens.
otherFunction();
}
function otherFunction():void {
// Stuff that is not dependent on the Event object goes here, an can be called from anywhere.
}
What is a callback function?
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).
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 --> 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.
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.
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.
Simple Explanation by Analogy
Everyday, I get to work. The boss tells me:
Oh, and when you're done with that, I have an extra task for you:
Great. He hands me a note with a task on it - this task is a call back function. It could be anything:
ben.doWork( and_when_finished_wash_my_car)
Tomorrow it could be:
ben.doWork( and_tell_me_how_great_i_am)
The key point is that the call back must be done AFTER I finish work....and that's it!
Now that you understand the concept (hopefully), you would do well to read the code contained in other answers.
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.
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++"
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.
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?
Functions which takes Funs(i.e. functional objects) as arguments, or which return Funs 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
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".
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.
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.
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.
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
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
“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<stdio.h>
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<stdio.h>
#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<stdio.h>
#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<OnClickEventListener> mClickEventListener = new ArrayList<OnClickEventListener>();
ArrayList<OnLongClickEventListener> mLongClickEventListener = new ArrayList<OnLongClickEventListener>();
#Override
public void clickEvent() {
// TODO Auto-generated method stub
Iterator<OnClickEventListener> it = mClickEventListener.iterator();
while(it.hasNext()){
OnClickEventListener li = it.next();
li.onClick(this);
}
}
#Override
public void longClickEvent() {
// TODO Auto-generated method stub
Iterator<OnLongClickEventListener> 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 & 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.
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...).
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).
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.
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.
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.
I'm 13 years late to the game on this answer but after learning it myself I thought I'd drop another answer in here in case anyone is baffled like I was.
The other answers sum up the crux of the question "What is a callback?"
It's just a function that calls another function when something is completed.
What got me was the examples, "You did this now do that."
Like WHY would I use it like that when I can just call a method or a function myself?
So here's a quick, real world example that hopefully makes it "click" for someone.
Ultra pseudocode
First the core issue you'll run into....
Multithreaded Method(Some arguments)
{
Do fancy multithreaded stuff....
}
Main()
{
Some stuff I wanna do = some tasks
Multhreaded Method(Some stuff I wanna do)
}
If you run that without any callback your program will look like it just exits.
Because the "Fancy multithreaded stuff" is running on another process.
So you scratch your head and think "Well hell, How do I know when it's done??"
BOOM... CALLBACK
IsItDone = false
Callback()
{
print("Hey, I'm done")
IsItDone = true
}
Multithreaded Method(Some arguments, Function callback)
{
Do fancy multithreaded stuff....
}
Main()
{
Some stuff I wanna do = some tasks
Multhreaded Method(Some stuff I wanna do,Callback)
while(!IsItDone)
Wait a bit
}
This is 100% not the best way to implement it, I just wanted to give a clear example.
So this isn't the bare "What is a callback?"
It's "What is a callback, and what does it do that benefits me???"