Is it a function?
Is it a function being called from the source?
Or, is it a function being returned from the destination?
Or, is it just executing a function at the destination?
Or, is it a value returned from a function passed to the destination?
A callback is the building block of asynchronous processing.
Think of it this way: when you call someone and they don't answer, you leave a message and your phone number. Later on, the person calls you back based on the phone number you left.
A callback works in a similar manner.
You ask an API for a long running operation and you provide a method from within your code to be called with the result of the operation. The API does its work and when the result is ready, it calls your callback method.
From the great Wikipedia:
In computer programming, a callback is
executable code that is passed as an
argument to other code. It allows a
lower-level software layer to call a
subroutine (or function) defined in a
higher-level layer.
Said another way, when you pass a callback to your method, it's as if you are providing additional instructions (e.g., what you should do next). An attempt at making a simple human example follows:
Paint this wall this shade of green (where "paint" is analagous to the method called, while "wall" and "green" are similar to arguments).
When you have finished painting, call me at this number to let me know that you're done and I'll tell you what to do next.
In terms of practical applications, one place where you will sometimes see callbacks is in situations with asynchronous message passing. You might want to register a particular message as an item of interest for class B.
However, without something like a callback, there's no obvious way for class A to know that class B has received the message. With a callback, you can tell class B, here's the message that I want you to listen for and this is the method in class A that I want you to call when you receive it.
Here is a Java example of a callback from a related question.
Related
I have seen a few different documentations refer to the term "callbacks" (Flux.jl and some SciML packages) but it is not clear what that means in the context of Julia.
I know that a callback is a function passed as an argument to another function but is there more to it than that? What would the use case for such a paradigm be?
A classic example of use of a callback is the progress bar. A callback is supplied to the function that is doing work within some kind of sequence. At regular intervals during the sequence the callback is called with some kind of information about the work being done (the percent completed in the case of the progress bar, which updates a progress display).
Flux can call a callback each time it completes a segment of training, or in the example in the source code, every 10 seconds:
https://github.com/FluxML/Flux.jl/blob/b78a27b01c9629099adb059a98657b995760b617/src/optimise/train.jl#L80-L95
By reading some text, especially the iOS document about delegate, all the protocol method are called hook that the custom delegate object need to implement. But some other books, name these hook as callback, what is the difference between them? Are they just different name but the same mechanism? In addition to Obj-C, some other programming languages, such as C, also got the hook, same situation with Obj-C?
The terminology here is a bit fuzzy. In general the two attempt to achieve similar results.
In general, a callback is a function (or delegate) that you register with the API to be called at the appropriate time in the flow of processing (e.g to notify you that the processing is at a certain stage)
A hook traditionally means something a bit more general that serves the purpose of modifying calls to the API (e.g. modify the passed parameters, monitor the called functions). In this meaning it is usually much lower level than what can be achieved by higher-level languages like Java.
In the context of iOS, the word hook means the exact same thing as callback above
Let me chime in with a Javascript answer. In Javascript, callbacks, hooks and events are all used. In this order, they are each higher level concepts than the other.
Unfortunately, they are often used improperly which leads to confusion.
Callbacks
From a control flow perspective, a callback is a function, usually given as an argument, that you execute before returning from your function.
This is usually used in asynchoronous situations when you need to wait for I/O (e.g. HTTP request, a file read, a database query etc.). You don't want to wait with a synchronous while loop, so other functions can be executed in the meantime.
When you get your data, you (permanently) relinquish control and call the callback with the result.
function myFunc(someArg, callback) {
// ...
callback(error, result);
}
Because the callback function may be some code that hasn't been executed yet, and you don't know what's above your function in the call stack, generally instead of throwing errors you pass on the error to the callback as an argument. There are error-first and result-first callback conventions.
Mostly callbacks have been replaced by Promises in the Javascript world and since ES2017+, you can natively use async/await to get rid of callback-rich spaghetti code and make asynchronous control flow look like it was synchronous.
Sometimes, in special cascading control flows you run callbacks in the middle of the function. E.g. in Koa (web server) middleware or Redux middleware you run next() which returns after all the other middlewares in the stack have been run.
Hooks
Hooks are not really a well-defined term, but in Javascript practice, you provide hooks when you want a client (API/library user, child classes etc.) to take optional actions at well-defined points in your control flow.
So a hook may be some function (given as e.g. an argument or a class method) that you call at a certain point e.g. during a database update:
data = beforeUpdate(data);
// ...update
afterUpdate(result);
Usually the point is that:
Hooks can be optional
Hooks usually are waited for i.e. they are there to modify some data
There is at most one function called per hook (contrary to events)
React makes use of hooks in its Hooks API, and they - quoting their definition - "are functions that let you “hook into” React state and lifecycle features", i.e. they let you change React state and also run custom functions each time when certain parts of the state change.
Events
In Javascript, events are emitted at certain points in time, and clients can subscribe to them. The functions that are called when an event happens are called listeners - or for added confusion, callbacks. I prefer to shun the term "callback" for this, and use the term "listener" instead.
This is also a generic OOP pattern.
In front-end there's a DOM interface for events, in node.js you have the EventEmitter interface. A sophisticated asynchronous version is implemented in ReactiveX.
Properties of events:
There may be multiple listeners/callbacks subscribed (to be executed) for the same event.
They usually don't receive a callback, only some event information and are run synchronously
Generally, and unlike hooks, they are not for modifying data inside the event emitter's control flow. The emitter doesn't care 'if there is anybody listening'. It just calls the listeners with the event data and then continues right away.
Examples: events happen when a data stream starts or ends, a user clicks on a button or modifies an input field.
The two term are very similar and are sometimes used interchangably. A hook is an option in a library were the user code can link a function to change the behavior of the library. The library function need not run concurrent with the user code; as in a destructor.
A callback is a specific type of hook where the user code is going to initiate the library call, usually an I/O call or GUI call, which gives contol over to the kernel or GUI subsystem. The controlling process then 'calls back' the user code on an interupt or signal so the user code can supply the handler.
Historically, I've seen hook used for interupt handlers and callback used for GUI event handlers. I also see hook used when the routine is to be static linked and callback used in dynamic code.
Two great answers already, but I wanted to throw in one more piece of evidence the terms "hook" and "callback" are the same, and can be used interchangeably: FreeRTOS favors the term "hook" but recognizes "callback" as an equivalent term, when they say:
The idle task can optionally call an application defined hook (or callback) function - the idle hook.
The tick interrupt can optionally call an application defined hook (or callback) function - the tick hook.
The memory allocation schemes implemented by heap_1.c, heap_2.c, heap_3.c, heap_4.c and heap_5.c can optionally include a malloc() failure hook (or callback) function that can be configured to get called if pvPortMalloc() ever returns NULL.
Source: https://www.freertos.org/a00016.html
In the past, I have used libraries that would allow me to register a callback so that the library can call my method when some event happens (e.g. it is common to see in code that use GUI libraries to look like button.onClick(clickHandler)).
Naively, I suppose the library's handling mechanism could be implemented like:
while(1){
if (event1) { event1Handler(); }
if (event2) { event2Handler(); }
...
}
but that would be really wasteful right? Or is that really how it is done (for instance do well known GUI libraries like java swing, or GTK+ do it this way)?
background:
This question hadn't really occured to me until I encountered curses. I thought about implementing my own callback system, until I realized I didn't know how.
The while loop will typically wait for an interrupt from the user (GetMessage in Windows). When an interrupt arrives GetMessage returns and then it ends up in the callback function. The if statements are typically implemented as a switch-case. See Windows Message Loop on Wikipedia.
In more detail, what happens is the following:
The user application calls GetMessage, which forces the process to sleep until an input message for that application arrives from the systems queue. When a message arrives, the user app calls DispatchMessage, which calls the callback function associated with the window that the message was aimed at.
Windows API uses one callback which handles all events in a switch case. Other libraries use one callback per event class instead.
The function pointers themselves are stored together with other window data in a struct.
Callback system implementation probably has different implementation in different technologies, however, I suppose they should be working this way:
A data structure stores the callback IDs and pointers to the handlers.
A callback handler has a validator
Event handlers have callback callers, which know what are the possible callbacks and check their validity this way:
for each callback in event.callbacks
if (callback.isValid())
call callback()
end if
end for
When you add a handler to a function the system will automatically know where the callback is valid and will add the callback to the datastructure described in 1.
Correct me if I'm wrong, this description is just a guess.
I know that a block is a reusable chunk of executable code in Objective-C. Is there a reason I shouldn't put that same chunk of code in a function and just called the function when I need that code to run?
It depends on what you're trying to accomplish. One of the cool things about blocks is that they capture local scope. You can achieve the same end result with a function, but you end up having to do something like pass around a context object full of relevant values. With a block, you can do this:
int num1 = 42;
void (^myBlock)(void) = ^{
NSLog(#"num1 is %d", num1);
};
num1 = 0; // Changed after block is created
// Sometime later, in a different scope
myBlock(); // num1 is 42
So simply by using the variable num1, its value at the time myBlock was defined is captured.
From Apple's documentation:
Blocks are a useful alternative to traditional callback functions for
two main reasons:
They allow you to write code at the point of invocation that is
executed later in the context of the method implementation. Blocks are
thus often parameters of framework methods.
They allow access to local variables. Rather than using callbacks
requiring a data structure that embodies all the contextual
information you need to perform an operation, you simply access local
variables directly.
As Brad Larson comments in response to this answer:
Blocks will let you define actions that take place in response to an
event, but rather than have you write a separate method or function,
they allow you to write the handling code right where you set up the
listener for that event. This can save a mess of code and make your
application much more organized.
A good example of which i can give you is of alert view, it will be good if i decided at time of creation of alert view what will happen when i dismiss that instead i write the delegate method and wait for that to call. So it will be much easier to understand and implement and also it provides fast processing.
While developing a web app using ZF, I had an haha! moment regarding the _forward method in Zend_Controller_Action. As stated in the Programmer's Reference Guide, when calling the _forward method inside an action, the requested action will not be executed until the current action completes. This begs the question:
When would you use the _forward action to intentionally make sure your current action completes before starting another, aside from form processing (although you would probably place the _forward request at the end of the action anyway)? What are some clear cut examples of this? Any pitfalls or advantages to using this approach as apposed to an ActionStack?
_forward() just replaces module/controller/action parameters in Request object.
It just allows to change your mind on the go (without another request).
This has different consequences, depending on which dispatch loop state it is called. Some time setDispatched() is needed to execute.
Consider those scenarios:
First:
$this->_forward('some')
Second:
return $this->_forward('some');
Third:
$this->someAction();
// ececuted?
Fourth:
return $this->someAction();
// executed?
I really only use _forward for two reasons:
I want to redirect but don't want the user's URL to change.
I want to pass some (non-string) object to another action.
$this->_forward('index', null, null, array('create_task_form' => $form));
In each case, the target action can stand by itself, without the originator, and usually just marshals up the display.
When would you use the _forward action to intentionally make sure your current action completes before starting another
I don't think it's used to let the current action complete before _forward() is used. That looks more like a call to your domain logic (model, service, something like that) than handling a request which is what an action is for.
yourAction
if(conditionsAreNotMet()) {
return _forward(anotherAction);
}
I think _forward() is provided to have an early exit point in your action (which logic is all focused on one thing, for example presenting news); without the need of performing another request (like _redirect()) and thus putting more load on the web server.