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
Related
I'm using hunchentoot session values to make my server code re-entrant. Problem is that session values are, by definition, retained during the session, i.e., from one call from the same browser to the next, whereas what I really am looking for is what amount to thread-specific re-entrancy, so that all the values disappear between calls -- I want to treat each click as a separate "from scratch" event, even if they are from the same session . Easy enough to have the driver either set to nil, or delete my session values, but I'm wondering if there's a "correct" way to do this? I don't see any thread-based analog to hunchentoot:session-value in the documentation.
Thanks in advance for any guidance you can offer.
If you want a value to be "thread specific" and at the same time to be "from scratch" on every request, that requires that every request must be dispatched in a brand new thread. This is not the case according to the Hunchentoot documentation, which says that two models are supported: a single-threaded taskmaster and a thread-per-connection taskmaster.
If your configuration is multi-threaded, then a thread-specific variable bound in a request-handling can therefore be expected to be per-connection. In a single-threaded Hunchentoot setup, it will effectively be global, tied to the request servicing thread.
A thread-based analog to hunchentoot:session-value probably doesn't exist because it would only introduce behaviors into the web app which surprisingly change if the threading model is reconfigured, or if the request pattern from the browser changes. A browser can make multiple requests using the same connection, or close the connection between requests.
To extend the request objects with custom per-request, I would look into, perhaps, subclassing from the acceptor (how to do this is described in the docs). My custom acceptor would have a custom method of the process-connection generic function which would create extended/subclasses request objects carrying the extra stuff I wanted to put into a request.
Another way would be to have some global weak hash which binds request objects as keys to additional information.
I'm having a difficult time trying to understand how the control flow in protractor work in relation to how JS event loop works. Here is what I know so far:
Protractor control flow stores commands that return promises in a queue. The first command will be at the front of the queue and the last command will be at the back. No command will be executed until the command in front of it has its promise resolved.
JS event loop stores asynchronous task (callbacks to be specific). Callbacks are not executed until all functions in the stack have completed and the stack is empty. Before running each callback, there is a check on whether the stack is empty or not.
so lets take this code for example. The code is basically clicking a search button and a api request is made. Then after data is returned, it checks whether the field that stores the returned data exists.
elem('#searchButton').click(); //will execute a api call to retrieve data
browser.wait(ExpectedConditions.presenceOf(elem('#resultDataField'),3000));
expect(elem('#resultDataField').isPresent()).toBeTruthy();
So with this code, I'm able to get it to work. But I don't know how it does it. How is the event loop applied in this scenario?
The core of the ControlFlow implementation is in runEventLoop_ (in Selenium's promise.js implementation).
As I understand it, the ControlFlow registers a call to runEventLoop_ with the JS event loop (e.g., with a 0-second timeout or somesuch). The call to runEventLoop_ can be thought of as a single iteration of a normal event loop. It registers code to actually run a scheduled task (i.e., actually do the work you queued up during your it). Once that task completes or fails (e.g., by hooking its async promise callbacks) the next iteration of runEventLoop_ is scheduled (see the calls to scheduleEventLoop in runEventLoop_).
There is some complexity when a callback ends up registering new promises (those need to be "inserted" before the old next event, this is accomplished by creating a "nested" control flow. Mostly you should never have to know this.)
While re-reading scala.lan.org's page detailing Future here, I have stumbled up on the following sentence:
In the event that some of the callbacks never complete (e.g. the callback contains an infinite loop), the other callbacks may not be executed at all. In these cases, a potentially blocking callback must use the blocking construct (see below).
Why may the other callbacks not be executed at all? I may install a number of callbacks for a given Future. The thread that completes the Future, may or may not execute the callbacks. But, because one callback is not playing footsie, the rest should not be penalized, I think.
One possibility I can think of is the way ExecutionContext is configured. If it is configured with one thread, then this may happen, but that is a specific behaviour and a not generally expected behaviour.
Am I missing something obvious here?
Callbacks are called within an ExecutionContext that has an eventually limited number of threads - if not by the specific context implementation, then by the underlying operating system and/or hardware itself.
Let's say your system's limit is OS_LIMIT threads. You create OS_LIMIT + 1 callbacks. From those, OS_LIMIT callbacks immediately get a thread each - and none ever terminate.
How can you guarantee that the remaining 1 callback ever gets a thread?
Sure, there could be some detection mechanisms built into the Scala library, but it's not possible in the general case to make an optimal implementation: maybe you want the callback to run for a month.
Instead (and this seems to be the approach in the Scala library), you could provide facilities for handling situations that you, the developer, know are risky. This removes the element of surprise from the system.
Perhaps most importantly - it enables the developer to "bake in" the necessary information about handler/task characteristics directly into his/her program, rather than relying on some obscure piece of language functionality (which may change from version to version).
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.
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.