How to explain callbacks in plain english? How are

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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++"

Answer 4

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.

Answer 5

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:

1. Check the window every 5 minutes to see if Joe is out
2. 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!

Answer 6

[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.