Question

Events are really awesome, and I wouldn’t know what I would do without them, but they’re a mystery to me.

I’m talking about events in a sense, a function(s) is called if a property, or value, a special event happens.

I have only the vaguest idea how these actually work. I know it’s an observer pattern, but I don’t truly know how it works and/or how to implement it.

Can someone explain that to me?

Answer 1

As I understand it you are asking how events work under the covers. You haven’t specified which language/platform you are asking about so I’m going to answer with what I know (.Net), although I’m sure a lot of platforms are similar bear in mind that what I say might not be true everywhere.

I’ll start from the lowest level and work upwards.

Function pointers

In most languages there is the concept of a function pointer. In languages like C++ you can literally store a pointer to the memory address of a method. In functional languages like Lisp or F# functions are key and it’s a crucial part of the language that you can store and pass function references around. In .net, function pointers are implemented using delegates.

Delegates

In .Net events are implemented using delegates. A delegate is a type safe function pointer. It’s a pointer to a function, that is restricted to a specific type, and checked against that type at compile time. You can trigger the delegate and that will call the function it points to.

Multicast

A multicast delegate is a class that forms a collection of delegates. It uses a list internally to store multiple delegates. When you call add or do += you are just adding your new delegate (function pointer) into the multicast’s internal list. Multicast delegate instances can be triggered and it simply moves down the list and triggers internally each delegate in sequence.

Event

An event is just a keyword that adds a few extra restrictions on top of the multicast delegate that underpins the event. For example (amongst other things) by using the event keyword when you declare the multicast delegate instance it restricts it so that it can only be triggered from within the class it is declared in.

So, to sum up. Events are just a list of function pointers. When you subscribe you simple add a pointer to your function to the list. When the event is triggered it simply moves down the list and triggers each function pointer it knows about.

Obviously, like I said at the start, every language/environment will differ, but I wouldn’t be surprised if the idea of maintaining a simple lists of function pointers is probably fairly common.

Jon Skeet has an excellent article on events in .Net that you should read for more information if this is the platform you are interested in.