I think the C# way is the best approach as far as language features go. Being able to declare a funcdef / delegate type in-place would be a huge help, as would the ability to take a handle to a member function by storing a "this" pointer if necessary.

It could be a problem if the object that owns the method is destroyed before the delegate, though (I think this is what InvalidPointer is talking about with regards to garbage collection). It probably wouldn't be a good idea to have the delegate keep a strong reference to the object to keep it alive. Maybe there could be a way to invalidate the delegate when its object is destroyed, so calling it would throw an "Unbound function" exception just like calling an empty function handle.

I agree with the decision to keep multicasting out of the engine proper. With the addition of a function call operator it would be reasonably simple to create a custom class that acted as a multicast delegate.

I made some delegates system in C++ that I exposed to Angel script in my project with minor modifications of a v2.22.0 WIP of AS (in order to be able to specify a method adress).

This delegate system works with events and subscribers, only subscribers has been exposed to my angelscript (ie AS can catch some Engine event but not raise some event).
To expose briefly how it's used:
I register a funcdef in the ASEngine which will be my signature (for instance void MouseMoveEvent(CMouse@, const CVec2f&in))
Then I expose to the script an "event subscriber" (for instance: CMouseMoveEventHandler) that declares a method "SetCallback" which takes in our exemple (IEventHandler@, MouseMoveEvent@). IEventHandler is an empty interface that I declared so AS classes can register and catch events simply by inherit from it (there may be better solutions, and maybe there already is a hidden common parent class to all script classes)

After that all Game Engine that have events expose a RegisterToEventxxx and a UnregisterFromEventxxx (in our exemple RegisterToEventMouseMove(CMouseEventHandler@+) / UnregisterFromEventMouseMove(CMouseEventHandler@+)

Then the class script declares an EventHandler, implements the "callback" method and all is done
(this is pseudo code and may contain syntax errors)

class CScriptCatcher : IEventHandler
{
CScriptCatcher()
{
m_MouseMoved.SetCallback(this, @OnMouseMove);
GetGame().RegisterToEventMouseMove(m_MouseMoved);
}
~CScriptCatcher()
{
GetGame().UnregisterFromEventMouseMove(m_MouseMoved);
}
private void OnMouseMove(CMouse@ _pMouse, const CVec2f& _vDelta)
{
//Do some stuff here to hande mouse move
}
private CMouseMoveEventHandler m_MouseMoved;
}


There certainly a better way and a better interface to use the idea to be a little more lightweight for the user, but it might help you to have an idea on how things could be done.

For the delegate implementation in C++, it's an event class that holds a list of generics subscribers using templates (because the system is primarily used with C++). Subscribers are only a "this" pointer and method pointer OR a function pointer so delegates can be either method members with a this or a static method / a function.

Hope this might help ;)

The only modifications I made is being able to point a method address: I added a "IsSignatureExceptNameAndObjectEqual" that is the same as "IsSignatureExceptNameEqual" and does not care about the object type, so a funcdef is not dependent on the object type but only on the method signature. It may not be ideal but it was the fastest way for me to do what I wanted.

And actually yes, this method provide a compile time verification of the callback signature, if you implement a callback with the wrong signature, it will not compile at the "SetCallback" as the second argument is a pointer to a funcdef. It would produce this kind of error: (example taken for my script sorry if names are not very relevant)

Compiling BPE
MainMenuForm.bhass (3, 5): INFO: Compiling CMainMenuForm::CMainMenuForm()
MainMenuForm.bhass (6, 27): ERROR: No matching signatures to 'CButtonEventHandler::SetCallback(CMainMenuForm&, OnChallengesClick@const)'
MainMenuForm.bhass (6, 27): INFO: Candidates are:
MainMenuForm.bhass (6, 27): INFO: void CButtonEventHandler::SetCallback(IEventHandler@, ButtonEvent@)
(just added an int parameter to the callback method)

So even if the error is not very clear with current implementation, it is triggered when the callback signature is incorrect at compile time.

I didn't think of the CScriptHandle that would have been cleaner indeed. But now the product has been released so it may be for the next one .

If I may request a feature to be included in this: When taking the handle of an object method, it would be awesome to be able to get accessor functions by their short names. For example:


// This
myDelegate = @myObject.prop;
// Instead of this
myDelegate = @myObject.get_prop;


I have no experience with writing compilers, so I don't know if this is feasible to implement.

Is Delegate a confirmed feature for AngelScript?

A function pointer and an ad-don just as well do the same job.


void GlobalFunction()
{
}

Object obj1;
Object2 obj2;

Signal signal;

signal << function(Object::DoStuff, @obj1);
signal << function(Object2::DoOtherStuff, @obj2);
signal << function(GlobalFunction);

signal.call(); // call all functions, also accepts parameters and refuses to call functions with wrong parameters.

I already do this but with strings and cached method pointers.
It just need to be faster with function pointers.

I don't see the point of adding delegates. Or any Observer pattern specific functionality to core language.