I'm working on a drap and drop system and trying to implement a hooking system (similar to what WordPress uses) so I can trigger action when certain events occurs.

I'm having multiple problems with my delegate functions:

• cannot convert UIElementDragger.CallBackFunction expression to type ListObserver.CallBackFunction
• The best overloaded method match for ListObserver.ListObserver(string, ListObserver.CallBackFunction) has some invalid arguments

Do you have any suggestions as to how either correct or otherwise implement a "hook system" so I can have functions with parameters called upon certain custom-defined events?

I have truncated the code bellow to leave the relevant sections, but if you need the whole thing I'd be happy to provide it.

The code to bind the events is:

Note: UIElementDragger is attached to the EventSystem.

//... removed "using"

public class GameSyllab : MonoBehaviour { 

//... removed attributes

private UIElementDragger UIElementDragger;

// Use this for initialization
void Start () {

    //Hook the functions
    UIElementDragger = GetComponent<UIElementDragger> ("UIElementDragger");
    UIElementDragger.HookOnto("DropComplete", DropComplete);
    UIElementDragger.HookOnto("DropCancel", DropCancel);

    //... removed unrelated code
}   

void DropComplete(Object objDraggable, Object objDropZone){
    // Do an action based on the element being dropped
}
void DropCancel(Object objDraggable, Object objDropZone){
    // Play a cancel sound
}

//...removed unrelated class code

The code from the UIElementDragger class that adds the draggable behaviour:

Note: This class is also attached to the EventSystem

using System.Collections;
using System.Collections.Generic;
using System.Linq;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;

public class UIElementDragger : MonoBehaviour {
    //A list with observers that are waiting for something to happen
    List<ListObserver> observers = new List<ListObserver>();
    public delegate void CallBackFunction(Object objDraggable, Object objDropZone);

    // ... removed extra attributes

    // Update is called once per frame
    void Update () {

        //... removed Mouse down code

        if (Input.GetMouseButtonUp (0)) {
            if (objectToDrag != null) {
                Transform objectToDrop = GetDraggableTransformUnderMouse(DROPZONE_TAG);

                if (objectToDrop != null) {
                    //Drop the dragged object on it's new drop zone
                    objectToDrag.position = objectToDrop.position;
                    NotifyObservers ("DropComplete", objectToDrag, objectToDrop);
                } else {
                    //Move the item back to its original position
                    objectToDrag.position = originalPosition;
                    NotifyObservers ("DropCancelled", objectToDrag, null);
                }

                objectToDragImage.raycastTarget = true;
                objectToDrag = null;
            }
            dragging = false;
        }


    }




    //Send notifications if something has happened
    public void NotifyObservers(string hookName, Object objectInstance, Object dropZoneInstance)
    {
        for (int i = 0; i < observers.Count; i++)
        {
            if (observers [i].GetHookName () == hookName) {
                observers [i].Call (objectInstance, dropZoneInstance);
            }
        }
    }

    //Add observer to the list
    public void HookOnto(string newHookName, CallBackFunction newCallBackFunction)
    {
        ListObserver newObserver = new ListObserver (newHookName, newCallBackFunction);
        observers.Add(newObserver);
    }
}

//In the same file, but a separate class
public class ListObserver:MonoBehaviour{
    private string hookName;
    public delegate void CallBackFunction(Object objDraggable, Object objDropZone);
    private CallBackFunction callBackFunction;

    public ListObserver(string newHookName, CallBackFunction newCallBackFunction){
        hookName = newHookName;
        callBackFunction = newCallBackFunction;
    }

    public void Call(Object objDraggable, Object objDropZone){
        callBackFunction (objDraggable, objDropZone);
    }

    public string GetHookName(){
        return hookName;
    }
}