Service locator pattern with callback system derived from EventManager script found in Unity FPS Microgame.

ServiceLocator.cs

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MIT License

Copyright (c) 2023 Calvin Rien

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using System;
using System.Collections.Generic;
using UnityEngine;

public interface IService { }

public static class ServiceLocator
{
    private static readonly Dictionary<Type, IService> k_Services = new Dictionary<Type, IService>();

    private static readonly Dictionary<Type, Action<IService>> k_Callbacks = new();

    private static readonly Dictionary<Delegate, Action<IService>> k_RegisteredCallbacks = new();

    public static bool Register<T>(T service) where T : class, IService
    {
        var type = typeof(T);

        if (!k_Services.TryAdd(type, service))
        {
            // Throw exception or just warn and return false?
            Debug.LogWarning($"Service of type {type.Name} already registered");
            return false;
        }

        if (k_Callbacks.TryGetValue(type, out var serviceCallback))
        {
            serviceCallback?.Invoke(service);
        }

        return true;
    }

    public static bool Unregister<T>(T service) where T : class, IService
    {
        var type = typeof(T);

        if (k_Services.TryGetValue(type, out var temp) && service == temp)
        {
            k_Services.Remove(type);
            return true;
        }

        Debug.LogWarning($"This instance of {type.Name} is not registered");
        return false;
    }

    public static T GetService<T>() where T : class, IService
    {
        if (k_Services.TryGetValue(typeof(T), out var service))
        {
            return (T)service;
        }

        return null;
    }

    public static bool TryGetService<T>(out T result) where T : class, IService
    {
        if (k_Services.TryGetValue(typeof(T), out var service))
        {
            result = (T)service;
            return true;
        }

        result = default;
        return false;
    }

    public static void AddServiceListener<T>(Action<T> serviceCallback) where T : class, IService
    {
        if (k_RegisteredCallbacks.ContainsKey(serviceCallback))
        {
            Debug.LogWarning("double registered");
            return;
        }

        if (TryGetService<T>(out var service))
        {
            serviceCallback?.Invoke(service);
            return;
        }

        var type = typeof(T);

        Action<IService> newAction = (e) => serviceCallback((T)e);

        if (k_Callbacks.TryGetValue(type, out var result))
        {
            k_Callbacks[type] = result += newAction;
        }
        else
        {
            k_Callbacks[type] = newAction;
        }

        k_RegisteredCallbacks.Add(serviceCallback, newAction);
    }

    public static void RemoveServiceListener<T>(Action<T> serviceCallback) where T : class, IService
    {
        if (!k_RegisteredCallbacks.TryGetValue(serviceCallback, out var action))
        {
            return;
        }

        var type = typeof(T);

        if (k_Callbacks.TryGetValue(type, out var tempAction))
        {
            tempAction -= action;
            if (tempAction == null)
            {
                k_Callbacks.Remove(type);
            }
            else
            {
                k_Callbacks[type] = tempAction;
            }
        }

        k_RegisteredCallbacks.Remove(serviceCallback);
    }
}