ln.threading/ln.threading/Promise.cs

using System;
using System.Threading;

namespace ln.threading
{
    public class PromiseNotPendingException : Exception {}
    public class PromiseRejectedException : Exception
    {
        public PromiseRejectedException(){}
        public PromiseRejectedException(Exception caughtExcpetion) : base("Promise was rejected", caughtExcpetion) {}

    }

    public enum PromiseState { PENDING, RESOLVED, REJECTED }
    public class Promise<T>
    {
        public event Action<T> Resolved;
        public event Action<Exception> Rejected;
        public event Action<Promise<T>> Settled;

        public bool IsSettled { get; private set; }

        public PromiseState State { get; private set; } = PromiseState.PENDING;
        T value;
        Exception rejectingException;


        public Promise() {}
        public Promise(Action<Action<T>,Action<Exception>> resolver)
        {
            Resolve(resolver);
        }

        void Resolve(Action<Action<T>,Action<Exception>> resolver)
        {
            DynamicThreadPool.DefaultPool.Enqueue(()=>{
                try{
                    resolver(Resolve, Reject);
                } catch (Exception e)
                {
                    Reject(e);
                }
            });            
        }


        public void Resolve(T value)
        {
            lock (this)
            {
                if (State == PromiseState.PENDING)
                {
                    this.value = value;
                    State = PromiseState.RESOLVED;
                    IsSettled = true;
                    Monitor.PulseAll(this);
                    DynamicThreadPool.DefaultPool.Enqueue(()=> {
                        Resolved?.Invoke(this.value);
                        Settled?.Invoke(this);
                    });
                } else
                {
                    throw new PromiseNotPendingException();
                }
            }
        }

        public void Reject() => Reject(null);
        public void Reject(Exception rejectingException)
        {
            lock (this)
            {
                if (State != PromiseState.PENDING)
                    throw new PromiseNotPendingException();

                this.rejectingException = rejectingException;
                IsSettled = true;

                State = PromiseState.REJECTED;
                Monitor.PulseAll(this);
                DynamicThreadPool.DefaultPool.Enqueue(()=> {
                    Rejected?.Invoke(this.rejectingException);
                    Settled?.Invoke(this);
                });
            }
        }

        public T Value {
            get {
                lock (this)
                {
                    while (State == PromiseState.PENDING)
                        Monitor.Wait(this);

                    if (State == PromiseState.REJECTED)
                        throw new PromiseRejectedException(rejectingException);
                    if (State == PromiseState.RESOLVED)
                        return value;
                    
                    throw new Exception("serious bug in Promise implementation");
                }
            }
        }


        public Promise<S> Then<S>(Func<T,S> resolved) => Then(resolved, null);
        public Promise<S> Then<S>(Func<T,S> resolved, Func<Exception,S> rejected)
        {
            lock (this)
            {
                Promise<S> chainedPromise = new Promise<S>();
                Action resolveAction = ()=>{
                    lock (this)
                    {
                        switch (State)
                        {
                            case PromiseState.PENDING:
                                throw new Exception("serious bug in Promise.Then(..)");
                            case PromiseState.RESOLVED:
                                try{
                                    chainedPromise.Resolve(resolved(value));
                                } catch (Exception e)
                                {
                                    chainedPromise.Reject(e);
                                }
                                break;
                            case PromiseState.REJECTED:
                                if (rejected == null)
                                    chainedPromise.Reject(rejectingException);
                                else
                                {
                                    try{
                                        chainedPromise.Resolve(rejected(rejectingException));
                                    } catch (Exception e)
                                    {
                                        chainedPromise.Reject(e);
                                    }
                                }
                                break;
                        }
                    }
                };

                if (IsSettled)
                    DynamicThreadPool.DefaultPool.Enqueue(resolveAction);
                else
                    Settled += (p) => resolveAction();

                return chainedPromise;
            }
        }

        public Promise<T> Then(Action<T> resolved) => Then(resolved, null);
        public Promise<T> Then(Action<T> resolved, Func<Exception,T> rejected)
        {
            lock (this)
            {
                Promise<T> chainedPromise = new Promise<T>();
                Action resolveAction = ()=>{
                    lock (this)
                    {
                        switch (State)
                        {
                            case PromiseState.PENDING:
                                throw new Exception("serious bug in Promise.Then(..)");
                            case PromiseState.RESOLVED:
                                try{
                                    resolved(value);
                                    chainedPromise.Resolve(value);
                                } catch (Exception e)
                                {
                                    chainedPromise.Reject(e);
                                }
                                break;
                            case PromiseState.REJECTED:
                                if (rejected == null)
                                    chainedPromise.Reject(rejectingException);
                                else
                                {
                                    try{
                                        chainedPromise.Resolve(rejected(rejectingException));
                                    } catch (Exception e)
                                    {
                                        chainedPromise.Reject(e);
                                    }
                                }
                                break;
                        }
                    }
                };

                if (IsSettled)
                    DynamicThreadPool.DefaultPool.Enqueue(resolveAction);
                else
                    Settled += (p) => resolveAction();

                return chainedPromise;
            }
        }


        public Promise<T> Catch(Func<Exception,T> rejectedHandler)
        {
            lock (this)
            {
                Promise<T> chainedPromise = new Promise<T>();
                Action resolveAction = ()=>{
                    lock (this)
                    {
                        switch (State)
                        {
                            case PromiseState.PENDING:
                                throw new Exception("serious bug in Promise.Catch(..)");
                            case PromiseState.RESOLVED:
                                chainedPromise.Resolve(value);
                                break;
                            case PromiseState.REJECTED:
                                try
                                {
                                    chainedPromise.Resolve( rejectedHandler(rejectingException));
                                } catch (Exception e)
                                {
                                    chainedPromise.Reject(e);
                                }
                                break;
                        }
                    }
                };

                if (IsSettled)
                    DynamicThreadPool.DefaultPool.Enqueue(resolveAction);
                else
                    Settled += (p) => resolveAction();
                
                return chainedPromise;
            }
        }

        public Promise<T> Finally(Action finallyHandler)
        {
            Promise<T> chainedPromise = new Promise<T>();
            lock (this)
            {
                Action resolveAction = ()=>{
                    lock (this)
                    {
                        if (State == PromiseState.PENDING)
                            throw new Exception("serious bug in Promise.Finally(..)");

                        try{
                            finallyHandler();

                            switch (State)
                            {
                                case PromiseState.RESOLVED:
                                    chainedPromise.Resolve(value);
                                    break;
                                case PromiseState.REJECTED:
                                    chainedPromise.Reject(rejectingException);
                                    break;
                            }
                        } catch (Exception e)
                        {
                            chainedPromise.Reject(e);
                        }
                    }
                };

                if (IsSettled)
                    DynamicThreadPool.DefaultPool.Enqueue(resolveAction);
                else
                    Settled += (p) => resolveAction();
                
                return chainedPromise;                
            }
        }



    }
}