namespace Unosquare.Swan.Abstractions
{
    using System;
    using System.Threading;
    using System.Threading.Tasks;

    /// <summary>
    /// A base implementation of an Application service containing a worker task that performs background processing.
    /// </summary>
    /// <example>
    /// The following code describes how to implement the <see cref="AppWorkerBase"/> class.
    /// <code>
    /// using System;
    /// using System.Threading.Tasks;
    /// using Unosquare.Swan;
    /// using Unosquare.Swan.Abstractions;
    /// 
    /// class Worker : AppWorkerBase
    /// {
    ///     // an action that will be executed if the worker is stopped
    ///     public Action OnExit { get; set; }
    ///      
    ///     // override the base loop method, this is the code will
    ///     // execute until the cancellation token is canceled.
    ///     protected override Task WorkerThreadLoop()
    ///     {
    ///         // delay a second and then proceed
    ///         await Task.Delay(TimeSpan.FromMilliseconds(1000), CancellationToken);
    ///             
    ///         // just print out this
    ///         $"Working...".WriteLine();
    ///     }
    ///     
    ///     // Once the worker is stopped this code will be executed
    ///     protected override void OnWorkerThreadExit()
    ///     {
    ///         // execute the base method
    ///         base.OnWorkerThreadExit();
    ///         
    ///         // then if the OnExit Action is not null execute it
    ///         OnExit?.Invoke();
    ///     }
    /// }
    /// </code>
    /// </example>
    public abstract class AppWorkerBase
        : IWorker, IDisposable
    {
        private readonly object _syncLock = new object();
        private AppWorkerState _workerState = AppWorkerState.Stopped;
        private CancellationTokenSource _tokenSource;

        /// <summary>
        /// Initializes a new instance of the <see cref="AppWorkerBase"/> class.
        /// </summary>
        protected AppWorkerBase()
        {
            State = AppWorkerState.Stopped;
            IsBusy = false;
        }

        /// <summary>
        /// Occurs when [state changed].
        /// </summary>
        public event EventHandler<AppWorkerStateChangedEventArgs> StateChanged;

        #region Properties

        /// <summary>
        /// Gets the state of the application service.
        /// In other words, useful to know whether the service is running.
        /// </summary>
        /// <value>
        /// The state.
        /// </value>
        public AppWorkerState State
        {
            get => _workerState;

            private set
            {
                lock (_syncLock)
                {
                    if (value == _workerState) return;

                    $"Service state changing from {State} to {value}".Debug(GetType().Name);
                    var newState = value;
                    var oldState = _workerState;
                    _workerState = value;

                    StateChanged?.Invoke(this, new AppWorkerStateChangedEventArgs(oldState, newState));
                }
            }
        }

        /// <summary>
        /// Gets the cancellation token.
        /// </summary>
        /// <value>
        /// The cancellation token.
        /// </value>
        public CancellationToken CancellationToken => _tokenSource?.Token ?? default;

        /// <summary>
        /// Gets a value indicating whether the thread is busy.
        /// </summary>
        /// <value>
        ///   <c>true</c> if this instance is busy; otherwise, <c>false</c>.
        /// </value>
        public bool IsBusy { get; private set; }

        #endregion

        #region AppWorkerBase Methods

        /// <summary>
        /// Performs internal service initialization tasks required before starting the service.
        /// </summary>
        /// <exception cref="InvalidOperationException">Service cannot be initialized because it seems to be currently running.</exception>
        public virtual void Initialize() => CheckIsRunning();

        /// <inheritdoc/>
        /// <exception cref="InvalidOperationException">Service cannot be started because it seems to be currently running.</exception>
        public virtual void Start()
        {
            CheckIsRunning();

            CreateWorker();
            State = AppWorkerState.Running;
        }

        /// <inheritdoc/>
        /// <exception cref="InvalidOperationException">Service cannot be stopped because it is not running.</exception>
        public virtual void Stop()
        {
            if (State != AppWorkerState.Running)
                return;

            _tokenSource?.Cancel();
            "Service stop requested.".Debug(GetType().Name);
            State = AppWorkerState.Stopped;
        }

        /// <inheritdoc />
        public void Dispose() => _tokenSource?.Dispose();

        #endregion

        #region Abstract and Virtual Methods

        /// <summary>
        /// Called when an unhandled exception is thrown.
        /// </summary>
        /// <param name="ex">The ex.</param>
        protected virtual void OnWorkerThreadLoopException(Exception ex)
            => "Service exception detected.".Debug(GetType().Name, ex);

        /// <summary>
        /// This method is called when the user loop has exited.
        /// </summary>
        protected virtual void OnWorkerThreadExit() => "Service thread is stopping.".Debug(GetType().Name);

        /// <summary>
        /// Implement this method as a loop that checks whether CancellationPending has been set to true
        /// If so, immediately exit the loop.
        /// </summary>
        /// <returns>A task representing the execution of the worker.</returns>
        protected abstract Task WorkerThreadLoop();

        private void CheckIsRunning()
        {
            if (State != AppWorkerState.Stopped)
                throw new InvalidOperationException("Service cannot be initialized because it seems to be currently running.");
        }

        private void CreateWorker()
        {
            _tokenSource = new CancellationTokenSource();
            _tokenSource.Token.Register(() =>
            {
                IsBusy = false;
                OnWorkerThreadExit();
            });

            Task.Run(async () =>
                {
                    IsBusy = true;

                    try
                    {
                        while (!CancellationToken.IsCancellationRequested)
                        {
                            await WorkerThreadLoop().ConfigureAwait(false);
                        }
                    }
                    catch (AggregateException)
                    {
                        // Ignored
                    }
                    catch (Exception ex)
                    {
                        ex.Log(GetType().Name);
                        OnWorkerThreadLoopException(ex);

                        if (!_tokenSource.IsCancellationRequested)
                            _tokenSource.Cancel();
                    }
                },
                _tokenSource.Token);
        }

        #endregion
    }
}