using System;
using System.Threading;
using System.Threading.Tasks;

namespace Unosquare.Swan.Abstractions {
  /// <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() {
      this.State = AppWorkerState.Stopped;
      this.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 => this._workerState;

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

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

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

    /// <summary>
    /// Gets the cancellation token.
    /// </summary>
    /// <value>
    /// The cancellation token.
    /// </value>
    public CancellationToken CancellationToken => this._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 Boolean 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() => this.CheckIsRunning();

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

      this.CreateWorker();
      this.State = AppWorkerState.Running;
    }

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

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

    /// <inheritdoc />
    public void Dispose() => this._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(this.GetType().Name, ex);

    /// <summary>
    /// This method is called when the user loop has exited.
    /// </summary>
    protected virtual void OnWorkerThreadExit() => "Service thread is stopping.".Debug(this.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(this.State != AppWorkerState.Stopped) {
        throw new InvalidOperationException("Service cannot be initialized because it seems to be currently running.");
      }
    }

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

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

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

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

    #endregion
  }
}