using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Controls;
using Tango.Core;
using Tango.Core.DI;

namespace Tango.SharedUI
{
    /// <summary>
    /// Represents an abstract view model.
    /// </summary>
    /// <seealso cref="Tango.Core.ExtendedObject" />
    /// <seealso cref="System.ComponentModel.INotifyDataErrorInfo" />
    public abstract class ViewModel : ExtendedObject, INotifyDataErrorInfo
    {
        private List<KeyValuePair<String, String>> _currentErrors = new List<KeyValuePair<string, string>>(); //Holds the current validation errors.

        private bool _hasErrors;
        /// <summary>
        /// Gets a value that indicates whether the entity has validation errors.
        /// </summary>
        public bool HasErrors
        {
            get { return _hasErrors; }
            set { _hasErrors = value; RaisePropertyChangedAuto(); }
        }

        private bool _isBusy;
        /// <summary>
        /// Gets or sets a value indicating whether this view model is currently busy doing operations.
        /// </summary>
        public bool IsBusy
        {
            get { return _isBusy; }
            set { _isBusy = value; RaisePropertyChangedAuto(); RaisePropertyChanged(nameof(IsFree)); InvalidateRelayCommands(); }
        }

        /// <summary>
        /// Gets or sets a value indicating whether this view model is free to perform actions.
        /// </summary>
        public bool IsFree
        {
            get { return !IsBusy; }
            set { IsBusy = !value; }
        }

        private ObservableCollection<String> _validationErrors;
        /// <summary>
        /// Gets or sets the validation errors.
        /// </summary>
        public ObservableCollection<String> ValidationErrors
        {
            get { return _validationErrors; }
            protected set { _validationErrors = value; RaisePropertyChangedAuto(); }
        }

        /// <summary>
        /// Occurs when the validation errors have changed for a property or for the entire entity.
        /// </summary>
        public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;

        /// <summary>
        /// Initializes a new instance of the <see cref="ViewModel"/> class.
        /// </summary>
        public ViewModel()
        {
            IsFree = true;
        }

        /// <summary>
        /// Gets the validation errors for a specified property or for the entire entity.
        /// </summary>
        /// <param name="propertyName">The name of the property to retrieve validation errors for; or null or <see cref="F:System.String.Empty" />, to retrieve entity-level errors.</param>
        /// <returns>
        /// The validation errors for the property or entity.
        /// </returns>
        public virtual IEnumerable GetErrors(string propertyName)
        {
            return _currentErrors.Where(x => x.Key == propertyName).Select(x => x.Value).ToList();
        }

        /// <summary>
        /// Invoked the <see cref="ErrorsChanged"/> event.
        /// </summary>
        /// <param name="propName">Name of the property.</param>
        protected void RaiseError(String propName)
        {
            ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propName));
        }

        /// <summary>
        /// Validates this view model by <see cref="ValidationAttribute">validation attributes</see>.
        /// </summary>
        /// <returns></returns>
        protected bool Validate()
        {
            HasErrors = false;
            _currentErrors.Clear();

            if (ValidationErrors == null)
            {
                ValidationErrors = new ObservableCollection<string>();
            }

            ValidationErrors.Clear();

            OnValidating();

            foreach (var prop in this.GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance))
            {
                foreach (var validation in prop.GetCustomAttributes<ValidationAttribute>())
                {
                    if (!validation.IsValid(prop.GetValue(this)))
                    {
                        _currentErrors.Add(new KeyValuePair<string, string>(prop.Name, validation.ErrorMessage));
                        ValidationErrors.Add(validation.ErrorMessage);
                    }
                }

                HasErrors = _currentErrors.Count > 0;
                RaiseError(prop.Name);
            }

            return !HasErrors;
        }

        /// <summary>
        /// Inserts the specified property error (Use inside OnValidating).
        /// </summary>
        /// <param name="propName">Name of the property.</param>
        /// <param name="error">The error.</param>
        protected void InsertError(String propName, String error)
        {
            _currentErrors.Add(new KeyValuePair<string, string>(propName, error));
        }

        /// <summary>
        /// Called before validating.
        /// </summary>
        protected virtual void OnValidating()
        {

        }

        /// <summary>
        /// Posts the specified message using the messenger service.
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="message">The message.</param>
        /// <returns></returns>
        protected virtual T PostMessage<T>(T message)
        {
            TangoMessenger.Default.Send<T>(message);
            return message;
        }

        /// <summary>
        /// Registers a messenger message handler.
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="handler">The handler.</param>
        protected virtual void RegisterMessage<T>(Action<T> handler)
        {
            TangoMessenger.Default.Register<T>(handler);
        }
    }

    /// <summary>
    /// Represents an abstract view model which expects an <see cref="IView"/> of the specified type T.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <seealso cref="Tango.Core.ExtendedObject" />
    /// <seealso cref="System.ComponentModel.INotifyDataErrorInfo" />
    public abstract class ViewModel<T> : ViewModel where T : IView
    {
        /// <summary>
        /// Gets or sets the view.
        /// </summary>
        public T View { get; set; }

        /// <summary>
        /// Initializes a new instance of the <see cref="ViewModel{T}"/> class.
        /// </summary>
        public ViewModel() : base()
        {
            TangoIOC.Default.GetInstanceWhenAvailable<T>((view) =>
            {
                View = view;
                OnViewAttached();
            });
        }

        /// <summary>
        /// Called when the <see cref="IView"/> is loaded and attached.
        /// </summary>
        protected virtual void OnViewAttached()
        {

        }
    }
}