namespace Caliburn.Micro.Core {
    using System;
    using System.Collections.Generic;

    /// <summary>
    /// A base implementation of <see cref = "IViewAware" /> which is capable of caching views by context.
    /// </summary>
    public class ViewAware : PropertyChangedBase, IViewAware {
        readonly IDictionary<object, object> views;

        /// <summary>
        /// The default view context.
        /// </summary>
        public static readonly object DefaultContext = new object();

        /// <summary>
        /// The view chache for this instance.
        /// </summary>
        protected IDictionary<object, object> Views {
            get { return views; }
        }

        /// <summary>
        /// Creates an instance of <see cref="ViewAware"/>.
        /// </summary>
        public ViewAware() {
            views = new WeakValueDictionary<object, object>();
        }

        /// <summary>
        /// Raised when a view is attached.
        /// </summary>
        public event EventHandler<ViewAttachedEventArgs> ViewAttached = delegate { };

        void IViewAware.AttachView(object view, object context) {
            Views[context ?? DefaultContext] = view;

            var nonGeneratedView = PlatformProvider.Current.GetFirstNonGeneratedView(view);
            PlatformProvider.Current.ExecuteOnFirstLoad(nonGeneratedView, OnViewLoaded);
            OnViewAttached(nonGeneratedView, context);
            ViewAttached(this, new ViewAttachedEventArgs {View = nonGeneratedView, Context = context});

            var activatable = this as IActivate;
            if (activatable == null || activatable.IsActive) {
                PlatformProvider.Current.ExecuteOnLayoutUpdated(nonGeneratedView, OnViewReady);
            }
            else {
                AttachViewReadyOnActivated(activatable, nonGeneratedView);
            }
        }

        static void AttachViewReadyOnActivated(IActivate activatable, object nonGeneratedView) {
            var viewReference = new WeakReference(nonGeneratedView);
            EventHandler<ActivationEventArgs> handler = null;
            handler = (s, e) => {
                ((IActivate)s).Activated -= handler;
                var view = viewReference.Target;
                if (view != null) {
                    PlatformProvider.Current.ExecuteOnLayoutUpdated(view, ((ViewAware)s).OnViewReady);
                }
            };
            activatable.Activated += handler;
        }

        /// <summary>
        /// Called when a view is attached.
        /// </summary>
        /// <param name="view">The view.</param>
        /// <param name="context">The context in which the view appears.</param>
        protected virtual void OnViewAttached(object view, object context) {
        }

        /// <summary>
        /// Called when an attached view's Loaded event fires.
        /// </summary>
        /// <param name = "view"></param>
        protected virtual void OnViewLoaded(object view) {
        }

        /// <summary>
        /// Called the first time the page's LayoutUpdated event fires after it is navigated to.
        /// </summary>
        /// <param name = "view"></param>
        protected virtual void OnViewReady(object view) {
        }

        /// <summary>
        /// Gets a view previously attached to this instance.
        /// </summary>
        /// <param name = "context">The context denoting which view to retrieve.</param>
        /// <returns>The view.</returns>
        public virtual object GetView(object context = null) {
            object view;
            Views.TryGetValue(context ?? DefaultContext, out view);
            return view;
        }
    }
}