using System.Collections;
using System.Collections.Specialized;
using System.ComponentModel;
using Prism.Common;

namespace Prism.Regions.Behaviors;

/// <summary>
/// Defines a behavior that forwards the <see cref="Xaml.RegionManager.RegionContextProperty"/> 
/// to the views in the region.
/// </summary>
public class BindRegionContextToVisualElementBehavior : IRegionBehavior
{
    /// <summary>
    /// The key of this behavior.
    /// </summary>
    public const string BehaviorKey = "ContextToVisualElement";

    /// <summary>
    /// Behavior's attached region.
    /// </summary>
    public IRegion Region { get; set; }

    /// <summary>
    /// Attaches the behavior to the specified region.
    /// </summary>
    public void Attach()
    {
        Region.Views.CollectionChanged += Views_CollectionChanged;
        Region.PropertyChanged += Region_PropertyChanged;
        SetContextToViews(Region.Views, Region.Context);
        AttachNotifyChangeEvent(Region.Views);
    }

    private static void SetContextToViews(IEnumerable views, object context)
    {
        foreach (var view in views)
        {
            if (view is VisualElement visualElement)
            {
                ObservableObject<object> contextWrapper = RegionContext.GetObservableContext(visualElement);
                contextWrapper.Value = context;
            }
        }
    }

    private void AttachNotifyChangeEvent(IEnumerable views)
    {
        foreach (var view in views)
        {
            if (view is VisualElement visualElement)
            {
                ObservableObject<object> viewRegionContext = RegionContext.GetObservableContext(visualElement);
                viewRegionContext.PropertyChanged += ViewRegionContext_OnPropertyChangedEvent;
            }
        }
    }

    private void DetachNotifyChangeEvent(IEnumerable views)
    {
        foreach (var view in views)
        {
            if (view is VisualElement visualElement)
            {
                ObservableObject<object> viewRegionContext = RegionContext.GetObservableContext(visualElement);
                viewRegionContext.PropertyChanged -= ViewRegionContext_OnPropertyChangedEvent;
            }
        }
    }

    private void ViewRegionContext_OnPropertyChangedEvent(object sender, PropertyChangedEventArgs args)
    {
        if (args.PropertyName == "Value")
        {
            var context = (ObservableObject<object>)sender;
            Region.Context = context.Value;
        }
    }

    private void Views_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.Action == NotifyCollectionChangedAction.Add)
        {
            SetContextToViews(e.NewItems, Region.Context);
            AttachNotifyChangeEvent(e.NewItems);
        }
        else if (e.Action == NotifyCollectionChangedAction.Remove && Region.Context != null)
        {
            DetachNotifyChangeEvent(e.OldItems);
            SetContextToViews(e.OldItems, null);
        }
    }

    private void Region_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "Context")
        {
            SetContextToViews(Region.Views, Region.Context);
        }
    }
}
