Repeater View

RepeaterView.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using Xamarin.Forms;

public delegate void RepeaterViewItemAddedEventHandler(object sender, RepeaterViewItemAddedEventArgs args);

/// <summary>
/// Don't set IsVisible to false or you will have a bad time
/// this won't relayout its child elements when you change the visibility
/// </summary>
// in lieu of an actual Xamarin Forms ItemsControl, this is a heavily modified version of code from https://forums.xamarin.com/discussion/21635/xforms-needs-an-itemscontrol
public class RepeaterView : StackLayout
{
    public RepeaterView()
    {
    }

    public static readonly BindableProperty ItemsSourceProperty =
        BindableProperty.Create(
            "ItemsSource",
            typeof(IEnumerable),
            typeof(RepeaterView),
            defaultValue: null,
            defaultBindingMode: BindingMode.OneWay,
            propertyChanged: ItemsChanged);


    public static readonly BindableProperty ItemTemplateProperty =
        BindableProperty.Create(
            "ViewModel",
            typeof(DataTemplate),
            typeof(RepeaterView),
            defaultValue: null,
            defaultBindingMode: BindingMode.OneWay);


    bool waitingForBindingContext = false;
    public event RepeaterViewItemAddedEventHandler ItemCreated;

    public IEnumerable ItemsSource
    {
        get { return (IEnumerable)GetValue(ItemsSourceProperty); }
        set { SetValue(ItemsSourceProperty, value); }
    }

    public DataTemplate ItemTemplate
    {
        get { return (DataTemplate)GetValue(ItemTemplateProperty); }
        set { SetValue(ItemTemplateProperty, value); }
    }


    protected override void OnBindingContextChanged()
    {
        base.OnBindingContextChanged();

        if (BindingContext != null && waitingForBindingContext && ItemsSource != null)
        {
            ItemsChanged(this, null, ItemsSource);
        }
    }

    private static void ItemsChanged(BindableObject bindable, object old, object newVal)
    {
        IEnumerable oldValue = old as IEnumerable;
        IEnumerable newValue = newVal as IEnumerable;

        var control = (RepeaterView)bindable;

        var oldObservableCollection = oldValue as INotifyCollectionChanged;

        if (oldObservableCollection != null)
        {
            oldObservableCollection.CollectionChanged -= control.OnItemsSourceCollectionChanged;
        }

        //HACK:SHANE
        if (control.BindingContext == null)
        {
            control.waitingForBindingContext = true;
            //this means this control has been removed from the visual tree
            //so don't update it other wise you get random null reference exceptions
            return;
        }

        control.waitingForBindingContext = false;

        var newObservableCollection = newValue as INotifyCollectionChanged;

        if (newObservableCollection != null)
        {
            newObservableCollection.CollectionChanged += control.OnItemsSourceCollectionChanged;
        }

        try
        {
            control.Children.Clear();

            if (newValue != null)
            {
                foreach (var item in newValue)
                {
                    var view = control.CreateChildViewFor(item);
                    control.Children.Add(view);
                    control.OnItemCreated(view);
                }
            }

            control.UpdateChildrenLayout();
            control.InvalidateLayout();
        }
        catch (NullReferenceException)
        {
            try
            {
                LogHost.Default.Error(
                    String.Format("RepeaterView: NullReferenceException Parent:{0} ParentView:{1} IsVisible:{2}",
                        control.Parent, control.Parent, control.IsVisible)
                );
            }
            catch (Exception exc)
            {
                LogHost.Default.ErrorException("NullReferenceException Logging Failed", exc);
            }
        }
    }



    protected virtual void OnItemCreated(View view)
    {

        if (this.ItemCreated != null)
        {
            ItemCreated.Invoke(this, new RepeaterViewItemAddedEventArgs(view, view.BindingContext));
        }


    }

    private void OnItemsSourceCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        try
        {
            var invalidate = false;

            List<View> createdViews = new List<View>();
            if (e.Action == NotifyCollectionChangedAction.Reset)
            {
                var list = sender as IEnumerable;


                this.Children.SyncList(
                    list,
                    (item) =>
                {
                    var view = this.CreateChildViewFor(item);
                    createdViews.Add(view);
                    return view;
                }, (item, view) => view.BindingContext == item,
                    null);

                foreach (View view in createdViews)
                {
                    OnItemCreated(view);
                }

                invalidate = true;
            }

            if (e.OldItems != null)
            {
                this.Children.RemoveAt(e.OldStartingIndex);
                invalidate = true;
            }

            if (e.NewItems != null)
            {
                for (var i = 0; i < e.NewItems.Count; ++i)
                {
                    var item = e.NewItems[i];
                    var view = this.CreateChildViewFor(item);

                    this.Children.Insert(i + e.NewStartingIndex, view);
                    OnItemCreated(view);
                }

                invalidate = true;
            }

            if (invalidate)
            {
                this.UpdateChildrenLayout();
                this.InvalidateLayout();
            }
        }
        catch (NullReferenceException)
        {
            try
            {
                this.Log().Error(
                    String.Format("RepeaterView.OnItemsSourceCollectionChanged: NullReferenceException Parent:{0} ParentView:{1} IsVisible:{2} BindingContext: {3}",
                        Parent, Parent, IsVisible, BindingContext)
                );
            }
            catch (Exception exc)
            {
                this.Log().ErrorException("OnItemsSourceCollectionChanged: NullReferenceException Logging Failed", exc);
            }
        }
    }

    private View CreateChildViewFor(object item)
    {
        this.ItemTemplate.SetValue(BindableObject.BindingContextProperty, item);

        if (this.ItemTemplate is DataTemplateSelector)
        {
            var dts = (DataTemplateSelector)this.ItemTemplate;
            return (View)dts.SelectTemplate(item, null).CreateContent();
        }
        else
        {
            return (View)this.ItemTemplate.CreateContent();
        }
    }
}

public class RepeaterViewItemAddedEventArgs : EventArgs
{
    private readonly View view;
    private readonly object model;

    public RepeaterViewItemAddedEventArgs(View view, object model)
    {
        this.view = view;
        this.model = model;
    }

    public View View { get { return view; } }

    public object Model { get { return model; } }
}




public static class IListMixIns
    {

        public static void SyncList<T>(
            this IList<T> This,
            IEnumerable<T> sourceList)
        {
            This.SyncList<T, T>(sourceList, x => x, (x, y) => x.Equals(y), null);
        }


        public static void SyncList<T>(
			this IList<T> This,
			IEnumerable sourceList,
			Func<object, T> selector,
			Func<object, T, bool> areEqual,
			Action<object, T> updateExisting,
			bool dontRemove = false)
		{
			var sourceListEnum = sourceList.OfType<Object>().ToList();
 
			foreach (T dest in This.ToList())
			{
				var match = sourceListEnum.FirstOrDefault(p => areEqual(p, dest));
				if (match != null)
				{
					if (updateExisting != null)
						updateExisting(match, dest);
				}
				else if (!dontRemove)
				{
					This.Remove(dest);
				}
			}

			sourceListEnum.Where(x => !This.Any(p => areEqual(x, p)))
				.ToList().ForEach(p =>
			{
                if (This.Count >= sourceListEnum.IndexOf(p))
                    This.Insert(sourceListEnum.IndexOf(p), selector(p));
                else
                {
                    var result = selector(p);
                    if (!EqualityComparer<T>.Default.Equals(result, default(T)))
                        This.Add(result);
                }
			});
		}

        public static bool IsEmpty<T>(this IEnumerable<T> list)
        {
            return !list.Any();
        }

        public static bool IsEmpty<T>(this Array list)
        {
            return list.Length == 0;
        }

        public static bool IsEmpty<T>(this List<T> list)
        {
            return list.Count == 0;
        }

        public static void  ForEach<T>(this IEnumerable<T> list, Action<T> doMe)
        {
            foreach(var item in list)
            {
                doMe(item);
            }
        }

        public static void SyncList<T, Source>(
            this IList<T> This,
            IEnumerable<Source> sourceList,
            Func<Source, T> selector,
            Func<Source, T, bool> areEqual,
            Action<Source, T> updateExisting,
            bool dontRemove = false)
        { 
            foreach (T dest in This.ToList())
            {
                var match = sourceList.FirstOrDefault(p => areEqual(p, dest));
                if (!EqualityComparer<Source>.Default.Equals(match, default(Source)))
                {
                    updateExisting?.Invoke(match, dest);
                }
                else if (!dontRemove)
                {
                    This.Remove(dest);
                }
            }

            sourceList.Where(x => !This.Any(p => areEqual(x, p)))
                .ToList().ForEach(p =>
                {
                    var result = selector(p);
                    if(!EqualityComparer<T>.Default.Equals(result, default(T)))
                        This.Add(result);
                });
        }
    }