namespace System.Windows.Forms
{
    public partial class CheckedListBox : ListBox
    {
        private static string template =
            "<DataTemplate xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'>" +
                "<StackPanel Orientation=\"Horizontal\">" +
                "<CheckBox IsChecked = \"{Binding IsChecked}\"/>" +
                "<TextBlock Text=\"{Binding Value}\" Margin=\"2,0,0,0\" IsHitTestVisible=\"False\"/>" +
                "</StackPanel>" +
            "</DataTemplate>";
        private static System.Windows.DataTemplate dataTemplate = (System.Windows.DataTemplate)System.Windows.Markup.XamlReader.Parse(template);

        public new ObjectCollection Items { get; }

        public new object SelectedItem
        {
            get => SelectedIndex == -1 ? null : Items[SelectedIndex];
            set => SelectedIndex = Items.IndexOf(value);
        }

        private CheckedItemCollection checkedItems;
        public CheckedItemCollection CheckedItems 
        { 
            get
            {
                if (checkedItems == null)
                    BuildCheckedItems();
                return checkedItems;
            }
        }

        private CheckedIndexCollection checkedIndices;
        public CheckedIndexCollection CheckedIndices
        {
            get
            {
                if (checkedIndices == null)
                    BuildCheckedIndices();
                return checkedIndices;
            }
        }

        public bool CheckOnClick { get; set; } // TODO?

        public event ItemCheckEventHandler ItemCheck;

        private void BuildCheckedItems()
        {
            checkedItems = new CheckedItemCollection();
            for (int i = 0; i < control.Items.Count; i++)
            {
                var item = control.Items[i] as Item;
                if (item.IsChecked == true)
                    checkedItems.Add(item);
            }
        }

        private void BuildCheckedIndices()
        {
            checkedIndices = new CheckedIndexCollection();
            for (int i = 0; i < control.Items.Count; i++)
            {
                var item = control.Items[i] as Item;
                if (item.IsChecked == true)
                    checkedIndices.Add(i);
            }
        }

        private void ClearCheckedItems()
        {
            checkedItems = null;
            checkedIndices = null;
        }

        protected internal override object GetItem(int index) => Items[index];

        protected virtual void OnItemCheck(ItemCheckEventArgs args)
        {
            ClearCheckedItems();
            ItemCheck?.Invoke(this, args);
        }

        public void SetItemCheckState(int index, CheckState value)
        {
            ((Item)control.Items[index]).SetState(value);
        }

        public void SetItemChecked(int index, bool value)
        {
            SetItemCheckState(index, value ? CheckState.Checked : CheckState.Unchecked);
        }

        public CheckedListBox()
        {
            control.ItemTemplate = dataTemplate;
            Items = new ObjectCollection(this, control.Items);
        }
    }
}