Gallery/Gallery.iOS/Renderers/AppShellSection/AppShellSectionRootHeader.cs

using System;
using System.Collections.Specialized;
using System.ComponentModel;
using CoreGraphics;
using Foundation;
using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

namespace Gallery.iOS.Renderers.AppShellSection
{
    public class AppShellSectionRootHeader : UICollectionViewController, IAppearanceObserver, IShellSectionRootHeader
    {
        #region IAppearanceObserver

        readonly Color _defaultBackgroundColor = new(0.964);
        readonly Color _defaultForegroundColor = Color.Black;
        readonly Color _defaultUnselectedColor = Color.Black.MultiplyAlpha(0.7);

        void IAppearanceObserver.OnAppearanceChanged(ShellAppearance appearance)
        {
            if (appearance == null)
                ResetAppearance();
            else
                SetAppearance(appearance);
        }

        protected virtual void ResetAppearance()
        {
            SetValues(_defaultBackgroundColor, _defaultForegroundColor, _defaultUnselectedColor);
        }

        protected virtual void SetAppearance(ShellAppearance appearance)
        {
            SetValues(appearance.BackgroundColor.IsDefault ? _defaultBackgroundColor : appearance.BackgroundColor,
                appearance.ForegroundColor.IsDefault ? _defaultForegroundColor : appearance.ForegroundColor,
                appearance.UnselectedColor.IsDefault ? _defaultUnselectedColor : appearance.UnselectedColor);
        }

        void SetValues(Color backgroundColor, Color foregroundColor, Color unselectedColor)
        {
            CollectionView.BackgroundColor = new Color(backgroundColor.R, backgroundColor.G, backgroundColor.B, .663).ToUIColor();
            _bar.BackgroundColor = foregroundColor.ToUIColor();

            bool reloadData = _selectedColor != foregroundColor || _unselectedColor != unselectedColor;

            _selectedColor = foregroundColor;
            _unselectedColor = unselectedColor;

            if (reloadData)
                CollectionView.ReloadData();
        }

        #endregion IAppearanceObserver

        static readonly NSString CellId = new("HeaderCell");

        readonly IShellContext _shellContext;
        UIView _bar;
        UIView _bottomShadow;
        Color _selectedColor;
        Color _unselectedColor;
        bool _isDisposed;

        public AppShellSectionRootHeader(IShellContext shellContext) : base(new UICollectionViewFlowLayout())
        {
            _shellContext = shellContext;
        }

        public double SelectedIndex { get; set; }
        public ShellSection ShellSection { get; set; }
        IShellSectionController ShellSectionController => ShellSection;

        public UIViewController ViewController => this;

        public override bool CanMoveItem(UICollectionView collectionView, NSIndexPath indexPath)
        {
            return false;
        }

        public override UICollectionViewCell GetCell(UICollectionView collectionView, NSIndexPath indexPath)
        {
            var reusedCell = (UICollectionViewCell)collectionView.DequeueReusableCell(CellId, indexPath);

            if (reusedCell is not ShellSectionHeaderCell headerCell)
                return reusedCell;

            var selectedItems = collectionView.GetIndexPathsForSelectedItems();

            var shellContent = ShellSectionController.GetItems()[indexPath.Row];
            headerCell.Label.Text = shellContent.Title;
            headerCell.Label.SetNeedsDisplay();

            headerCell.SelectedColor = _selectedColor.ToUIColor();
            headerCell.UnSelectedColor = _unselectedColor.ToUIColor();

            if (selectedItems.Length > 0 && selectedItems[0].Row == indexPath.Row)
                headerCell.Selected = true;
            else
                headerCell.Selected = false;

            return headerCell;
        }

        public override nint GetItemsCount(UICollectionView collectionView, nint section)
        {
            return ShellSectionController.GetItems().Count;
        }

        public override void ItemDeselected(UICollectionView collectionView, NSIndexPath indexPath)
        {
            if (CollectionView.CellForItem(indexPath) is ShellSectionHeaderCell cell)
                cell.Label.TextColor = _unselectedColor.ToUIColor();
        }

        public override void ItemSelected(UICollectionView collectionView, NSIndexPath indexPath)
        {
            var row = indexPath.Row;

            var item = ShellSectionController.GetItems()[row];

            if (item != ShellSection.CurrentItem)
                ShellSection.SetValueFromRenderer(ShellSection.CurrentItemProperty, item);

            if (CollectionView.CellForItem(indexPath) is ShellSectionHeaderCell cell)
                cell.Label.TextColor = _selectedColor.ToUIColor();
        }

        public override nint NumberOfSections(UICollectionView collectionView)
        {
            return 1;
        }

        public override bool ShouldSelectItem(UICollectionView collectionView, NSIndexPath indexPath)
        {
            var row = indexPath.Row;
            var item = ShellSectionController.GetItems()[row];
            IShellController shellController = _shellContext.Shell;

            if (item == ShellSection.CurrentItem)
                return true;
            return shellController.ProposeNavigation(ShellNavigationSource.ShellContentChanged, (ShellItem)ShellSection.Parent, ShellSection, item, ShellSection.Stack, true);
        }

        public override void ViewDidLayoutSubviews()
        {
            if (_isDisposed)
                return;

            base.ViewDidLayoutSubviews();

            LayoutBar();

            _bottomShadow.Frame = new CGRect(0, CollectionView.Frame.Bottom, CollectionView.Frame.Width, 0.5);
        }

        public override void ViewDidLoad()
        {
            if (_isDisposed)
                return;

            base.ViewDidLoad();

            CollectionView.ScrollsToTop = false;
            CollectionView.Bounces = false;
            CollectionView.AlwaysBounceHorizontal = false;
            CollectionView.ShowsHorizontalScrollIndicator = false;
            CollectionView.ClipsToBounds = false;

            _bar = new UIView(new CGRect(0, 0, 20, 20));
            _bar.Layer.ZPosition = 9001; //its over 9000!
            CollectionView.AddSubview(_bar);

            _bottomShadow = new UIView(new CGRect(0, 0, 10, 1))
            {
                BackgroundColor = Color.Black.MultiplyAlpha(0.3).ToUIColor()
            };
            _bottomShadow.Layer.ZPosition = 9002;
            CollectionView.AddSubview(_bottomShadow);

            var flowLayout = Layout as UICollectionViewFlowLayout;
            flowLayout.ScrollDirection = UICollectionViewScrollDirection.Horizontal;
            flowLayout.MinimumInteritemSpacing = 0;
            flowLayout.MinimumLineSpacing = 0;
            flowLayout.EstimatedItemSize = new CGSize(70, 35);

            CollectionView.RegisterClassForCell(GetCellType(), CellId);

            ((IShellController)_shellContext.Shell).AddAppearanceObserver(this, ShellSection);
            ShellSectionController.ItemsCollectionChanged += OnShellSectionItemsChanged;

            UpdateSelectedIndex();
            ShellSection.PropertyChanged += OnShellSectionPropertyChanged;
        }

        protected virtual Type GetCellType()
        {
            return typeof(ShellSectionHeaderCell);
        }

        protected override void Dispose(bool disposing)
        {
            if (_isDisposed)
                return;

            if (disposing)
            {
                ((IShellController)_shellContext.Shell).RemoveAppearanceObserver(this);
                ShellSectionController.ItemsCollectionChanged -= OnShellSectionItemsChanged;
                ShellSection.PropertyChanged -= OnShellSectionPropertyChanged;

                ShellSection = null;
                _bar.RemoveFromSuperview();
                RemoveFromParentViewController();
                _bar.Dispose();
                _bar = null;
            }

            _isDisposed = true;
            base.Dispose(disposing);
        }

        protected void LayoutBar()
        {
            if (SelectedIndex < 0)
                return;

            if (ShellSectionController.GetItems().IndexOf(ShellSection.CurrentItem) != SelectedIndex)
                return;

            var layout = CollectionView.GetLayoutAttributesForItem(NSIndexPath.FromItemSection((int)SelectedIndex, 0));

            if (layout == null)
                return;

            var frame = layout.Frame;

            if (_bar.Frame.Height != 2)
            {
                _bar.Frame = new CGRect(frame.X, frame.Bottom - 2, frame.Width, 2);
            }
            else
            {
                UIView.Animate(.25, () => _bar.Frame = new CGRect(frame.X, frame.Bottom - 2, frame.Width, 2));
            }
        }

        protected virtual void OnShellSectionPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            if (e.PropertyName == ShellSection.CurrentItemProperty.PropertyName)
            {
                UpdateSelectedIndex();
            }
        }

        protected virtual void UpdateSelectedIndex(bool animated = false)
        {
            if (ShellSection.CurrentItem == null)
                return;

            SelectedIndex = ShellSectionController.GetItems().IndexOf(ShellSection.CurrentItem);

            if (SelectedIndex < 0)
                return;

            LayoutBar();

            CollectionView.SelectItem(NSIndexPath.FromItemSection((int)SelectedIndex, 0), false, UICollectionViewScrollPosition.CenteredHorizontally);
        }

        void OnShellSectionItemsChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            if (_isDisposed)
                return;

            CollectionView.ReloadData();
        }

        public class ShellSectionHeaderCell : UICollectionViewCell
        {
            public UIColor SelectedColor { get; set; }
            public UIColor UnSelectedColor { get; set; }

            public ShellSectionHeaderCell()
            {

            }

            [Export("initWithFrame:")]
            public ShellSectionHeaderCell(CGRect frame) : base(frame)
            {
                Label = new UILabel
                {
                    TextAlignment = UITextAlignment.Center,
                    Font = UIFont.BoldSystemFontOfSize(14)
                };
                ContentView.AddSubview(Label);
            }

            public override bool Selected
            {
                get => base.Selected;
                set
                {
                    base.Selected = value;
                    Label.TextColor = value ? SelectedColor : UnSelectedColor;
                }
            }

            public UILabel Label { get; }

            public override void LayoutSubviews()
            {
                base.LayoutSubviews();

                Label.Frame = Bounds;
            }

            public override CGSize SizeThatFits(CGSize size)
            {
                return new CGSize(Label.SizeThatFits(size).Width + 30, 35);
            }
        }
    }
}