Add chapter 4 completed solution

Chapter04/Complete/MyMediaCollection/Services/DataService.cs

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+using MyMediaCollection.Enums;
+using MyMediaCollection.Interfaces;
+using MyMediaCollection.Model;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace MyMediaCollection.Services
+{
+    public class DataService : IDataService
+    {
+        private IList<MediaItem> _items;
+        private IList<ItemType> _itemTypes;
+        private IList<Medium> _mediums;
+        private IList<LocationType> _locationTypes;
+
+        public DataService()
+        {
+            PopulateItemTypes();
+            PopulateMediums();
+            PopulateLocationTypes();
+            PopulateItems();
+        }
+
+        private void PopulateItems()
+        {
+            var cd = new MediaItem
+            {
+                Id = 1,
+                Name = "Classical Favorites",
+                MediaType = ItemType.Music,
+                MediumInfo = _mediums.FirstOrDefault(m => m.Name == "CD"),
+                Location = LocationType.InCollection
+            };
+
+            var book = new MediaItem
+            {
+                Id = 2,
+                Name = "Classic Fairy Tales",
+                MediaType = ItemType.Book,
+                MediumInfo = _mediums.FirstOrDefault(m => m.Name == "Hardcover"),
+                Location = LocationType.InCollection
+            };
+
+            var bluRay = new MediaItem
+            {
+                Id = 3,
+                Name = "The Mummy",
+                MediaType = ItemType.Video,
+                MediumInfo = _mediums.FirstOrDefault(m => m.Name == "Blu Ray"),
+                Location = LocationType.InCollection
+            };
+
+            _items = new List<MediaItem>
+            {
+                cd,
+                book,
+                bluRay
+            };
+        }
+
+        private void PopulateMediums()
+        {
+            var cd = new Medium { Id = 1, MediaType = ItemType.Music, Name = "CD" };
+            var vinyl = new Medium { Id = 2, MediaType = ItemType.Music, Name = "Vinyl" };
+            var hardcover = new Medium { Id = 3, MediaType = ItemType.Book, Name = "Hardcover" };
+            var paperback = new Medium { Id = 4, MediaType = ItemType.Book, Name = "Paperback" };
+            var dvd = new Medium { Id = 5, MediaType = ItemType.Video, Name = "DVD" };
+            var bluRay = new Medium { Id = 6, MediaType = ItemType.Video, Name = "Blu Ray" };
+
+            _mediums = new List<Medium>
+            {
+                cd,
+                vinyl,
+                hardcover,
+                paperback,
+                dvd,
+                bluRay
+            };
+        }
+
+        private void PopulateItemTypes()
+        {
+            _itemTypes = new List<ItemType>
+            {
+                ItemType.Book,
+                ItemType.Music,
+                ItemType.Video
+            };
+        }
+
+        private void PopulateLocationTypes()
+        {
+            _locationTypes = new List<LocationType>
+            {
+                LocationType.InCollection,
+                LocationType.Loaned
+            };
+        }
+
+        public int AddItem(MediaItem item)
+        {
+            item.Id = _items.Max(i => i.Id) + 1;
+            _items.Add(item);
+
+            return item.Id;
+        }
+
+        public MediaItem GetItem(int id)
+        {
+            return _items.FirstOrDefault(i => i.Id == id);
+        }
+
+        public IList<MediaItem> GetItems()
+        {
+            return _items;
+        }
+
+        public IList<ItemType> GetItemTypes()
+        {
+            return _itemTypes;
+        }
+
+        public IList<Medium> GetMediums()
+        {
+            return _mediums;
+        }
+
+        public IList<Medium> GetMediums(ItemType itemType)
+        {
+            return _mediums
+                .Where(m => m.MediaType == itemType)
+                .ToList();
+        }
+
+        public IList<LocationType> GetLocationTypes()
+        {
+            return _locationTypes;
+        }
+
+        public void UpdateItem(MediaItem item)
+        {
+            var idx = -1;
+            var matchedItem =
+                (from x in _items
+                 let ind = idx++
+                 where x.Id == item.Id
+                 select ind).FirstOrDefault();
+
+            if (idx == -1)
+            {
+                throw new Exception("Unable to update item. Item not found in collection.");
+            }
+
+            _items[idx] = item;
+        }
+
+        public Medium GetMedium(string name)
+        {
+            return _mediums.FirstOrDefault(m => m.Name == name);
+        }
+    }
+}

Chapter04/Complete/MyMediaCollection/Services/NavigationService.cs

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+using Microsoft.UI.Xaml.Controls;
+using MyMediaCollection.Interfaces;
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace MyMediaCollection.Services
+{
+    public class NavigationService : INavigationService
+    {
+        public NavigationService(Frame rootFrame)
+        {
+            AppFrame = rootFrame;
+        }
+
+        private readonly IDictionary<string, Type> _pages = new ConcurrentDictionary<string, Type>();
+
+        public const string RootPage = "(Root)";
+
+        public const string UnknownPage = "(Unknown)";
+
+        private static Frame AppFrame;
+
+        public void Configure(string page, Type type)
+        {
+            if (_pages.Values.Any(v => v == type))
+            {
+                throw new ArgumentException($"The {type.Name} view has already been registered under another name.");
+            }
+
+            _pages[page] = type;
+        }
+
+        /// <summary>
+        /// Gets the name of the currently displayed page.
+        /// </summary>
+        public string CurrentPage
+        {
+            get
+            {
+                var frame = AppFrame;
+
+                if (frame.BackStackDepth == 0)
+                    return RootPage;
+
+                if (frame.Content == null)
+                    return UnknownPage;
+
+                var type = frame.Content.GetType();
+
+                if (_pages.Values.All(v => v != type))
+                    return UnknownPage;
+
+                var item = _pages.Single(i => i.Value == type);
+
+                return item.Key;
+            }
+        }
+
+        public void NavigateTo(string page)
+        {
+            NavigateTo(page, null);
+        }
+
+        public void NavigateTo(string page, object parameter)
+        {
+            if (!_pages.ContainsKey(page))
+            {
+                throw new ArgumentException($"Unable to find a page registered with the name {page}.");
+            }
+
+            AppFrame.Navigate(_pages[page], parameter);
+        }
+
+        public void GoBack()
+        {
+            if (AppFrame?.CanGoBack == true)
+            {
+                AppFrame.GoBack();
+            }
+        }
+    }
+}