adding ability for IFileSystem to tell me when something changes on disk.

first pass at implementing the windows version of the new interface methods
IViewPort now has a public method to call whenever you want to notify it that its underlying file has been changed.
Added 3 tests showing how the Editor and ViewPort should behave with this new functionality.
- Editor should hook up ViewPorts whenever a new tab is opened
- Editor should clear out ViewPorts whenever a tab is closed
- ViewPort should reload when the file is changed and it is not.

Note that the model isn't implemented yet. This is just some failing tests.

Gen3Hex/Model/IFileSystem.cs

@@ -1,4 +1,6 @@
-namespace HavenSoft.Gen3Hex.Model {
+using System;
+
+namespace HavenSoft.Gen3Hex.Model {
    public interface IFileSystem {
       string CopyText { get; set; }
 
@@ -34,6 +36,22 @@
       /// </returns>
       LoadedFile OpenFile(params string[] extensionOptions);
 
+      /// <summary>
+      /// Have the filesystem open a specific file.
+      /// </summary>
+      /// <returns>
+      /// If the file exists, it is loaded and returned.
+      /// If it doesn't exist, returns null.
+      /// </returns>
+      LoadedFile LoadFile(string fileName);
+
+      /// <summary>
+      /// When a file changes, the filesystem will call all listeners for that file.
+      /// </summary>
+      void AddListenerToFile(string fileName, Action<IFileSystem> listener);
+
+      void RemoveAllListenersForFile(string fileName);
+
       /// <summary>
       /// Saves the file without prompting the user for permission.
       /// </summary>

Gen3Hex/View/WindowsFileSystem.cs

@@ -1,11 +1,15 @@
 using HavenSoft.Gen3Hex.Model;
 using Microsoft.Win32;
+using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Windows;
 
 namespace HavenSoft.Gen3Hex.View {
    public class WindowsFileSystem : IFileSystem {
+      private readonly Dictionary<string, List<FileSystemWatcher>> watchers = new Dictionary<string, List<FileSystemWatcher>>();
+
       public string CopyText {
          get => Clipboard.ContainsText() ? Clipboard.GetText() : string.Empty;
          set => Clipboard.SetText(value);
@@ -15,9 +19,31 @@ namespace HavenSoft.Gen3Hex.View {
          var dialog = new OpenFileDialog { Filter = CreateFilterFromOptions(extensionOptions) };
          var result = dialog.ShowDialog();
          if (result != true) return null;
-         if (!File.Exists(dialog.FileName)) return null;
-         var data = File.ReadAllBytes(dialog.FileName);
-         return new LoadedFile(dialog.FileName, data);
+         return LoadFile(dialog.FileName);
+      }
+
+      public LoadedFile LoadFile(string fileName) {
+         if (!File.Exists(fileName)) return null;
+         var data = File.ReadAllBytes(fileName);
+         return new LoadedFile(fileName, data);
+      }
+
+      public void AddListenerToFile(string fileName, Action<IFileSystem> listener) {
+         var watcher = new FileSystemWatcher(Path.GetDirectoryName(fileName)) {
+            NotifyFilter = NotifyFilters.LastAccess | NotifyFilters.LastWrite | NotifyFilters.FileName,
+         };
+         watcher.Changed += (sender, e) => {
+            if (e.FullPath.EndsWith(fileName)) listener(this);
+         };
+         watcher.EnableRaisingEvents = true;
+
+         if (!watchers.ContainsKey(fileName)) watchers[fileName] = new List<FileSystemWatcher>();
+         watchers[fileName].Add(watcher);
+      }
+
+      public void RemoveAllListenersForFile(string fileName) {
+         if (!watchers.ContainsKey(fileName)) return;
+         watchers[fileName].Clear();
       }
 
       public string RequestNewName(string currentName, params string[] extensionOptions) {

Gen3Hex/ViewModel/ITabContent.cs

@@ -1,4 +1,5 @@
-using System;
+using HavenSoft.Gen3Hex.Model;
+using System;
 using System.Windows.Input;
 
 namespace HavenSoft.Gen3Hex.ViewModel {

Gen3Hex/ViewModel/IViewPort.cs

@@ -22,5 +22,6 @@ namespace HavenSoft.Gen3Hex.ViewModel {
       IReadOnlyList<int> Find(string search);
       IChildViewPort CreateChildView(int offset);
       void FollowLink(int x, int y);
+      void ConsiderReload(IFileSystem fileSystem);
    }
 }

Gen3Hex/ViewModel/SearchResultsViewPort.cs

@@ -116,6 +116,8 @@ namespace HavenSoft.Gen3Hex.ViewModel {
          RequestTabChange?.Invoke(this, parent);
       }
 
+      public void ConsiderReload(IFileSystem fileSystem) { }
+
       private void NotifyCollectionChanged() {
          if (children.Count == 0) return;
          UpdateHeaders();

Gen3Hex/ViewModel/ViewPort.cs

@@ -301,6 +301,10 @@ namespace HavenSoft.Gen3Hex.ViewModel {
 
       public void FollowLink(int x, int y) { }
 
+      public void ConsiderReload(IFileSystem fileSystem) {
+         
+      }
+
       private void Edit(char input) {
          var point = GetEditPoint();
          var element = currentView[point.X, point.Y];

HexTests/GeneralAppTests.cs

@@ -204,7 +204,6 @@ namespace HavenSoft.HexTests {
          Assert.Equal(7, count);
       }
 
-
       [Theory]
       [InlineData(nameof(EditorViewModel.Copy))]
       [InlineData(nameof(EditorViewModel.Delete))]
@@ -323,6 +322,58 @@ namespace HavenSoft.HexTests {
          Assert.Equal(1, editor.SelectedIndex);
       }
 
+      [Fact]
+      public void EditorAddsOpenedFilesToFileSystemWatch() {
+         var fileSystem = new StubFileSystem();
+         string name = null;
+         fileSystem.AddListenerToFile = (fileName, action) => name = fileName;
+         var editor = new EditorViewModel(fileSystem);
+
+         editor.Open.Execute(new LoadedFile("InputFile.txt", new byte[20]));
+
+         Assert.Equal("InputFile.txt", name);
+      }
+
+      [Fact]
+      public void EditorRemovesFileSystemWatchWhenTabsClose() {
+         var fileSystem = new StubFileSystem();
+         string name = null;
+         fileSystem.RemoveAllListenersForFile = fileName => name = fileName;
+         var editor = new EditorViewModel(fileSystem);
+         editor.Open.Execute(new LoadedFile("InputFile.txt", new byte[20]));
+
+         editor.Close.Execute();
+
+         Assert.Equal("InputFile.txt", name);
+      }
+
+      [Fact]
+      public void ViewPortReloadsIfNoLocalChangesWhenFileChanges() {
+         var fileSystem = new StubFileSystem();
+         string file = null;
+         fileSystem.LoadFile = input => { file = input; return new LoadedFile(input, new byte[] { 0x10, 0x20 }); };
+         var viewPort = new ViewPort(new LoadedFile("file.txt", new byte[] { 0x00, 0x00 }));
+
+         viewPort.ConsiderReload(fileSystem);
+
+         Assert.Equal("file.txt", file);
+         Assert.Equal(0x10, viewPort[0, 0].Value);
+      }
+
+      [Fact]
+      public void ViewPortDoesNotReloadIfLocalChangesWhenFileChanges() {
+         var fileSystem = new StubFileSystem();
+         string file = null;
+         fileSystem.LoadFile = input => { file = input; return new LoadedFile(input, new byte[] { 0x10, 0x20 }); };
+         var viewPort = new ViewPort(new LoadedFile("file.txt", new byte[] { 0x00, 0x00 }));
+
+         viewPort.Edit("05");
+         viewPort.ConsiderReload(fileSystem);
+
+         Assert.Null(file);
+         Assert.Equal(0x05, viewPort[0, 0].Value);
+      }
+
       private StubTabContent CreateClosableTab() {
          var tab = new StubTabContent();
          var close = new StubCommand { CanExecute = arg => true };