Added a background file system watcher.

FileManager/AudibleFileStorage.cs

@@ -41,6 +41,8 @@ namespace FileManager
                 return Directory.CreateDirectory(Configuration.Instance.Books).FullName;
             }
         }
+
+        private static BackgroundFileSystem BookDirectoryFiles { get; } = new BackgroundFileSystem();
         #endregion
 
         #region instance
@@ -53,7 +55,7 @@ namespace FileManager
 		{
 			extensions_noDots = Extensions.Select(ext => ext.Trim('.')).ToList();
 			extAggr = extensions_noDots.Aggregate((a, b) => $"{a}|{b}");
-		}
+        }
 
         /// <summary>
         /// Example for full books:
@@ -63,16 +65,31 @@ namespace FileManager
         /// </summary>
         public bool Exists(string productId) => GetPath(productId) != null;
 
-		public string GetPath(string productId)
+        public string GetPath(string productId)
         {
             var cachedFile = FilePathCache.GetPath(productId, FileType);
             if (cachedFile != null)
                 return cachedFile;
 
-            var firstOrNull =
+            string storageDir = StorageDirectory;
-				Directory
+            string regexPattern = $@"{productId}.*?\.({extAggr})$";
-				.EnumerateFiles(StorageDirectory, "*.*", SearchOption.AllDirectories)
+            string firstOrNull;
-				.FirstOrDefault(s => Regex.IsMatch(s, $@"{productId}.*?\.({extAggr})$", RegexOptions.IgnoreCase));
+
+            if (storageDir == BooksDirectory)
+            {
+                //If user changed the BooksDirectory, reinitialize.
+                if (storageDir != BookDirectoryFiles.RootDirectory)
+                    BookDirectoryFiles.Init(storageDir, "*.*", SearchOption.AllDirectories);
+
+                firstOrNull = BookDirectoryFiles.FindFile(regexPattern, RegexOptions.IgnoreCase);
+            }
+            else
+            {
+                firstOrNull =
+                    Directory
+                    .EnumerateFiles(storageDir, "*.*", SearchOption.AllDirectories)
+                    .FirstOrDefault(s => Regex.IsMatch(s, regexPattern, RegexOptions.IgnoreCase));
+            }
 
 			if (firstOrNull is null)
 				return null;

FileManager/BackgroundFileSystem.cs

@@ -0,0 +1,107 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Text.RegularExpressions;
+using System.Threading.Tasks;
+
+namespace FileManager
+{
+    class BackgroundFileSystem
+    {
+        public string RootDirectory { get; private set; }
+        public string SearchPattern { get; private set; }
+        public SearchOption SearchOption { get; private set; }
+
+        private FileSystemWatcher fileSystemWatcher { get; set; }
+        private BlockingCollection<FileSystemEventArgs> directoryChangesEvents { get; set; }
+        private Task backgroundScanner { get; set; }
+        private List<string> fsCache { get; set; }
+
+        public string FindFile(string regexPattern, RegexOptions options) =>
+            fsCache.FirstOrDefault(s => Regex.IsMatch(s, regexPattern, options));
+
+
+        public void Init(string rootDirectory, string searchPattern, SearchOption searchOptions)
+        {
+            RootDirectory = rootDirectory;
+            SearchPattern = searchPattern;
+            SearchOption = searchOptions;
+
+            //Calling CompleteAdding() will cause background scanner to terminate.
+            directoryChangesEvents?.CompleteAdding();
+            fsCache?.Clear();
+            directoryChangesEvents?.Dispose();
+            fileSystemWatcher?.Dispose();
+
+            fsCache = Directory.EnumerateFiles(RootDirectory, SearchPattern, SearchOption).ToList();
+
+            directoryChangesEvents = new BlockingCollection<FileSystemEventArgs>();
+            fileSystemWatcher = new FileSystemWatcher(RootDirectory);
+            fileSystemWatcher.Created += FileSystemWatcher_Changed;
+            fileSystemWatcher.Deleted += FileSystemWatcher_Changed;
+            fileSystemWatcher.Renamed += FileSystemWatcher_Changed;
+            fileSystemWatcher.Error += FileSystemWatcher_Error;
+            fileSystemWatcher.IncludeSubdirectories = true;
+            fileSystemWatcher.EnableRaisingEvents = true;
+
+            //Wait for background scanner to terminate before reinitializing.
+            backgroundScanner?.Wait();
+            backgroundScanner = new Task(BackgroundScanner);
+            backgroundScanner.Start();
+        }
+
+        private void FileSystemWatcher_Error(object sender, ErrorEventArgs e)
+        {
+            Init(RootDirectory, SearchPattern, SearchOption);
+        }
+
+        private void FileSystemWatcher_Changed(object sender, FileSystemEventArgs e)
+        {
+            directoryChangesEvents.Add(e);
+        }
+
+        private void UpdateLocalCache(FileSystemEventArgs change) 
+        {
+            if (change.ChangeType == WatcherChangeTypes.Deleted)
+            {
+                RemovePath(change.FullPath);
+            }
+            else if (change.ChangeType == WatcherChangeTypes.Created)
+            {
+                AddPath(change.FullPath);
+            }
+            else if (change.ChangeType == WatcherChangeTypes.Renamed)
+            {
+                var renameChange = change as RenamedEventArgs;
+
+                RemovePath(renameChange.OldFullPath);
+                AddPath(renameChange.FullPath);
+            }
+        }
+
+        private void RemovePath(string path)
+        {
+            var pathsToRemove = fsCache.Where(p => p.StartsWith(path)).ToArray();
+
+            foreach (var p in pathsToRemove)
+                fsCache.Remove(p);
+        }
+
+        private void AddPath(string path)
+        {
+            if (File.GetAttributes(path).HasFlag(FileAttributes.Directory))
+                fsCache.AddRange(Directory.EnumerateFiles(path, SearchPattern, SearchOption));
+            else
+                fsCache.Add(path);
+        }
+
+        private void BackgroundScanner()
+        {
+            while (directoryChangesEvents.TryTake(out FileSystemEventArgs change, -1))
+                UpdateLocalCache(change);
+        }
+    }
+}