project: reorganize the structure of the project.

* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.

src/Models/AvatarManager.cs

@@ -0,0 +1,164 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+
+using Avalonia.Media.Imaging;
+using Avalonia.Threading;
+
+namespace SourceGit.Models
+{
+    public interface IAvatarHost
+    {
+        void OnAvatarResourceChanged(string md5);
+    }
+
+    public static class AvatarManager
+    {
+        public static string SelectedServer
+        {
+            get;
+            set;
+        } = "https://www.gravatar.com/avatar/";
+
+        static AvatarManager()
+        {
+            _storePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), "SourceGit", "avatars");
+            if (!Directory.Exists(_storePath))
+                Directory.CreateDirectory(_storePath);
+
+            Task.Run(() =>
+            {
+                while (true)
+                {
+                    var md5 = null as string;
+
+                    lock (_synclock)
+                    {
+                        foreach (var one in _requesting)
+                        {
+                            md5 = one;
+                            break;
+                        }
+                    }
+
+                    if (md5 == null)
+                    {
+                        Thread.Sleep(100);
+                        continue;
+                    }
+
+                    var localFile = Path.Combine(_storePath, md5);
+                    var img = null as Bitmap;
+                    try
+                    {
+                        var client = new HttpClient() { Timeout = TimeSpan.FromSeconds(2) };
+                        var task = client.GetAsync($"{SelectedServer}{md5}?d=404");
+                        task.Wait();
+
+                        var rsp = task.Result;
+                        if (rsp.IsSuccessStatusCode)
+                        {
+                            using (var stream = rsp.Content.ReadAsStream())
+                            {
+                                using (var writer = File.OpenWrite(localFile))
+                                {
+                                    stream.CopyTo(writer);
+                                }
+                            }
+
+                            using (var reader = File.OpenRead(localFile))
+                            {
+                                img = Bitmap.DecodeToWidth(reader, 128);
+                            }
+                        }
+                    }
+                    catch { }
+
+                    lock (_synclock)
+                    {
+                        _requesting.Remove(md5);
+                    }
+
+                    Dispatcher.UIThread.InvokeAsync(() =>
+                    {
+                        if (_resources.ContainsKey(md5))
+                            _resources[md5] = img;
+                        else
+                            _resources.Add(md5, img);
+                        NotifyResourceChanged(md5);
+                    });
+                }
+            });
+        }
+
+        public static void Subscribe(IAvatarHost host)
+        {
+            _avatars.Add(host);
+        }
+
+        public static void Unsubscribe(IAvatarHost host)
+        {
+            _avatars.Remove(host);
+        }
+
+        public static Bitmap Request(string md5, bool forceRefetch = false)
+        {
+            if (forceRefetch)
+            {
+                if (_resources.ContainsKey(md5))
+                    _resources.Remove(md5);
+
+                var localFile = Path.Combine(_storePath, md5);
+                if (File.Exists(localFile))
+                    File.Delete(localFile);
+
+                NotifyResourceChanged(md5);
+            }
+            else
+            {
+                if (_resources.TryGetValue(md5, out var value))
+                    return value;
+
+                var localFile = Path.Combine(_storePath, md5);
+                if (File.Exists(localFile))
+                {
+                    try
+                    {
+                        using (var stream = File.OpenRead(localFile))
+                        {
+                            var img = Bitmap.DecodeToWidth(stream, 128);
+                            _resources.Add(md5, img);
+                            return img;
+                        }
+                    }
+                    catch { }
+                }
+            }
+
+            lock (_synclock)
+            {
+                if (!_requesting.Contains(md5))
+                    _requesting.Add(md5);
+            }
+
+            return null;
+        }
+
+        private static void NotifyResourceChanged(string md5)
+        {
+            foreach (var avatar in _avatars)
+            {
+                avatar.OnAvatarResourceChanged(md5);
+            }
+        }
+
+        private static readonly object _synclock = new object();
+        private static readonly string _storePath = string.Empty;
+        private static readonly List<IAvatarHost> _avatars = new List<IAvatarHost>();
+        private static readonly Dictionary<string, Bitmap> _resources = new Dictionary<string, Bitmap>();
+        private static readonly HashSet<string> _requesting = new HashSet<string>();
+    }
+}