code_review: PR #1185

- make `SourceGit` running in singleton mode
- `TrySendArgsToExistingInstance` should not be called before `BuildAvaloniaApp().StartWithClassicDesktopLifetime(args)` since we may want to launch `SourceGit` as a core editor.
- avoid `preference.json` to be saved by multiple instances.
- move IPC code to models.

Signed-off-by: leo <longshuang@msn.cn>

src/Models/IpcChannel.cs

@@ -0,0 +1,97 @@
+using System;
+using System.IO;
+using System.IO.Pipes;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace SourceGit.Models
+{
+    public class IpcChannel : IDisposable
+    {
+        public bool IsFirstInstance
+        {
+            get => _server != null;
+        }
+
+        public event Action<string> MessageReceived;
+
+        public IpcChannel()
+        {
+            try
+            {
+                _server = new NamedPipeServerStream("SourceGitIPCChannel", PipeDirection.In, 1, PipeTransmissionMode.Byte, PipeOptions.Asynchronous | PipeOptions.CurrentUserOnly);
+                _cancellationTokenSource = new CancellationTokenSource();
+                Task.Run(StartServer);
+            }
+            catch
+            {
+                // IGNORE
+            }
+        }
+
+        public void SendToFirstInstance(string cmd)
+        {
+            try
+            {
+                using (var client = new NamedPipeClientStream(".", "SourceGitIPCChannel", PipeDirection.Out))
+                {
+                    client.Connect(1000);
+                    if (client.IsConnected)
+                    {
+                        using (var writer = new StreamWriter(client))
+                        {
+                            writer.Write(Encoding.UTF8.GetBytes(cmd));
+                            writer.Flush();
+                        }
+                    }
+                }
+            }
+            catch
+            {
+                // IGNORE
+            }
+        }
+
+        public void Dispose()
+        {
+            _server?.Close();
+        }
+
+        private async void StartServer()
+        {
+            var buffer = new byte[1024];
+
+            while (true)
+            {
+                try
+                {
+                    await _server.WaitForConnectionAsync(_cancellationTokenSource.Token);
+
+                    using (var stream = new MemoryStream())
+                    {
+                        while (true)
+                        {
+                            var readed = await _server.ReadAsync(buffer.AsMemory(0, 1024), _cancellationTokenSource.Token);
+                            if (readed == 0)
+                                break;
+
+                            stream.Write(buffer, 0, readed);
+                        }
+
+                        stream.Seek(0, SeekOrigin.Begin);
+                        MessageReceived?.Invoke(Encoding.UTF8.GetString(stream.ToArray()).Trim());
+                        _server.Disconnect();
+                    }
+                }
+                catch
+                {
+                    // IGNORE
+                }
+            }
+        }
+
+        private NamedPipeServerStream _server = null;
+        private CancellationTokenSource _cancellationTokenSource = null;
+    }
+}