Add UseReactDevelopmentServer() middleware. Factor out common code.

src/Microsoft.AspNetCore.SpaServices.Extensions/Util/EventedStreamReader.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.IO;
+using System.Text;
+using System.Text.RegularExpressions;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.NodeServices.Util
+{
+    /// <summary>
+    /// Wraps a <see cref="StreamReader"/> to expose an evented API, issuing notifications
+    /// when the stream emits partial lines, completed lines, or finally closes.
+    /// </summary>
+    internal class EventedStreamReader
+    {
+        public delegate void OnReceivedChunkHandler(ArraySegment<char> chunk);
+        public delegate void OnReceivedLineHandler(string line);
+        public delegate void OnStreamClosedHandler();
+
+        public event OnReceivedChunkHandler OnReceivedChunk;
+        public event OnReceivedLineHandler OnReceivedLine;
+        public event OnStreamClosedHandler OnStreamClosed;
+
+        private readonly StreamReader _streamReader;
+        private readonly StringBuilder _linesBuffer;
+
+        public EventedStreamReader(StreamReader streamReader)
+        {
+            _streamReader = streamReader ?? throw new ArgumentNullException(nameof(streamReader));
+            _linesBuffer = new StringBuilder();
+            Task.Factory.StartNew(Run);
+        }
+
+        public Task<Match> WaitForMatch(Regex regex, TimeSpan timeout = default)
+        {
+            var tcs = new TaskCompletionSource<Match>();
+            var completionLock = new object();
+
+            OnReceivedLineHandler onReceivedLineHandler = null;
+            OnStreamClosedHandler onStreamClosedHandler = null;
+
+            void ResolveIfStillPending(Action applyResolution)
+            {
+                lock (completionLock)
+                {
+                    if (!tcs.Task.IsCompleted)
+                    {
+                        OnReceivedLine -= onReceivedLineHandler;
+                        OnStreamClosed -= onStreamClosedHandler;
+                        applyResolution();
+                    }
+                }
+            }
+
+            onReceivedLineHandler = line =>
+            {
+                var match = regex.Match(line);
+                if (match.Success)
+                {
+                    ResolveIfStillPending(() => tcs.SetResult(match));
+                }
+            };
+
+            onStreamClosedHandler = () =>
+            {
+                ResolveIfStillPending(() => tcs.SetException(new EndOfStreamException()));
+            };
+
+            OnReceivedLine += onReceivedLineHandler;
+            OnStreamClosed += onStreamClosedHandler;
+
+            if (timeout != default)
+            {
+                var timeoutToken = new CancellationTokenSource(timeout);
+                timeoutToken.Token.Register(() =>
+                {
+                    ResolveIfStillPending(() => tcs.SetCanceled());
+                });
+            }
+
+            return tcs.Task;
+        }
+
+        private async Task Run()
+        {
+            var buf = new char[8 * 1024];
+            while (true)
+            {
+                var chunkLength = await _streamReader.ReadAsync(buf, 0, buf.Length);
+                if (chunkLength == 0)
+                {
+                    OnClosed();
+                    break;
+                }
+
+                OnChunk(new ArraySegment<char>(buf, 0, chunkLength));
+
+                var lineBreakPos = Array.IndexOf(buf, '\n', 0, chunkLength);
+                if (lineBreakPos < 0)
+                {
+                    _linesBuffer.Append(buf, 0, chunkLength);
+                }
+                else
+                {
+                    _linesBuffer.Append(buf, 0, lineBreakPos + 1);
+                    OnCompleteLine(_linesBuffer.ToString());
+                    _linesBuffer.Clear();
+                    _linesBuffer.Append(buf, lineBreakPos + 1, chunkLength - (lineBreakPos + 1));
+                }
+            }
+        }
+
+        private void OnChunk(ArraySegment<char> chunk)
+        {
+            var dlg = OnReceivedChunk;
+            dlg?.Invoke(chunk);
+        }
+
+        private void OnCompleteLine(string line)
+        {
+            var dlg = OnReceivedLine;
+            dlg?.Invoke(line);
+        }
+
+        private void OnClosed()
+        {
+            var dlg = OnStreamClosed;
+            dlg?.Invoke();
+        }
+    }
+}