refactor "ChainTo" to instance method "Then"

Author: EamonHetherton

src/Serilog.Sinks.File/Sinks/File/FileLifeCycleHookChain.cs

@@ -0,0 +1,46 @@
+// Copyright 2019 Serilog Contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using System;
+using System.IO;
+using System.Text;
+
+namespace Serilog.Sinks.File
+{
+    class FileLifeCycleHookChain : FileLifecycleHooks
+    {
+        private readonly FileLifecycleHooks _first;
+        private readonly FileLifecycleHooks _second;
+
+        public FileLifeCycleHookChain(FileLifecycleHooks first, FileLifecycleHooks second)
+        {
+            _first = first ?? throw new ArgumentNullException(nameof(first));
+            _second = second ?? throw new ArgumentNullException(nameof(second));
+        }
+
+        public override Stream OnFileOpened(Stream underlyingStream, Encoding encoding)
+        {
+            var firstStreamResult = _first.OnFileOpened(underlyingStream, encoding);
+            var secondStreamResult = _second.OnFileOpened(firstStreamResult, encoding);
+
+            return secondStreamResult;
+        }
+
+        public override void OnFileDeleting(string path)
+        {
+            _first.OnFileDeleting(path);
+            _second.OnFileDeleting(path);
+        }
+    }
+}

src/Serilog.Sinks.File/Sinks/File/FileLifecycleHooks.cs

@@ -43,5 +43,22 @@ public abstract class FileLifecycleHooks
         /// </summary>
         /// <param name="path">The full path to the file being deleted.</param>
         public virtual void OnFileDeleting(string path) {}
+
+        /// <summary>
+        /// Creates a chain of <see cref="FileLifecycleHooks"/> that have their methods called sequentially
+        /// Can be used to compose <see cref="FileLifecycleHooks"/> together; e.g. add header information to each log file and
+        /// compress it.
+        /// </summary>
+        /// <example>
+        /// <code>
+        /// var hooks = new GZipHooks().Then(new HeaderWriter("File Header"));
+        /// </code>
+        /// </example>
+        /// <param name="next">The next <see cref="FileLifecycleHooks"/> to have its methods called in the chain</param>
+        /// <returns></returns>
+        public FileLifecycleHooks Then(FileLifecycleHooks next)
+        {
+            return new FileLifeCycleHookChain(this, next);
+        }
     }
 }