Introduce a protected virtual CreateJsonReader method in JsonEventFormatter

(Currently just for Json.NET)

Fixes #151

Signed-off-by: Jon Skeet <jonskeet@google.com>

src/CloudNative.CloudEvents.NewtonsoftJson/JsonEventFormatter.cs

@@ -524,7 +524,24 @@ namespace CloudNative.CloudEvents.NewtonsoftJson
             }
         }
 
-        internal static JsonReader CreateJsonReader(Stream stream, Encoding encoding) =>
+        /// <summary>
+        /// Creates a <see cref="JsonReader"/> for the given stream. This may be overridden in derived classes to
+        /// customize the JSON parsing process, subject to the constraints listed in the remarks.
+        /// </summary>
+        /// <remarks>
+        /// <para>
+        /// The default implementation always creates an instance of <see cref="JsonTextReader"/>, and derived classes
+        /// may assume that (calling this implementation and casting the result before modifying it).
+        /// </para>
+        /// <para>
+        /// Implementations should ensure that <see cref="JsonReader.DateParseHandling"/> is set to <see cref="DateParseHandling.None"/>,
+        /// as timestamp parsing is performed in a CloudEvent-specific way, and Json.NET's own implementation can obscure that.
+        /// </para>
+        /// </remarks>
+        /// <param name="stream">The stream to read from. Will not be null.</param>
+        /// <param name="encoding">The expected text encoding. May be null, in which case UTF-8 should be assumed.</param>
+        /// <returns>A JsonReader suitable for reading the </returns>
+        protected virtual JsonReader CreateJsonReader(Stream stream, Encoding encoding) =>
             new JsonTextReader(new StreamReader(stream, encoding ?? Encoding.UTF8, detectEncodingFromByteOrderMarks: true, bufferSize: 8192, leaveOpen: true))
             {
                 DateParseHandling = DateParseHandling.None

test/CloudNative.CloudEvents.UnitTests/NewtonsoftJson/SpecializedJsonReaderTest.cs

@@ -0,0 +1,97 @@
+// Copyright 2021 Cloud Native Foundation. 
+// Licensed under the Apache 2.0 license.
+// See LICENSE file in the project root for full license information.
+
+using CloudNative.CloudEvents.UnitTests;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using Xunit;
+
+namespace CloudNative.CloudEvents.NewtonsoftJson.UnitTests
+{
+    /// <summary>
+    /// Tests for the specialization of <see cref="JsonEventFormatter.CreateJsonReader(System.IO.Stream, Encoding)"/>
+    /// </summary>
+    public class SpecializedJsonReaderTest
+    {
+        [Fact]
+        public void DefaultImplementation_ReturnsJsonTextReader()
+        {
+            var formatter = new CreateJsonReaderExposingFormatter();
+            var reader = formatter.CreateJsonReaderPublic(CreateJsonStream(), null);
+            Assert.IsType<JsonTextReader>(reader);
+        }
+
+        [Fact]
+        public void DefaultImplementation_NoPropertyNameTable()
+        {
+            var formatter = new JsonEventFormatter();
+            var event1 = formatter.DecodeStructuredModeMessage(CreateJsonStream(), null, null);
+            var event2 = formatter.DecodeStructuredModeMessage(CreateJsonStream(), null, null);
+
+            JObject data1 = (JObject)event1.Data;
+            JObject data2 = (JObject)event2.Data;
+
+            var property1 = data1.Properties().Single();
+            var property2 = data2.Properties().Single();
+            Assert.Equal(property1.Name, property2.Name);
+            Assert.NotSame(property1.Name, property2.Name);
+        }
+
+        [Fact]
+        public void Specialization_WithPropertyNameTable()
+        {
+            var formatter = new PropertyNameTableFormatter();
+            var event1 = formatter.DecodeStructuredModeMessage(CreateJsonStream(), null, null);
+            var event2 = formatter.DecodeStructuredModeMessage(CreateJsonStream(), null, null);
+
+            JObject data1 = (JObject)event1.Data;
+            JObject data2 = (JObject)event2.Data;
+
+            var property1 = data1.Properties().Single();
+            var property2 = data2.Properties().Single();
+            Assert.Equal(property1.Name, property2.Name);
+            Assert.Same(property1.Name, property2.Name);
+        }
+
+        private Stream CreateJsonStream()
+        {
+            var cloudEvent = new CloudEvent
+            {
+                Data = new { DataName = "DataValue" }
+            }.PopulateRequiredAttributes();
+            var bytes = new JsonEventFormatter().EncodeStructuredModeMessage(cloudEvent, out _);
+            return new MemoryStream(bytes);
+        }
+
+        private class CreateJsonReaderExposingFormatter : JsonEventFormatter
+        {
+            public JsonReader CreateJsonReaderPublic(Stream stream, Encoding encoding) =>
+                base.CreateJsonReader(stream, encoding);
+        }
+
+        private class PropertyNameTableFormatter : JsonEventFormatter
+        {
+            private readonly DefaultJsonNameTable table;
+
+            public PropertyNameTableFormatter()
+            {
+                // Names aren't automatically cached by JsonTextReader, so we need to prepopulate the table.
+                table = new DefaultJsonNameTable();
+                table.Add("DataName");
+            }
+
+            protected override JsonReader CreateJsonReader(Stream stream, Encoding encoding)
+            {
+                var reader = (JsonTextReader) base.CreateJsonReader(stream, encoding);
+                reader.PropertyNameTable = table;
+                return reader;
+            }
+        }
+    }
+}