Add Copy/Move for content - Remove HtmlTextWriter

Currently we overload the definition of WriteTo on IHtmlContent
implementation classes to either represent a "real" write or a "flatten"
by checking if the writer inherits HtmlText writer.

This overloading is a bit of an odd fit and hides the real semantic we
want for flattening.

Additionally, we want to gradually make the concept of a pooled
backing-buffer for IHtmlContent first-class - using pooled buffers
dictates that we support move-semantics to some degree.

This change makes the work that we do for flattening into pooled buffers
explicit rather than hidden.

src/Microsoft.AspNetCore.Html.Abstractions/HtmlContentBuilder.cs

@@ -96,6 +96,70 @@ namespace Microsoft.AspNetCore.Html
             return this;
         }
 
+        /// <inheritdoc />
+        public void CopyTo(IHtmlContentBuilder destination)
+        {
+            if (destination == null)
+            {
+                throw new ArgumentNullException(nameof(destination));
+            }
+
+            for (var i = 0; i < Entries.Count; i++)
+            {
+                var entry = Entries[i];
+
+                string entryAsString;
+                IHtmlContentContainer entryAsContainer;
+                if ((entryAsString = entry as string)  != null)
+                {
+                    destination.Append(entryAsString);
+                }
+                else if ((entryAsContainer = entry as IHtmlContentContainer) != null)
+                {
+                    // Since we're copying, do a deep flatten.
+                    entryAsContainer.CopyTo(destination);
+                }
+                else
+                {
+                    // Only string, IHtmlContent values can be added to the buffer.
+                    destination.AppendHtml((IHtmlContent)entry);
+                }
+            }
+        }
+
+        /// <inheritdoc />
+        public void MoveTo(IHtmlContentBuilder destination)
+        {
+            if (destination == null)
+            {
+                throw new ArgumentNullException(nameof(destination));
+            }
+
+            for (var i = 0; i < Entries.Count; i++)
+            {
+                var entry = Entries[i];
+
+                string entryAsString;
+                IHtmlContentContainer entryAsContainer;
+                if ((entryAsString = entry as string) != null)
+                {
+                    destination.Append(entryAsString);
+                }
+                else if ((entryAsContainer = entry as IHtmlContentContainer) != null)
+                {
+                    // Since we're moving, do a deep flatten.
+                    entryAsContainer.MoveTo(destination);
+                }
+                else
+                {
+                    // Only string, IHtmlContent values can be added to the buffer.
+                    destination.AppendHtml((IHtmlContent)entry);
+                }
+            }
+
+            Entries.Clear();
+        }
+
         /// <inheritdoc />
         public void WriteTo(TextWriter writer, HtmlEncoder encoder)
         {

src/Microsoft.AspNetCore.Html.Abstractions/HtmlEncodedString.cs

@@ -17,17 +17,20 @@ namespace Microsoft.AspNetCore.Html
         /// </summary>
         public static readonly IHtmlContent NewLine = new HtmlEncodedString(Environment.NewLine);
 
-        private readonly string _value;
-
         /// <summary>
         /// Creates a new <see cref="HtmlEncodedString"/>.
         /// </summary>
         /// <param name="value">The HTML encoded value.</param>
         public HtmlEncodedString(string value)
         {
-            _value = value;
+            Value = value;
         }
 
+        /// <summary>
+        /// Gets the HTML encoded value.
+        /// </summary>
+        public string Value { get; }
+
         /// <inheritdoc />
         public void WriteTo(TextWriter writer, HtmlEncoder encoder)
         {
@@ -41,13 +44,13 @@ namespace Microsoft.AspNetCore.Html
                 throw new ArgumentNullException(nameof(encoder));
             }
 
-            writer.Write(_value);
+            writer.Write(Value);
         }
 
         /// <inheritdoc />
         public override string ToString()
         {
-            return _value ?? string.Empty;
+            return Value ?? string.Empty;
         }
     }
 }

src/Microsoft.AspNetCore.Html.Abstractions/HtmlTextWriter.cs

@@ -1,49 +0,0 @@
-// 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.IO;
-
-namespace Microsoft.AspNetCore.Html
-{
-    /// <summary>
-    /// A <see cref="TextWriter"/> which supports special processing of <see cref="IHtmlContent"/>.
-    /// </summary>
-    public abstract class HtmlTextWriter : TextWriter
-    {
-        /// <summary>
-        /// Writes an <see cref="IHtmlContent"/> value.
-        /// </summary>
-        /// <param name="value">The <see cref="IHtmlContent"/> value.</param>
-        public abstract void Write(IHtmlContent value);
-
-        /// <inheritdoc />
-        public override void Write(object value)
-        {
-            var htmlContent = value as IHtmlContent;
-            if (htmlContent == null)
-            {
-                base.Write(value);
-            }
-            else
-            {
-                Write(htmlContent);
-            }
-        }
-
-        /// <inheritdoc />
-        public override void WriteLine(object value)
-        {
-            var htmlContent = value as IHtmlContent;
-            if (htmlContent == null)
-            {
-                base.Write(value);
-            }
-            else
-            {
-                Write(htmlContent);
-            }
-
-            base.WriteLine();
-        }
-    }
-}

src/Microsoft.AspNetCore.Html.Abstractions/IHtmlContentBuilder.cs

@@ -6,7 +6,7 @@ namespace Microsoft.AspNetCore.Html
     /// <summary>
     /// A builder for HTML content.
     /// </summary>
-    public interface IHtmlContentBuilder : IHtmlContent
+    public interface IHtmlContentBuilder : IHtmlContentContainer
     {
         /// <summary>
         /// Appends an <see cref="IHtmlContent"/> instance.

src/Microsoft.AspNetCore.Html.Abstractions/IHtmlContentContainer.cs

@@ -0,0 +1,30 @@
+// 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.
+
+namespace Microsoft.AspNetCore.Html
+{
+    /// <summary>
+    /// Defines a contract for <see cref="IHtmlContent"/> instances made up of several components which
+    /// can be copied into an <see cref="IHtmlContentBuilder"/>.
+    /// </summary>
+    public interface IHtmlContentContainer : IHtmlContent
+    {
+        /// <summary>
+        /// Copies the contained content of this <see cref="IHtmlContentContainer"/> into <paramref name="builder"/>.
+        /// </summary>
+        /// <param name="builder">The <see cref="IHtmlContentBuilder"/>.</param>
+        void CopyTo(IHtmlContentBuilder builder);
+
+        /// <summary>
+        /// <para>
+        /// Moves the contained content of this <see cref="IHtmlContentContainer"/> into <paramref name="builder"/>.
+        /// </para>
+        /// <para>
+        /// After <see cref="MoveTo"/> is called, this <see cref="IHtmlContentContainer"/> instance should be left
+        /// in an empty state.
+        /// </para>
+        /// </summary>
+        /// <param name="builder">The <see cref="IHtmlContentBuilder"/>.</param>
+        void MoveTo(IHtmlContentBuilder builder);
+    }
+}

test/Microsoft.AspNetCore.Html.Abstractions.Test/HtmlContentBuilderExtensionsTest.cs

@@ -392,6 +392,20 @@ namespace Microsoft.AspNetCore.Html.Test
                 return this;
             }
 
+            public void CopyTo(IHtmlContentBuilder destination)
+            {
+                foreach (var entry in Entries)
+                {
+                    destination.AppendHtml(entry);
+                }
+            }
+
+            public void MoveTo(IHtmlContentBuilder destination)
+            {
+                CopyTo(destination);
+                Clear();
+            }
+
             public void WriteTo(TextWriter writer, HtmlEncoder encoder)
             {
                 foreach (var entry in Entries)

test/Microsoft.AspNetCore.Html.Abstractions.Test/HtmlContentBuilderTest.cs

@@ -1,6 +1,7 @@
 // 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.Encodings.Web;
 using Microsoft.AspNetCore.Html;
@@ -105,6 +106,106 @@ namespace Microsoft.Extensions.Internal
             Assert.Equal(0, content.Entries.Count);
         }
 
+        [Fact]
+        public void CopyTo_CopiesAllItems()
+        {
+            // Arrange
+            var source = new HtmlContentBuilder();
+            source.AppendHtml(new TestHtmlContent("hello"));
+            source.Append("Test");
+
+            var destination = new HtmlContentBuilder();
+            destination.Append("some-content");
+
+            // Act
+            source.CopyTo(destination);
+
+            // Assert
+            Assert.Equal(2, source.Entries.Count);
+            Assert.Equal(3, destination.Entries.Count);
+
+            Assert.Equal("some-content", Assert.IsType<string>(destination.Entries[0]));
+            Assert.Equal(new TestHtmlContent("hello"), Assert.IsType<TestHtmlContent>(destination.Entries[1]));
+            Assert.Equal("Test", Assert.IsType<string>(destination.Entries[2]));
+        }
+
+        [Fact]
+        public void CopyTo_DoesDeepCopy()
+        {
+            // Arrange
+            var source = new HtmlContentBuilder();
+
+            var nested = new HtmlContentBuilder();
+            source.AppendHtml(nested);
+            nested.AppendHtml(new TestHtmlContent("hello"));
+            source.Append("Test");
+
+            var destination = new HtmlContentBuilder();
+            destination.Append("some-content");
+
+            // Act
+            source.CopyTo(destination);
+
+            // Assert
+            Assert.Equal(2, source.Entries.Count);
+            Assert.Equal(1, nested.Entries.Count);
+            Assert.Equal(3, destination.Entries.Count);
+
+            Assert.Equal("some-content", Assert.IsType<string>(destination.Entries[0]));
+            Assert.Equal(new TestHtmlContent("hello"), Assert.IsType<TestHtmlContent>(destination.Entries[1]));
+            Assert.Equal("Test", Assert.IsType<string>(destination.Entries[2]));
+        }
+
+        [Fact]
+        public void MoveTo_CopiesAllItems_AndClears()
+        {
+            // Arrange
+            var source = new HtmlContentBuilder();
+            source.AppendHtml(new TestHtmlContent("hello"));
+            source.Append("Test");
+
+            var destination = new HtmlContentBuilder();
+            destination.Append("some-content");
+
+            // Act
+            source.MoveTo(destination);
+
+            // Assert
+            Assert.Equal(0, source.Entries.Count);
+            Assert.Equal(3, destination.Entries.Count);
+
+            Assert.Equal("some-content", Assert.IsType<string>(destination.Entries[0]));
+            Assert.Equal(new TestHtmlContent("hello"), Assert.IsType<TestHtmlContent>(destination.Entries[1]));
+            Assert.Equal("Test", Assert.IsType<string>(destination.Entries[2]));
+        }
+
+        [Fact]
+        public void MoveTo_DoesDeepMove()
+        {
+            // Arrange
+            var source = new HtmlContentBuilder();
+
+            var nested = new HtmlContentBuilder();
+            source.AppendHtml(nested);
+            nested.AppendHtml(new TestHtmlContent("hello"));
+            source.Append("Test");
+
+            var destination = new HtmlContentBuilder();
+            destination.Append("some-content");
+
+            // Act
+            source.MoveTo(destination);
+
+            // Assert
+            Assert.Equal(0, source.Entries.Count);
+            Assert.Equal(0, nested.Entries.Count);
+            Assert.Equal(3, destination.Entries.Count);
+
+            Assert.Equal("some-content", Assert.IsType<string>(destination.Entries[0]));
+            Assert.Equal(new TestHtmlContent("hello"), Assert.IsType<TestHtmlContent>(destination.Entries[1]));
+            Assert.Equal("Test", Assert.IsType<string>(destination.Entries[2]));
+        }
+
         [Fact]
         public void WriteTo_WritesAllItems()
         {
@@ -122,7 +223,7 @@ namespace Microsoft.Extensions.Internal
             Assert.Equal("Written from TestHtmlContent: HelloHtmlEncode[[Test]]", writer.ToString());
         }
 
-        private class TestHtmlContent : IHtmlContent
+        private class TestHtmlContent : IHtmlContent, IEquatable<TestHtmlContent>
         {
             private string _content;
 
@@ -140,6 +241,27 @@ namespace Microsoft.Extensions.Internal
             {
                 return "Written from TestHtmlContent: " + _content;
             }
+
+            public override int GetHashCode()
+            {
+                return _content.GetHashCode();
+            }
+
+            public override bool Equals(object obj)
+            {
+                var other = obj as TestHtmlContent;
+                if (other != null)
+                {
+                    return Equals(other);
+                }
+
+                return base.Equals(obj);
+            }
+
+            public bool Equals(TestHtmlContent other)
+            {
+                return string.Equals(_content, other._content);
+            }
         }
     }
 }