Make GifGraphicsControlExtension a struct

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -241,13 +241,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
             byte packed = this.buffer[1];
 
-            this.graphicsControlExtension = new GifGraphicsControlExtension
-            {
-                DelayTime = BitConverter.ToInt16(this.buffer, 2),
-                TransparencyIndex = this.buffer[4],
-                TransparencyFlag = (packed & 0x01) == 1,
-                DisposalMethod = (DisposalMethod)((packed & 0x1C) >> 2)
-            };
+            this.graphicsControlExtension = GifGraphicsControlExtension.Parse(this.buffer);
         }
 
         /// <summary>
@@ -430,8 +424,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
             }
             else
             {
-                if (this.graphicsControlExtension != null &&
-                    this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToPrevious)
+                if (this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToPrevious)
                 {
                     prevFrame = previousFrame;
                 }
@@ -494,8 +487,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 {
                     int index = Unsafe.Add(ref indicesRef, i);
 
-                    if (this.graphicsControlExtension == null ||
-                        this.graphicsControlExtension.TransparencyFlag == false ||
+                    if (this.graphicsControlExtension.TransparencyFlag == false ||
                         this.graphicsControlExtension.TransparencyIndex != index)
                     {
                         int indexOffset = index * 3;
@@ -518,8 +510,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
             previousFrame = currentFrame ?? image.Frames.RootFrame;
 
-            if (this.graphicsControlExtension != null &&
-                this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToBackground)
+            if (this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToBackground)
             {
                 this.restoreArea = new Rectangle(descriptor.Left, descriptor.Top, descriptor.Width, descriptor.Height);
             }
@@ -550,16 +541,13 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// <param name="meta">The meta data.</param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private void SetFrameMetaData(ImageFrameMetaData meta)
-        {
-            if (this.graphicsControlExtension != null)
+        {  
+            if (this.graphicsControlExtension.DelayTime > 0)
             {
-                if (this.graphicsControlExtension.DelayTime > 0)
-                {
-                    meta.FrameDelay = this.graphicsControlExtension.DelayTime;
-                }
-
-                meta.DisposalMethod = this.graphicsControlExtension.DisposalMethod;
+                meta.FrameDelay = this.graphicsControlExtension.DelayTime;
             }
+
+            meta.DisposalMethod = this.graphicsControlExtension.DisposalMethod;
         }
 
         /// <summary>

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -108,7 +108,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
                     quantized = this.quantizer.CreateFrameQuantizer<TPixel>().QuantizeFrame(frame);
                 }
 
-                this.WriteGraphicalControlExtension(frame.MetaData, writer, this.GetTransparentIndex(quantized));
+                this.WriteGraphicalControlExtension(frame.MetaData, stream, this.GetTransparentIndex(quantized));
                 this.WriteImageDescriptor(frame, writer);
                 this.WriteColorTable(quantized, writer);
                 this.WriteImageData(quantized, writer);
@@ -261,35 +261,21 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// Writes the graphics control extension to the stream.
         /// </summary>
         /// <param name="metaData">The metadata of the image or frame.</param>
-        /// <param name="writer">The stream to write to.</param>
+        /// <param name="stream">The stream to write to.</param>
         /// <param name="transparencyIndex">The index of the color in the color palette to make transparent.</param>
-        private void WriteGraphicalControlExtension(ImageFrameMetaData metaData, EndianBinaryWriter writer, int transparencyIndex)
+        private void WriteGraphicalControlExtension(ImageFrameMetaData metaData, Stream stream, int transparencyIndex)
         {
-            var extension = new GifGraphicsControlExtension
-            {
-                DisposalMethod = metaData.DisposalMethod,
-                TransparencyFlag = transparencyIndex > -1,
-                TransparencyIndex = unchecked((byte)transparencyIndex),
-                DelayTime = metaData.FrameDelay
-            };
-
-            // Write the intro.
-            this.buffer[0] = GifConstants.ExtensionIntroducer;
-            this.buffer[1] = GifConstants.GraphicControlLabel;
-            this.buffer[2] = 4;
-            writer.Write(this.buffer, 0, 3);
-
-            var field = default(PackedField);
-            field.SetBits(3, 3, (int)extension.DisposalMethod); // 1-3 : Reserved, 4-6 : Disposal
+            var extension = new GifGraphicsControlExtension(
+                disposalMethod: metaData.DisposalMethod,
+                transparencyFlag: transparencyIndex > -1,
+                transparencyIndex: unchecked((byte)transparencyIndex),
+                delayTime: (ushort)metaData.FrameDelay
+            );
 
-            // TODO: Allow this as an option.
-            field.SetBit(6, false); // 7 : User input - 0 = none
-            field.SetBit(7, extension.TransparencyFlag); // 8: Has transparent.
+            extension.WriteTo(this.buffer);
 
-            writer.Write(field.Byte);
-            writer.Write((ushort)extension.DelayTime);
-            writer.Write(extension.TransparencyIndex);
-            writer.Write(GifConstants.Terminator);
+            stream.Write(this.buffer, 0, GifGraphicsControlExtension.Size);
+           
         }
 
         /// <summary>

src/ImageSharp/Formats/Gif/Sections/GifGraphicsControlExtension.cs

@@ -1,40 +1,94 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
+using System;
+using System.Buffers.Binary;
+
 namespace SixLabors.ImageSharp.Formats.Gif
 {
     /// <summary>
     /// The Graphic Control Extension contains parameters used when
     /// processing a graphic rendering block.
     /// </summary>
-    internal sealed class GifGraphicsControlExtension
+    internal readonly struct GifGraphicsControlExtension
     {
+        public const int Size = 8;
+
+        public GifGraphicsControlExtension(
+            DisposalMethod disposalMethod,
+            bool transparencyFlag,
+            ushort delayTime,
+            byte transparencyIndex)
+        {
+            this.DisposalMethod = disposalMethod;
+            this.TransparencyFlag = transparencyFlag;
+            this.DelayTime = delayTime;
+            this.TransparencyIndex = transparencyIndex;
+        }
+
         /// <summary>
-        /// Gets or sets the disposal method which indicates the way in which the
+        /// Gets the disposal method which indicates the way in which the
         /// graphic is to be treated after being displayed.
         /// </summary>
-        public DisposalMethod DisposalMethod { get; set; }
+        public DisposalMethod DisposalMethod { get; }
 
         /// <summary>
-        /// Gets or sets a value indicating whether transparency flag is to be set.
+        /// Gets a value indicating whether transparency flag is to be set.
         /// This indicates whether a transparency index is given in the Transparent Index field.
         /// (This field is the least significant bit of the byte.)
         /// </summary>
-        public bool TransparencyFlag { get; set; }
-
-        /// <summary>
-        /// Gets or sets the transparency index.
-        /// The Transparency Index is such that when encountered, the corresponding pixel
-        /// of the display device is not modified and processing goes on to the next pixel.
-        /// </summary>
-        public byte TransparencyIndex { get; set; }
+        public bool TransparencyFlag { get; }
 
         /// <summary>
-        /// Gets or sets the delay time.
+        /// Gets the delay time.
         /// If not 0, this field specifies the number of hundredths (1/100) of a second to
         /// wait before continuing with the processing of the Data Stream.
         /// The clock starts ticking immediately after the graphic is rendered.
         /// </summary>
-        public int DelayTime { get; set; }
+        public ushort DelayTime { get; }
+
+        /// <summary>
+        /// Gets the transparency index.
+        /// The Transparency Index is such that when encountered, the corresponding pixel
+        /// of the display device is not modified and processing goes on to the next pixel.
+        /// </summary>
+        public byte TransparencyIndex { get; }
+
+        public byte PackField()
+        {
+            PackedField field = default;
+
+            field.SetBits(3, 3, (int)this.DisposalMethod); // 1-3 : Reserved, 4-6 : Disposal
+
+            // TODO: Allow this as an option.
+            field.SetBit(6, false); // 7 : User input - 0 = none
+            field.SetBit(7, this.TransparencyFlag); // 8: Has transparent.
+
+            return field.Byte;
+        }
+
+        public void WriteTo(Span<byte> buffer)
+        {
+            buffer[0] = GifConstants.ExtensionIntroducer;
+            buffer[1] = GifConstants.GraphicControlLabel;
+            buffer[2] = 4;                                                                 // Block Size
+            buffer[3] = this.PackField();                                                  // Packed Field
+            BinaryPrimitives.WriteUInt16LittleEndian(buffer.Slice(4, 2), this.DelayTime);  // Delay Time
+            buffer[6] = this.TransparencyIndex;
+            buffer[7] = GifConstants.Terminator;
+        }
+
+        public static GifGraphicsControlExtension Parse(ReadOnlySpan<byte> buffer)
+        {
+            // We've already read the Extension Introducer introducer & Graphic Control Label
+            // Start from the block size (0)
+            byte packed = buffer[1];
+
+            return new GifGraphicsControlExtension(
+                disposalMethod: (DisposalMethod)((packed & 0x1C) >> 2),
+                delayTime: BinaryPrimitives.ReadUInt16LittleEndian(buffer.Slice(2, 2)),
+                transparencyIndex: buffer[4],
+                transparencyFlag: (packed & 0x01) == 1);
+        }
     }
-}
+}