ImageSharp-762: Added methods to pre-seed AoT compiler on iOS

src/ImageSharp/Formats/Gif/ImageExtensions.cs

@@ -5,6 +5,7 @@ using System.IO;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Formats.Gif;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Processors.Quantization;
 
 namespace SixLabors.ImageSharp
 {
@@ -35,5 +36,23 @@ namespace SixLabors.ImageSharp
         public static void SaveAsGif<TPixel>(this Image<TPixel> source, Stream stream, GifEncoder encoder)
             where TPixel : struct, IPixel<TPixel>
             => source.Save(stream, encoder ?? source.GetConfiguration().ImageFormatsManager.FindEncoder(GifFormat.Instance));
+
+        /// <summary>
+        /// This method doesn't actually do anything but serves an important purpose...
+        /// If you are running ImageSharp on iOS and try to call SaveAsGif, it will throw an excepion:
+        /// "Attempting to JIT compile method... OctreeFrameQuantizer.ConstructPalette... while running in aot-only mode."
+        /// The reason this happens is the SaveAsGif method makes haevy use of generics, which are too confusing for the AoT
+        /// compiler used on Xamarin.iOS. It spins up the JIT compiler to try and figure it out, but that is an illegal op on 
+        /// iOS so it bombs out. 
+        /// If you are getting the above error, you need to call this method, which will pre-seed the AoT compiler with the 
+        /// necessary methods to complete the SaveAsGif call. That's it, otherwise you should NEVER need this method!!!
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        public static void AotCompileOctreeQuantizer<TPixel>()
+            where TPixel : struct, IPixel<TPixel>
+        {
+            var test = new OctreeFrameQuantizer<TPixel>(new OctreeQuantizer(false));
+            test.AotGetPalette();
+        }
     }
 }

src/ImageSharp/Processing/Processors/Quantization/OctreeFrameQuantizer{TPixel}.cs

@@ -136,6 +136,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
             }
         }
 
+        internal TPixel[] AotGetPalette() => this.octree.Palletize(this.colors);
+
         /// <inheritdoc/>
         protected override TPixel[] GetPalette() => this.octree.Palletize(this.colors);