initial implementation for skia.

src/Avalonia.Visuals/Media/Imaging/WriteableBitmap.cs

@@ -1,5 +1,8 @@
 using System;
+using System.IO;
+using System.Threading.Tasks;
 using Avalonia.Platform;
+using Avalonia.Visuals.Media.Imaging;
 
 namespace Avalonia.Media.Imaging
 {
@@ -34,8 +37,50 @@ namespace Avalonia.Media.Imaging
         {
         }
 
+        private WriteableBitmap(IWriteableBitmapImpl impl) : base(impl)
+        {
+            
+        }
+
         public ILockedFramebuffer Lock() => ((IWriteableBitmapImpl) PlatformImpl.Item).Lock();
 
+        public static WriteableBitmap Decode(Stream stream)
+        {
+            var ri = AvaloniaLocator.Current.GetService<IPlatformRenderInterface>();
+
+            return new WriteableBitmap(ri.LoadWriteableBitmap(stream));
+        }
+        
+        /// <summary>
+        /// Loads a WriteableBitmap from a stream and decodes at the desired width. Aspect ratio is maintained.
+        /// This is more efficient than loading and then resizing.
+        /// </summary>
+        /// <param name="stream">The stream to read the bitmap from. This can be any supported image format.</param>
+        /// <param name="width">The desired width of the resulting bitmap.</param>
+        /// <param name="interpolationMode">The <see cref="BitmapInterpolationMode"/> to use should any scaling be required.</param>
+        /// <returns>An instance of the <see cref="Bitmap"/> class.</returns>
+        public static Bitmap DecodeToWidth(Stream stream, int width, BitmapInterpolationMode interpolationMode = BitmapInterpolationMode.HighQuality)
+        {
+            var ri = AvaloniaLocator.Current.GetService<IPlatformRenderInterface>();
+
+            return new WriteableBitmap(ri.LoadWriteableBitmapToWidth(stream, width, interpolationMode));
+        }
+
+        /// <summary>
+        /// Loads a Bitmap from a stream and decodes at the desired height. Aspect ratio is maintained.
+        /// This is more efficient than loading and then resizing.
+        /// </summary>
+        /// <param name="stream">The stream to read the bitmap from. This can be any supported image format.</param>
+        /// <param name="height">The desired height of the resulting bitmap.</param>
+        /// <param name="interpolationMode">The <see cref="BitmapInterpolationMode"/> to use should any scaling be required.</param>
+        /// <returns>An instance of the <see cref="Bitmap"/> class.</returns>
+        public static Bitmap DecodeToHeight(Stream stream, int height, BitmapInterpolationMode interpolationMode = BitmapInterpolationMode.HighQuality)
+        {
+            var ri = AvaloniaLocator.Current.GetService<IPlatformRenderInterface>();
+
+            return new WriteableBitmap(ri.LoadWriteableBitmapToHeight(stream, height, interpolationMode));
+        }
+
         private static IBitmapImpl CreatePlatformImpl(PixelSize size, in Vector dpi, PixelFormat? format, AlphaFormat? alphaFormat)
         {
             var ri = AvaloniaLocator.Current.GetService<IPlatformRenderInterface>();

src/Avalonia.Visuals/Platform/IPlatformRenderInterface.cs

@@ -100,6 +100,38 @@ namespace Avalonia.Platform
         /// <returns>An <see cref="IBitmapImpl"/>.</returns>
         IBitmapImpl LoadBitmap(Stream stream);
 
+        /// <summary>
+        /// Loads a WriteableBitmap implementation from a stream to a specified width maintaining aspect ratio.
+        /// </summary>
+        /// <param name="stream">The stream to read the bitmap from.</param> 
+        /// <param name="width">The desired width of the resulting bitmap.</param>
+        /// <param name="interpolationMode">The <see cref="BitmapInterpolationMode"/> to use should resizing be required.</param>
+        /// <returns>An <see cref="IBitmapImpl"/>.</returns>
+        IWriteableBitmapImpl LoadWriteableBitmapToWidth(Stream stream, int width, BitmapInterpolationMode interpolationMode = BitmapInterpolationMode.HighQuality);
+
+        /// <summary>
+        /// Loads a WriteableBitmap implementation from a stream to a specified height maintaining aspect ratio.
+        /// </summary>
+        /// <param name="stream">The stream to read the bitmap from.</param> 
+        /// <param name="height">The desired height of the resulting bitmap.</param>
+        /// <param name="interpolationMode">The <see cref="BitmapInterpolationMode"/> to use should resizing be required.</param>
+        /// <returns>An <see cref="IBitmapImpl"/>.</returns>
+        IWriteableBitmapImpl LoadWriteableBitmapToHeight(Stream stream, int height, BitmapInterpolationMode interpolationMode = BitmapInterpolationMode.HighQuality);
+        
+        /// <summary>
+        /// Loads a WriteableBitmap implementation from a file..
+        /// </summary>
+        /// <param name="fileName">The filename of the bitmap.</param>        
+        /// <returns>An <see cref="IBitmapImpl"/>.</returns>
+        IWriteableBitmapImpl LoadWriteableBitmap(string fileName);
+
+        /// <summary>
+        /// Loads a WriteableBitmap implementation from a file..
+        /// </summary>
+        /// <param name="stream">The stream to read the bitmap from.</param>        
+        /// <returns>An <see cref="IBitmapImpl"/>.</returns>
+        IWriteableBitmapImpl LoadWriteableBitmap(Stream stream);
+
         /// <summary>
         /// Loads a bitmap implementation from a stream to a specified width maintaining aspect ratio.
         /// </summary>

src/Skia/Avalonia.Skia/PlatformRenderInterface.cs

@@ -77,6 +77,31 @@ namespace Avalonia.Skia
             return new ImmutableBitmap(stream);
         }
 
+        public IWriteableBitmapImpl LoadWriteableBitmapToWidth(Stream stream, int width,
+            BitmapInterpolationMode interpolationMode = BitmapInterpolationMode.HighQuality)
+        {
+            return new WriteableBitmapImpl(stream, width, true, interpolationMode);
+        }
+
+        public IWriteableBitmapImpl LoadWriteableBitmapToHeight(Stream stream, int height,
+            BitmapInterpolationMode interpolationMode = BitmapInterpolationMode.HighQuality)
+        {
+            return new WriteableBitmapImpl(stream, height, false, interpolationMode);
+        }
+
+        public IWriteableBitmapImpl LoadWriteableBitmap(string fileName)
+        {
+            using (var stream = File.OpenRead(fileName))
+            {
+                return LoadWriteableBitmap(stream);
+            }
+        }
+
+        public IWriteableBitmapImpl LoadWriteableBitmap(Stream stream)
+        {
+            return new WriteableBitmapImpl(stream);
+        }
+
         /// <inheritdoc />
         public IBitmapImpl LoadBitmap(PixelFormat format, AlphaFormat alphaFormat, IntPtr data, PixelSize size, Vector dpi, int stride)
         {

src/Skia/Avalonia.Skia/WriteableBitmapImpl.cs

@@ -1,8 +1,10 @@
 using System;
 using System.IO;
 using System.Threading;
+using Avalonia.Media.Imaging;
 using Avalonia.Platform;
 using Avalonia.Skia.Helpers;
+using Avalonia.Visuals.Media.Imaging;
 using SkiaSharp;
 
 namespace Avalonia.Skia
@@ -15,7 +17,70 @@ namespace Avalonia.Skia
         private static readonly SKBitmapReleaseDelegate s_releaseDelegate = ReleaseProc;
         private readonly SKBitmap _bitmap;
         private readonly object _lock = new object();
+        
+        /// <summary>
+        /// Create immutable bitmap from given stream.
+        /// </summary>
+        /// <param name="stream">Stream containing encoded data.</param>
+        public WriteableBitmapImpl(Stream stream)
+        {
+            using (var skiaStream = new SKManagedStream(stream))
+            {
+                _bitmap = SKBitmap.Decode(skiaStream);
+
+                if (_bitmap == null)
+                {
+                    throw new ArgumentException("Unable to load bitmap from provided data");
+                }
+
+                PixelSize = new PixelSize(_bitmap.Width, _bitmap.Height);
+                Dpi = new Vector(96, 96);
+            }
+        }
+
+        public WriteableBitmapImpl(Stream stream, int decodeSize, bool horizontal, BitmapInterpolationMode interpolationMode)
+        {
+            using (var skStream = new SKManagedStream(stream))
+            using (var codec = SKCodec.Create(skStream))
+            {
+                var info = codec.Info;
 
+                // get the scale that is nearest to what we want (eg: jpg returned 512)
+                var supportedScale = codec.GetScaledDimensions(horizontal ? ((float)decodeSize / info.Width) : ((float)decodeSize / info.Height));
+
+                // decode the bitmap at the nearest size
+                var nearest = new SKImageInfo(supportedScale.Width, supportedScale.Height);
+                var bmp = SKBitmap.Decode(codec, nearest);
+
+                // now scale that to the size that we want
+                var realScale = horizontal ? ((double)info.Height / info.Width) : ((double)info.Width / info.Height);
+
+                SKImageInfo desired;
+
+
+                if (horizontal)
+                {
+                    desired = new SKImageInfo(decodeSize, (int)(realScale * decodeSize));
+                }
+                else
+                {
+                    desired = new SKImageInfo((int)(realScale * decodeSize), decodeSize);
+                }
+
+                if (bmp.Width != desired.Width || bmp.Height != desired.Height)
+                {
+                    var scaledBmp = bmp.Resize(desired, interpolationMode.ToSKFilterQuality());
+                    bmp.Dispose();
+                    bmp = scaledBmp;
+                }
+
+                _bitmap = bmp;
+
+                PixelSize = new PixelSize(bmp.Width, bmp.Height);
+                Dpi = new Vector(96, 96);
+            }
+        }
+        
         /// <summary>
         /// Create new writeable bitmap.
         /// </summary>