BitmapFactory.DecodeByteArrayAsync

 private async void OnAsyncDownImage(object sender, EventArgs e)
        {
            this._webclient = new WebClient();
            var url = new Uri("http://pic1.sc.chinaz.com/files/pic/pic9/201404/apic105.jpg");
            byte[] bytes = null;
            try
            {
                bytes = await this._webclient.DownloadDataTaskAsync(url);
                this._webclient.DownloadProgressChanged += this.OnProgressChanged;
            }
            catch (TaskCanceledException)
            {
//                Console.WriteLine("i cancel the async download ~~~~~~~~~~~~~~~~~");
                Log.Warn("async canceled", "i can cancel the async methord");
                return;
            }
            catch (Exception error)
            {
                //                throw;
                Console.Out.WriteLine("this is error message :" + error.Message);
               return;
            }
            var options = new BitmapFactory.Options { InJustDecodeBounds = true, };
            if (bytes.Length != 0)
            {
                Console.Out.WriteLine("length not 0");
                await BitmapFactory.DecodeByteArrayAsync(bytes, 0, bytes.Length, options);

                Console.Out.WriteLine(
                    "output width :{0}; output height :{1}; imageview width :{2}; imageheight :{3}",
                    options.OutWidth,
                    options.OutHeight,
                    this._imagevew.Width,
                    this._imagevew.Height);
                //simplesize calculate 

                /*  options.InSampleSize = options.OutWidth > options.OutHeight
                                           ? options.OutHeight / _imagevew.Height
                                           : options.OutWidth / _imagevew.Width;*/

                options.InSampleSize = calculateInSampleSize(options, this._imagevew.Width, this._imagevew.Height);
                options.InJustDecodeBounds = false;
                Bitmap image = await BitmapFactory.DecodeByteArrayAsync(bytes, 0, bytes.Length, options);
                Console.Out.WriteLine("image the width is :{0}; image the height is :{1}", image.Width, image.Height);
                this._imagevew.SetImageBitmap(image);
            }
            else
            {
                Console.Out.WriteLine("length is 0");
            }

            Console.Out.WriteLine("i end the download image");
        }

如果不使用 calculateInSampleSize ，那么imageview的图像就会一会打一会小

  public static int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight)
        {
            // Raw height and width of image
            int height = options.OutHeight;
            int width = options.OutWidth;
            int inSampleSize = 1;

            if (height > reqHeight || width > reqWidth)
            {
                int halfHeight = height / 2;
                int halfWidth = width / 2;

                // Calculate the largest inSampleSize value that is a power of 2 and keeps both
                // height and width larger than the requested height and width.
                while ((halfHeight / inSampleSize) > reqHeight && (halfWidth / inSampleSize) > reqWidth)
                {
                    inSampleSize *= 2;
                }
            }

            return inSampleSize;
        }