通常情况下，我们的程序需要从服务器读取图片，但如果需要不止一次读取某一张图片的话，就需要做本地缓存了，这样既为用户省一点流量，又能显得你的APP很快。

　　假如你已经知道了某一张图片的地址，那么第一件事就是要把这张图片下载下来；当然如果是一次性读取的话，可以直接把图片地址给Image控件或者给Bitmapimage对象（实际上这二者是没有去别的），但这无法存到本地，只作为显示用；但是我们要做的是保存到本地，这样肯定是不行的。现在我们就要用到HTTP的东西了，请看下面的代码：

           
 async static public Task<IInputStream> GetStreamAsync(string url)
        {

            httpClient = new HttpClient();
            var response = await httpClient.GetInputStreamAsync(new Uri(url));
            return response;
        }

 async static public Task<IBuffer> GetBufferAsync(string url)
        {

            httpClient = new HttpClient();
           
            var ResultStr = await httpClient.GetBufferAsync(new Uri(url));
            return ResultStr;
        }

这两个静态方法分别获取url地址的buffer数据和输入流。有了buffer或者stream之后就可以进行下一步-保存。

　　当我们下载完成后，首先要做的很有可能是先显示出来，然后再保存，所以先把数据写入到图片对象中：

这里有两种方法：

1.WriteableBitmap

 protected async Task<WriteableBitmap> GetWriteableBitmapAsync(string url)
        {
            try
            {
                IBuffer buffer = await GetBufferAsync(url);
                if (buffer != null)
                {
                    BitmapImage bi = new BitmapImage();
                    WriteableBitmap wb = null; Stream stream2Write;
                    using (InMemoryRandomAccessStream stream = new InMemoryRandomAccessStream())
                    {

                        stream2Write = stream.AsStreamForWrite();

                        await stream2Write.WriteAsync(buffer.ToArray(), 0, (int)buffer.Length);

                        await stream2Write.FlushAsync();
                        stream.Seek(0);

                        await bi.SetSourceAsync(stream);

                        wb = new WriteableBitmap(bi.PixelWidth, bi.PixelHeight);
                        stream.Seek(0);
                        await wb.SetSourceAsync(stream);

                        return wb;
                    }
                }
                else
                {
                    return null;
                }
            }
            catch
            {
                return null;
            }
        }

2.SoftwareBitmap

public async Task<SoftwareBitmap> GetSoftwareBitmapAsync(string url)
        {
            try
            {
                IInputStream inputStream = await GetSteramAsync(url);
                IRandomAccessStream memStream = new InMemoryRandomAccessStream();
                await RandomAccessStream.CopyAsync(inputStream, memStream);
                BitmapDecoder decoder = await BitmapDecoder.CreateAsync(memStream);
                SoftwareBitmap sb = await decoder.GetSoftwareBitmapAsync(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Premultiplied);
                return sb;
            }
            catch 
            {
                return null;
            }
        }

这两种都可以作为展示图像的数据源，其中WriteableBitmap可以直接给Image.Source , SoftwareBitmap这需要转为SoftwareBitmap：

 　　　　　　　 　SoftwareBitmapSource sbs = new SoftwareBitmapSource();
                sbs.SetBitmapAsync(sb);

接下来就是保存了：WriteableBitmap：

 public async Task SaveImageAsync(WriteableBitmap image, string filename)
        {
            try
            {
                if (image == null)
                {
                    return;
                }
                Guid BitmapEncoderGuid = BitmapEncoder.JpegEncoderId;
                if (filename.EndsWith("jpg"))
                    BitmapEncoderGuid = BitmapEncoder.JpegEncoderId;
                else if (filename.EndsWith("png"))
                    BitmapEncoderGuid = BitmapEncoder.PngEncoderId;
                else if (filename.EndsWith("bmp"))
                    BitmapEncoderGuid = BitmapEncoder.BmpEncoderId;
                else if (filename.EndsWith("tiff"))
                    BitmapEncoderGuid = BitmapEncoder.TiffEncoderId;
                else if (filename.EndsWith("gif"))
                    BitmapEncoderGuid = BitmapEncoder.GifEncoderId;
                var folder = await _local_folder.CreateFolderAsync("images_cache", CreationCollisionOption.OpenIfExists);
                var file = await folder.CreateFileAsync(filename, CreationCollisionOption.ReplaceExisting);

                using (IRandomAccessStream stream = await file.OpenAsync(FileAccessMode.ReadWrite))
                {
                    BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoderGuid,