UWP開發中兩種網路圖片緩存方法

　　通常情況下，我們的程式需要從伺服器讀取圖片，但如果需要不止一次讀取某一張圖片的話，就需要做本地緩存了，這樣既為用戶省一點流量，又能顯得你的APP很快。

　　假如你已經知道了某一張圖片的地址，那麼第一件事就是要把這張圖片下載下來；當然如果是一次性讀取的話，可以直接把圖片地址給Image控制項或者給Bitmapimage對象（實際上這二者是沒有去別的），但這無法存到本地，只作為顯示用；但是我們要做的是保存到本地，這樣肯定是不行的。現在我們就要用到HTTP的東西了，請看下麵的代碼：

           
 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;
        }

這兩個靜態方法分別獲取url地址的buffer數據和輸入流。有了buffer或者stream之後就可以進行下一步-保存。

　　當我們下載完成後，首先要做的很有可能是先顯示出來，然後再保存，所以先把數據寫入到圖片對象中：

這裡有兩種方法：

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;
            }
        }

這兩種都可以作為展示圖像的數據源，其中WriteableBitmap可以直接給Image.Source , SoftwareBitmap這需要轉為SoftwareBitmap：

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

接下來就是保存了：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, stream);
                    Stream pixelStream = image.PixelBuffer.AsStream();
                    byte[] pixels = new byte[pixelStream.Length];
                    await pixelStream.ReadAsync(pixels, 0, pixels.Length);
                    encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore,
                              (uint)image.PixelWidth,
                              (uint)image.PixelHeight,
                              96.0,
                              96.0,
                              pixels);
                    await encoder.FlushAsync();
                }
            }
            catch
            {

            }
        }

public async Task WriteToFileAsync(StorageFolder folder,SoftwareBitmap sb,string fileName)
        {
           
            if (sb != null)
            {
                // save image file to cache
                StorageFile file = await folder.CreateFileAsync(fileName, CreationCollisionOption.OpenIfExists);
                using (IRandomAccessStream stream = await file.OpenAsync(FileAccessMode.ReadWrite))
                {
                    BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, stream);
                    encoder.SetSoftwareBitmap(sb);
                    await encoder.FlushAsync();
                }
            }
        }

怎麼樣，是不是很簡單？