原文:
UWP开发中两种网络图片缓存方法
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, 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(); } } }
怎么样，是不是很简单？
