UWP开发中两种网络图片缓存方法-白红宇

原文:

　　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
     
       GetStreamAsync(string url)        {            httpClient = new HttpClient();            var response = await httpClient.GetInputStreamAsync(new Uri(url));            return response;        } async static public Task
      
        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
     
       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
     
       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？