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(), , (int)buffer.Length); await stream2Write.FlushAsync();
stream.Seek(); await bi.SetSourceAsync(stream); wb = new WriteableBitmap(bi.PixelWidth, bi.PixelHeight);
stream.Seek();
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, , 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？