etag.cs

/// <summary>
/// Gets stream data from the server. Uses ETag to cache data locally
/// </summary>
/// <param name="url">Url to get data from</param>
/// <returns>Stream data</returns>
[NotNull]
private async Task<Stream> GetData([NotNull] string url)
{
    var client = new HttpClient();

    var key = MD5.GetMd5String(url); //Urls can be long and create strange characters, using a hash instead
    var etag = (string)_settingsService.Get(key); //current ETag for the reques

    var request = new HttpRequestMessage
    {
        Method = HttpMethod.Get,
        RequestUri = new Uri(url)
    };

    if (!String.IsNullOrEmpty(etag)) //if there is a saved ETag, use it
    {
        request.Headers.TryAddWithoutValidation("If-None-Match", etag);
    }

    var response = await client.SendAsync(request);

    if (response.StatusCode == HttpStatusCode.NotModified) //ETag matches, no newer data available
    {
        var file = await ApplicationData.Current.LocalFolder.GetFileAsync(key);
        return await file.OpenStreamForReadAsync().ConfigureAwait(false); //return cached data
    }

    var newFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(key, CreationCollisionOption.ReplaceExisting);
    using (var stream = await response.Content.ReadAsStreamAsync().ConfigureAwait(false)) //get content
    {
        using (var outputStream = await newFile.OpenStreamForWriteAsync()) //save to cache
        {
            await stream.CopyToAsync(outputStream);
            _settingsService.Set(key, response.Headers.ETag.Tag); //save etag
        }
    }
    return await newFile.OpenStreamForReadAsync().ConfigureAwait(false); //return (fresh) from cache
}