public async Task <Stream> DoGetRequestAsync(string url)
{
string fileName;
// Cache lookup
var cacheLine = await _db.GetCacheLineAsync(url);
if (cacheLine != null && cacheLine.IsStale())
{
Debug.WriteLine("Requested item is stale and will be discarded");
await _db.RemoveCacheLineAsync(cacheLine);
cacheLine = null;
}
// Check if item is still valid so we can return it directly without asking the server
if (cacheLine != null && cacheLine.IsValid())
{
Debug.WriteLine("CACHE HIT with validity");
return(await _db.GetFileAsync(cacheLine));
}
var client = new HttpClient();
// Check for cache hits that must be verified with the server
if (cacheLine != null)
{
if (cacheLine.HasETag())
{
client.DefaultRequestHeaders.Add("If-None-Match", cacheLine.GetETag());
}
if (cacheLine.HasLastModified())
{
client.DefaultRequestHeaders.Add("If-Modified-Since", cacheLine.GetLastModified());
}
fileName = cacheLine.GetFileName();
}
else
{
fileName = _db.GetRandomFilename();
}
try {
var response = await client.GetAsync(url);
var headers = response.Headers;
var contentHeaders = response.Content.Headers;
if (response.StatusCode == HttpStatusCode.OK)
{
int contentLength;
int.TryParse(GetContentHeaderValue(response, "Content-Length"), out contentLength);
var newCacheLine = _db.CreateCacheLine();
newCacheLine.SetData(url,
GetContentHeaderValue(response, "Content-Type"),
contentLength,
GetHeaderValue(response, "ETag"),
GetHeaderValue(response, "Cache-Control"),
GetContentHeaderValue(response, "Expires"),
GetHeaderValue(response, "Last-Modified"),
fileName);
newCacheLine.ParseHeaders();
// Save in cache for future use
await _db.AddCacheLineAsync(newCacheLine);
var stream = await response.Content.ReadAsStreamAsync();
await _db.SaveFileAsync(stream, newCacheLine.GetFileName());
// return newly downloaded file as stream
return(await _db.GetFileAsync(newCacheLine));
}
else if (response.StatusCode == HttpStatusCode.NotModified)
{
Debug.WriteLine("CACHE HIT from server");
// return cached file as stream
return(await _db.GetFileAsync(cacheLine));
}
}
catch (UnauthorizedAccessException e) {
Debug.WriteLine(e);
}
return(null);
}
