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NSURLCache does not work when response header value for transfer-encoding is chunked

I found an issue with (possibly) NSURLCache today while inspecting request and response headers in Charles Proxy. The issue is a little perplexing, but I'm able to repro it consistently:

In a nutshell, the issue has to do with caching of NSURLRequests using iOS's native NSURLCache with the default policy. It turns out that the request is not cached whenever the response has the header transfer-encoding: chunked. But if the response header is content-length: xxx instead, caching works fine. Specifically, it seems that when the response is chunked, NSURLCache doesn't save the eTag and also neglects appending the if-none-match header to subsequent requests to the same url, and consequently, caching fails (as it should), i.e. a 200 is returned instead of a 304.

I'm testing on the iOS8.2 simulator. Even if you don't have a solution, I'd love to hear if you've experienced the same issue. I've found at least one similar report), and here's a related thread posted by my back-end engineer.

like image 796
rainypixels Avatar asked Mar 18 '15 01:03

rainypixels


People also ask

How do I stop transfer encoding chunked?

Try adding "&headers=false" to your request. That should shorten it up and cause the response to be less likely to be chunked. Also, are you sending a HTTP/1.1 or HTTP/1.0 request? Try sending a HTTP/1.0 if your device cannot handle a HTTP/1.1 request.

How does transfer encoding chunked work?

In chunked transfer encoding, the data stream is divided into a series of non-overlapping "chunks". The chunks are sent out and received independently of one another. No knowledge of the data stream outside the currently-being-processed chunk is necessary for both the sender and the receiver at any given time.

What is Transfer Encoding header?

Transfer-Encoding is a hop-by-hop header, that is applied to a message between two nodes, not to a resource itself. Each segment of a multi-node connection can use different Transfer-Encoding values. If you want to compress data over the whole connection, use the end-to-end Content-Encoding header instead.


1 Answers

It should work if you manually add the response data to the cache. I've got an image loading class where I want to make sure everything is cached, so I do something like this:

- (void)getImageWithURL:(NSURL *)url onCompletion:(void (^)(UIImage *image, NSError *error))completion {
    NSURLRequest *request = [NSURLRequest requestWithURL:url];

    UIImage *cachedImage = [self cachedImageForURLRequest:request];
    if (cachedImage) {
        NSLog(@"Got image from cache.");
        completion(cachedImage, nil);
        return;
    }

    [[[NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]] dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        // Manually cache the response.
        NSCachedURLResponse *cachedResponse = [[NSCachedURLResponse alloc] initWithResponse:response data:data userInfo:nil storagePolicy:NSURLCacheStorageAllowed];
        [[NSURLCache sharedURLCache] storeCachedResponse:cachedResponse forRequest:request];
        NSLog(@"Got a fresh image.");
        completion([UIImage imageWithData:data], error);
    }] resume];
}

- (UIImage *)cachedImageForURLRequest:(NSURLRequest *)urlRequest {
    NSCachedURLResponse *cachedResponse = [[NSURLCache sharedURLCache] cachedResponseForRequest:urlRequest];
    return [UIImage imageWithData:cachedResponse.data];
}
like image 196
Alex Robinson Avatar answered Oct 02 '22 14:10

Alex Robinson