NSURLConnection gzip magic

Sun, 28. Jun 2009

Categories: en development Tags: Cocoa compression gzip htaccess iPhone NSURLConnection Objective C wget

For quite some time I ranted about not being able to use compressed network communcation out-of-the-box on the iPhone.

Despite being undocumented (or I just overlooked the hint), NSURLConnection does gzip decompression transparently!

That’s how to use it:

  1. ensure the webserver sends gzipped content, use e.g. wget to verify:

    $ wget –header=‘Accept-Encoding: gzip’ 
    –server-response http://example.com/demo.xmlz
    It should (in case of a gzipped xml document) look like
    HTTP/1.1 200 OK
    Content-Type: text/xml
    Content-Encoding: gzip

  2. If your webserver doesn’t support transparent compression, you can still upload gzipped content and tell the server to send the correct response headers by setting up a .htaccess file:

    AddType text/xml .xml .xmlz
    AddEncoding gzip .gz .xmlz
    # inspired by
    # http://betterexplained.com/articles/
    #  how-to-optimize-your-site-with-gzip-compression/
    # compress all text & html:
    AddOutputFilterByType DEFLATE text/html text/plain text/xml
  3. to verify from within your app, log the response header in the NSURLConnection callbacks:

    - (void)connection:(NSURLConnection *)connection
      didReceiveResponse:(NSURLResponse )response
        NSLog(@“didReceiveResponse %@: %@“, [response URL],
    [(NSHTTPURLResponse)response allHeaderFields]);
        buffer = [[NSMutableData dataWithCapacity:1024*1024] retain];

    • (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)dat { [buffer appendData:dat]; }

    • (void)connectionDidFinishLoading:(NSURLConnection *)connection { NSLog(@“connectionDidFinishLoading %d bytes”, [buffer length]); [buffer release]; }

  4. It should look like

    2009-06-28 14:31:09.722 DemoApp[3981:20b] \
      didReceiveResponse http://example.com/demo.xmlz: {
    "Content-Type" = "text/xml";
    "Content-Encoding" = gzip;
    "Content-Length" = 123042;
    2009-06-28 14:31:11.619 DemoApp[3981:20b] \
      connectionDidFinishLoading 602979 bytes

As you can see we received way more bytes than went over the wire.

P.S.: It seems not to be necessary setting the request header yourself:

NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url
// set explicitly:
[request setValue:@"gzip" forHTTPHeaderField:@"Accept-Encoding"];