MRo Blog

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];
   }

   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.

4. enjoy lightning fast network communication!

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

NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url
    cachePolicy:NSURLRequestReloadIgnoringLocalCacheData
    timeoutInterval:60.0];
// set explicitly:
[request setValue:@"gzip" forHTTPHeaderField:@"Accept-Encoding"];