Which delegate of NSURLConnection to use

I have a doubt regarding downloading data from a web service. One way is to download it in a single line mentioned below.

NSString *returnString = [[NSString alloc] initWithData:[NSURLConnection sendSynchronousRequest:urlrequest returningResponse:nil error:nil] encoding:NSUTF8StringEncoding];

And the other way to get it is via connectionDidFinishLoading

    [..]
    NSURLConnection *theConnection = [[NSURLConnection alloc] initWithRequest:urlrequest delegate:self];  

    NSHTTPURLResponse *response;  
   [NSURLConnection sendSynchronousRequest: urlrequest returningResponse: &response error: nil];   

    if( theConnection )  
    {  
        webData = [[NSMutableData data] retain];  
    }  
    else  
    {  
        NSLog(@"theConnection is NULL");  
    }  

}  

-(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{   
    [webData setLength: 0];  
}  
-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data  
{  
    [webData appendData:data];  
}  
-(void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{  
    NSLog(@"ERROR w开发者_Go百科ith theConenction");  
    [connection release];  
    [webData release];  
}  
-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{  
    NSLog(@"DONE. Received Bytes: %d", [webData length]);  
    NSString *theXML = [[NSString alloc] initWithBytes: [webData mutableBytes] length:  [webData length] encoding:NSUTF8StringEncoding];  
}

Is there any difference between these two? NSURLConnection delegate method is not called when I use a separate class to parse the response.

You're using sendSynchronousRequest:returningResponse:error: which does not call any delegates methods because it doesn't need to: when you call it, the main thread stops until the request is finished and you get the response.

If you want to make an asynchronous request, use connectionWithRequest:delegate:. I recommend to always do asynchronous responses since the synchronous request blocks the main thread and your UI can't respond during that time. Animations will become interrupted. Scrolling becomes jerky. If you do want to use synchronous requests you should do it in a background thread.

The -sendSynchronousRequest:returningResponse:error: method blocks the main-thread (whenever it runs on the main-thread of course, since it's possible to run this method from any other thread, but I believe this is not recommended).

The methods using the delegates are asynchronous, the methods will fire and the results will (at some point in the future) be returned in the delegate methods. This gives the user a more smooth experience, since the main-thread will not be blocked.

Edit: personally I hardly ever use the -sendSynchronousRequest:returningResponse:error: method for the aforementioned reasons. Most of the time I use this method when I need to build something quickly, for example a proof-of-concept. I guess one could use the method for small downloads, yet if a timeout occurs (because for some reason the server is down) the whole UI will be blocked for (I believe) 2 minutes, which would be very annoying for the enduser.

An excellent demonstration to clarify your doubt is available in apple sample apps.You can refer Apple's sample app for a better understanding of asynchronous request and parsing data in separate class.