Image blank when using NSURLConnection but not when using dataWithContentsOfURL - ios

Question

When I use NSURLConnection asynchronously to try and get a NSData with my image data in it the image comes back as blank, but when I use dataWithContentsOfURL synchronously I have no problems and I get the image data correctly. Is there any reason why my asynchronous method would be failing?
This works:
NSData *data = [NSData dataWithContentsOfURL: url];
NSLog(#"TEST %#", data);
UIImage *map = [UIImage imageWithData:data];
mapView.image = map;
This doesn't:
//
// MapHttpRequest.m
// GTWeb
//
// Created by Graphic Technologies on 6/21/11.
// Copyright 2011 __MyCompanyName__. All rights reserved.
//
#import "MapHttpRequest.h"
#implementation MapHttpRequest
#synthesize receivedData;
#synthesize dataString;
#synthesize vc;
- (void)request:(NSString *)url fromView:(UIViewController *) theVC
{
vc = theVC;
// Create the request.
NSURLRequest *theRequest=[NSURLRequest requestWithURL:[NSURL URLWithString:url]
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:60.0];
NSLog(#"URL: %#", url);
// create the connection with the request
// and start loading the data
NSURLConnection *theConnection=[[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
if (theConnection) {
// Create the NSMutableData to hold the received data.
// receivedData is an instance variable declared elsewhere.
receivedData = [[NSMutableData data] retain];
} else {
// Inform the user that the connection failed.
}
}
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
// This method is called when the server has determined that it
// has enough information to create the NSURLResponse.
// It can be called multiple times, for example in the case of a
// redirect, so each time we reset the data.
// receivedData is an instance variable declared elsewhere.
[receivedData setLength:0];
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
// Append the new data to receivedData.
// receivedData is an instance variable declared elsewhere.
[receivedData appendData:data];
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
// release the connection, and the data object
[connection release];
// receivedData is declared as a method instance elsewhere
[receivedData release];
// inform the user
NSLog(#"Connection failed! Error - %# %#",
[error localizedDescription],
[[error userInfo] objectForKey:NSURLErrorFailingURLStringErrorKey]);
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
// do something with the data
// receivedData is declared as a method instance elsewhere
[vc mapImageConnectionFinished:receivedData];
// release the connection, and the data object
[dataString release];
[connection release];
[receivedData release];
}
#end

Answer 1

It looks like you started looking at the guide on http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/URLLoadingSystem/Tasks/UsingNSURLConnection.html but didnt finish reading it :)
You need to implement the methods that will received infomation about the data being received.
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
Its all described in the link i provided.

Answer 2

According to your code, you didn't schedule the connection to run:
[connection scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
[connection start];
Did you put traces in your delegate's callbacks to make sure they are being called?

Answer 3

Turns out it was giving me a bad request because there was a invisible carriage return or white space. Trimming it off with:
url = [url stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
fixed my problems.