Set progress bar for downloading NSData

Question

NSURL *url = [NSURL URLWithString:@"http://i0.kym-cdn.com/entries/icons/original/000/005/545/OpoQQ.jpg?1302279173"]; 
NSData *data = [NSData dataWithContentsOfURL:url]; 
imageView.image = [[[UIImage imageWithData:data];

I want to set progress bar while downloading.

Answer 1

To give a more detailed example:

in your .h file do

@interface YourClass : YourSuperclass<NSURLConnectionDataDelegate>

in your .m file do

@property (nonatomic) NSMutableData *imageData;
@property (nonatomic) NSUInteger totalBytes;
@property (nonatomic) NSUInteger receivedBytes;

And somewhere call

NSURL *url = [NSURL URLWithString:@"http://i0.kym-cdn.com/entries/icons/original/000/005/545/OpoQQ.jpg?1302279173"];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES];

And also implement the delegate methods

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *) urlResponse;
    NSDictionary *dict = httpResponse.allHeaderFields;
    NSString *lengthString = [dict valueForKey:@"Content-Length"];
    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
    NSNumber *length = [formatter numberFromString:lengthString];
    self.totalBytes = length.unsignedIntegerValue;

    self.imageData = [[NSMutableData alloc] initWithCapacity:self.totalBytes];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    [self.imageData appendData:data];
    self.receivedBytes += data.length;

    // Actual progress is self.receivedBytes / self.totalBytes
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    imageView.image = [UIImage imageWithData:self.imageData];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    //handle error
}

Answer 2

You can't get progress call backs by using that method.

You need to use an NSURLConnection and NSURLConnectionDataDelegate.

The NSURLConnection then runs asynchronously and will send callbacks to its delegate.

The main ones to look at are...

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response;
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error;
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data;

and

- (void)connectionDidFinishLoading:(NSURLConnection *)connection;

These are all used for getting the connection to do what you're already doing.

EDIT

Actually, see Marc's answer below. It is correct.