You should observe the fractionCompleted property of your NSProgress object using KVO:
NSURL *url = [NSURL URLWithString:@"http://www.hfrmovies.com/TheHobbitDesolationOfSmaug48fps.mp4"];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
AFHTTPSessionManager *session = [AFHTTPSessionManager manager];
NSProgress *progress;
NSURLSessionDownloadTask *downloadTask = [session downloadTaskWithRequest:request progress:&progress destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
// …
} completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
[progress removeObserver:self forKeyPath:@"fractionCompleted" context:NULL];
// …
}];
[downloadTask resume];
[progress addObserver:self
forKeyPath:@"fractionCompleted"
options:NSKeyValueObservingOptionNew
context:NULL];
Then add the observer method:
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
if ([keyPath isEqualToString:@"fractionCompleted"]) {
NSProgress *progress = (NSProgress *)object;
NSLog(@"Progress… %f", progress.fractionCompleted);
} else {
[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
}
}
Of course, you should check keyPath and/or object parameters to decide if that’s the object/property you want to observe.
You can also use the setDownloadTaskDidWriteDataBlock: method from AFURLSessionManager (from which AFHTTPSessionManager inherits) to set a block for receiving download progress updates.
[session setDownloadTaskDidWriteDataBlock:^(NSURLSession *session, NSURLSessionDownloadTask *downloadTask, int64_t bytesWritten, int64_t totalBytesWritten, int64_t totalBytesExpectedToWrite) {
NSLog(@"Progress… %lld", totalBytesWritten);
}];
This AFNetworking method maps the URLSession:downloadTask:didWriteData:totalBytesWritten:totalBytesExpectedToWrite: method from NSURLSessionDownloadDelegate protocol to a more convenient block mechanism.
BTW, Apple’s KVO implementation is severely broken. I recommend using a better implementation like the one proposed by Mike Ash with MAKVONotificationCenter. If you are interested in reading why Apple’s KVO is broken, read Key-Value Observing Done Right by Mike Ash.