NSnotifications for multiple downloads - iphone

Question

I have a parser class and some view controller classes. In the parser class i am sending a request and receiving an asynchronous response. I want multiple downloads, say one per viewcontroller. So i register an observer in each of these classes :
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(dataDownloadComplete:) name:OP_DataComplete object:nil];
and then post a notification in :
-(void)connectionDidFinishLoading:(NSURLConnection *)connection method of the parser class.
[[NSNotificationCenter defaultCenter] postNotificationName:OP_DataComplete object:nil];
but then on running the code the first viewcontroller works fine but for the second one onwards after download and parser class posting notification infinitely the code enters the first class's dataDownloadComplete: method although i have specified a different method name in the selector each time. I don't understand what the error might be. Please help. Thanks in advance.

Answer 1

Both view controllers are listening for the notification so both methods should be being called, one after another.
There's a few ways to solve this. The easiest would be for the notification to contain some sort of identifier that the view controller can look at to see if it should ignore it or not. NSNotifications have a userInfo property for this.
NSDictionary *info = [NSDictionary dictionaryWithObjectsAndKeys:#"viewController1", #"id", nil];
[[NSNotificationCenter defaultCenter] postNotificationName:OP_DataComplete object:self userInfo:info];
and when you recieve the notification, check to see who it's for :
- (void)dataDownloadComplete:(NSNotification *)notification {
NSString *id = [[notification userInfo] objectForKey:#"id"];
if (NO == [id isEqualToString:#"viewController1"]) return;
// Deal with the notification here
...
}
There's a few other ways to deal with it but without knowing more about your code, I can't explain them well - basically you can specify the objects that you want to listen to notifications from (see how I have object:self but you sent object:nil) but sometimes your architecture won't allow that to happen.

Answer 2

it's better to create a protocol:
#protocol MONStuffParserRecipientProtocol
#required
- (void)parsedStuffIsReady:(NSDictionary *)parsedStuff;
#end
and to declare the view controller:
#class MONStuffDownloadAndParserOperation;
#interface MONViewController : UIViewController < MONStuffParserRecipientProtocol >
{
MONStuffDownloadAndParserOperation * operation; // add property
}
...
- (void)parsedStuffIsReady:(NSDictionary *)parsedStuff; // implement protocol
#end
and add some backend: to the view controller
- (void)displayDataAtURL:(NSURL *)url
{
MONStuffDownloadAndParserOperation * op = self.operation;
if (op) {
[op cancel];
}
[self putUpLoadingIndicator];
MONStuffDownloadAndParserOperation * next = [[MONStuffDownloadAndParserOperation alloc] initWithURL:url recipient:viewController];
self.operation = next;
[next release], next = 0;
}
and have the operation hold on to the view controller:
#interface MONStuffDownloadAndParserOperation : NSOperation
{
NSObject<MONStuffParserRecipientProtocol>* recipient; // << retained
}
- (id)initWithURL:(NSURL *)url Recipient:(NSObject<MONStuffParserRecipientProtocol>*)recipient;
#end
and have the operation message the recipient when the data is downloaded and parsed:
// you may want to message from the main thread, if there are ui updates
[recipient parsedStuffIsReady:parsedStuff];
there are a few more things to implement -- it's just a form. it's safer and involves direct messaging, ref counting, cancellation, and such.