Question

I am developing an iPhone application in which I need to use Facebook’s FQL to load the user’s notifications. As I need to load these notifications different places in the application I would like to make a subclass of NSObject to load and return these notifications as an array.
I understand that I should make a subclass (NotificationLoader) and then I could call a method inside this e.g. startLoading: and in an ideal world this method would just return an array but it cannot, as the notifications should be loaded asynchronous. I also have to take into account that the asynchrnonous request might return an error in connection:didFailWithError:

Can anyone give me a hint or an example of how I can make a class which does an asynchronous load and returns the result? I imagine this class should be called like this:

NotificationLoader *notificationLoader = [NotificationLoader alloc] init];
NSArray *notifications = [notificationLoader startLoading];

Though, I’m not sure that’s the best way to do it.

Answer 1

If you want it to be asynchronously you should use the delegation pattern. Define a protocol which needs to be implemented by the class which calls the NotificationLoader, When calling startLoading the method should start a separate thread (or using NSURL for starting an asychronous call) and loading all the stuff asynchronously. When it’s done it will call either the ‘finishedLoadingResults:(NSArray*)results’ method on the delegate (which is declared in the protocol) or the ‘didFailWithError’

so you just call

 -(void) someMethod{
       NotificationLoader *notificationLoader = [NotificationLoader alloc] init];
       notificationLoader.delegate = self;
       [notificationLoader startLoading];
 }

 -(void) notificationLoaderDidLoadResults:(NSArray*) results
 {
       // This is the place where you get your results.
 } 

 -(void) notificationLoaderDidFailWithError:....
 {
       // or there was an error...
 }

Example for your NotificationLoader:

 @protocol NotificationLoaderDelegate;
 @interface NotificationLoader : NSObject
 @property(nonatomic,retain) id<NotificationLoader> delegate;
 -(void) startLoading;
 @end

 // Define the methods for your delegate:
 @protocol NotificationLoaderDelegate <NSObject>

 -(void) notificationLoader:(NotificationLoader*) notifloader didFinishWithResults:(NSArray*) results;
 -(void) notificationLoader:(NotificationLoader*) notifloader didFailWithError;     

 @end

Implementation of the NotificationLoader

 @implementation NotificationLoader
 @synthesize delegate;

 -(void) startLoading{
       NSArray * myResults = ....;
       // Call delegate:
       [delegate notificationLoader:self didFinishWithResults:  myResults];
 }