Question

I’m trying to create a class which will let me get requested data from a web service. I’m stuck on how to return the values.

// FooClass.m
// DataGrabber is the class which is supposed to get values
dataGrabber = [[DataGrabber alloc] init];
xmlString = [dataGrabber getData:[NSDictionary dictionaryWithObjectsAndKeys:@"news", @"instruction", @"sport", @"section", nil]];

In this example, it’s supposed to get the sports news. The problem is that the DataGrabber gets the data asynchronously and ends up hopping from several NSURLConnection delegate methods. How do know in FooClass when data has been received?

Answer 1

The delegate pattern used with a strict protocol is very useful for this (that’s how DataGrabber would find out when NSURLConnection is done, right?). I have written a number of Web APIs that consume XML and JSON information this way.

// In my view controller
- (void) viewDidLoad
{
  [super viewDidLoad];
  DataGrabber *dataGrabber = [[DataGrabber alloc] init];
  dataGrabber.delegate = self;
  [dataGrabber getData:[NSDictionary dictionaryWithObjectsAndKeys:@"news", @"instruction", @"sport", @"section", nil]];
}

Then in your DataGrabber.h file:

@protocol DataGrabberDelegate
@required
- (void) dataGrabberFinished:(DataGrabber*)dataGrabber;
- (void) dataGrabber:(DataGrabber*)dataGrabber failedWithError:(NSError*)error;
@end

And in DataGrabber.m:

- (void) getData:(NSDictionary*)dict
{
  // ... Some code to process "dict" here and create an NSURLRequest ...
  NSURLConnection *connection = [NSURLConnection connectionWithRequest:req delegate:self];
}

- (void) connectionDidFinishLoading:(NSURLConnection*)connection
{
  // ... Do any processing with the returned data ...

  // Tell our view controller we are done
  [self.delegate dataGrabberFinished:self];
}

Then make sure that Foo is implements the DataGrabberDelegate protocol methods to handle each case.

Finally, your DataGrabber has a delegate property (make sure you use assign, not retain to avoid retain cycles):

@property (nonatomic, assign) id<DataGrabberDelegate> delegate;

And when the NSURLConnection asynchronous loads are finished inside of DataGrabber, they call back to your UIViewController in the protocol laid out above so that you can update the UI. If it’s ONE request, you could theoretically get rid of DataGrabber and put it inside your view controller, but I like to “separate my concerns” – API and View Controller stay separate. It generates an extra layer, but it keeps “text processing code” out of the view controllers (specifically for JSON and XML parsing code).

I’ve done this many times with success – one other key is that it’s good to provide the user with some feedback that a page is loading – turn on the activity indicator in the status bar, show them a UIActivityIndicator, etc., and then when your delegate callback comes back with either success or failure, you get rid of it.

Finally, I’ve written a more detailed blog post about this: Consuming Web APIs on the iPhone