Three20 JSON datasource implementation

Three20 is a nice iPhone library, but the documentation is pretty much limited to the TTCatalog example app code…one thing that’s left as an exercise for the reader is the implementation of a datasource that fetches its data over the network and how it interacts with its associated TTTableViewController.

One approach to this problem is to subclass a canned datasource, which gets us a bunch of stuff for free right off the bat:

@interface MyDataSource : TTListDataSource<TTURLRequestDelegate> {
 @private
  BOOL _loading;
  BOOL _loaded;
}

You’ll want to provide meaningful implementations of accessors like loadedTime:, isLoading:, isLoadingMore:, isLoaded:, etc. Your controller will query the datasource using those selectors to know what state to show (e.g. loading, error, data, no data).

Write a load:nextPage: implementation that hits the network:

- (void)load:(TTURLRequestCachePolicy)cachePolicy nextPage:(BOOL)nextPage {
  TTURLRequest *request =
   [TTURLRequest requestWithURL:@"http://test.com/test.json" delegate:self];

  request.cachePolicy = cachePolicy;
  request.response = [[[TTURLDataResponse alloc] init] autorelease];
  request.httpMethod = @"GET";
  [request send];
}

Write a requestDidFinishLoad: implementation that pulls JSON out of the response (maybe using Stig Brautaset’s very nice JSON framework) and let your controller know that data’s available via dataSourceDidFinishLoad:.

- (void)requestDidFinishLoad:(TTURLRequest*)request {
  TTURLDataResponse *response = request.response;
  NSString *json = [[NSString alloc] initWithData:response.data encoding:NSUTF8StringEncoding];

  // Load up self.items with json data however you'd like

  [json release];

  _loading = NO;
  _loaded = YES;  

  [self dataSourceDidFinishLoad];
}

Don’t forget to implement requestDidStartLoad:, request:didFailLoadWithError:, and requestDidCancelLoad: and have them notify any interested delegates of the request’s status. Here’s an example:

- (void)request:(TTURLRequest*)request didFailLoadWithError:(NSError*)error {
  _loading = NO;
  _loaded = YES;
  [self dataSourceDidFailLoadWithError:error];
}

In your controller, createDataSource: might look like this:

- (id<TTTableViewDataSource>)createDataSource {
  MyDataSource *dataSource = [[[MyDataSource alloc] init] autorelease];
  [dataSource.delegates addObject:self];
  [dataSource load:TTURLRequestCachePolicyDefault nextPage:NO];
  return dataSource;
}

Have fun out there.

Update, April 11 10:30 AM EDT
I’ve added a new post with a downloadable example project and (finally) cleaned up the above code. I apologize for previous typos; I really should make sure the code I post actually compiles instead of writing/tweaking it in a wordpress edit window.

Update, October 3 9:30 AM EDT
It’s come to my attention that some people are still using this code – please don’t! It was based on three20 as it existed back in March/April 2009 and is now very much out-of-date. If you’re looking for current and correct three20 sample code, Keith Lazuka’s github has what you need. Thanks!