cs193p – Assignment #6 Part #2

Please note, this blog entry is from a previous course. You might want to check out the current one.

… back to the actual task … prevent the “old” top-places table view controller from loading photos we do not need any more:

- (void)viewDidLoad
{
    [super viewDidLoad];
//    [self fetchPlaces];
}

Download Photos from Flickr using URLforRecentGeoreferencedPhotos:

Assignment #6 Task #1

Your application must work identically to last week except that where you are displaying the “top places” (as reported by Flickr), you are going to display the “top regions” in which photos have been taken. You will calculate the most popular regions from the data you gather periodically from the URLforRecentGeoreferencedPhotos.


… let’s start the initial fetch from the application delegate:

- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [self startFlickrFetch];
    return YES;
}

Let’s assume we have not read task #7, and just perform a “normal fetch” for which we use a helper function from the FlickrHelper class. As soon as we get a result show how many photos we got … (hopefully 250) …

- (void)startFlickrFetch
{
    [FlickrHelper loadRecentPhotosOnCompletion:^(NSArray *photos, NSError *error) {
        if (error) {
            NSLog(@"Flickr background fetch failed: %@", error.localizedDescription);
        } else {
            NSLog(@"%d photos fetched", [photos count]);
        }
    }];
}

The helper method just opens a new ephemeral session, accesses Flickr, tries to get photo data from the received JSON file, and runs the completion handler (which is for now a log message):

//  FlickrHelper.h
+ (void)loadRecentPhotosOnCompletion:(void (^)(NSArray *places, NSError *error))completionHandler;

//  FlickrHelper.m
+ (void)loadRecentPhotosOnCompletion:(void (^)(NSArray *places, NSError *error))completionHandler
{
    NSURLSessionConfiguration *config = [NSURLSessionConfiguration ephemeralSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:config];
    NSURLSessionDownloadTask *task = [session downloadTaskWithURL:[FlickrHelper URLforRecentGeoreferencedPhotos]
                                                completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
                                                    NSArray *photos;
                                                    if (!error) {
                                                        photos = [[NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfURL:location]
                                                                                                  options:0
                                                                                                    error:&error] valueForKeyPath:FLICKR_RESULTS_PHOTOS];
                                                    }
                                                    dispatch_async(dispatch_get_main_queue(), ^{
                                                        completionHandler(photos, error);
                                                    });
                                                }];
    [task resume];
}

… and if you run it … it will return nothing … Flickr just changed its API and does only allow https requests. The FlickrFetcher files provided by Stanford only use http URLs. Therefore you need to change all “http://” strings to “https://” in FlickrFetcher.m.

The complete code for tasks #1 to #7 is available on github.