Ambiguity in the way the Category methods are written to Extend the NSDictionary

Question

Hi I was going through the AFNetworking Tutorial on Ray Wenderlich blog and in this tutorial project he has written a few categories to extend the NSDictionary and one of the category looks like below:

#import "NSDictionary+weather_package.h"

@implementation NSDictionary (weather_package)

- (NSDictionary *)currentCondition
{
    NSDictionary *dict = self[@"data"];
    NSArray *ar = dict[@"current_condition"];
    return ar[0];
}

- (NSDictionary *)request
{
    NSDictionary *dict = self[@"data"];
    NSArray *ar = dict[@"request"];
    return ar[0];
}

- (NSArray *)upcomingWeather
{
    NSDictionary *dict = self[@"data"];
    return dict[@"weather"];
}

@end

I know for what the categories are used for in Objective-C. But really the code which he has written seems very high level to me and it's confusing. BTW I'm talking about these lines :

NSDictionary *dict = self[@"data"];
NSArray *ar = dict[@"current_condition"];

I'm really not understanding how self[@"data"] and dict[@"current_condition"] work.

So could someone please help me understand what's happening here ? It would be greatly helpful to me.

PS: BTW this is how Ray is calling his category method is called on dictionary :

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"WeatherCell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];

    NSDictionary *daysWeather = nil;

    switch (indexPath.section) {
        case 0: {
            daysWeather = [self.weather currentCondition];
            break;
        }
        default:
            break;
    }

Thanks in advance.

Answer 1

This line:

NSDictionary *dict = self[@"data"];

Is just a nice shortcut for :

NSDictionary *dict = [self objectForKey:@"data"]; 

Where the "data" is the key and the value is a NSDictionary object.

Same goes with the other line only that the returned value is an array.