Getting nil from property

Question

I have 2 classes, NSObject class DataParser and a ViewController. In DataParser class I have some methods that parses XML and results are saved into an array which is a property of DataParser class. I want to show results to user in my ViewController. I have here this property @property (strong, nonatomic) DataParser *parser; and in viewWillAppear method I created instance of DataParser class and I'm trying to get array with results this way

self.parser = [[DataParser alloc] init];
[self.parser downloadBooksXML];
NSLog(@"%@", self.parser.array);

but I'm still getting nil. Does anyone know where could be the problem?

- (void)downloadBooksXML
{
    NSURL *url = [NSURL URLWithString:@"http://..."];

    [self downloadDataFromURL:url withCompletionHandler:^(NSData *data) {
        if (data) {
            self.xmlParser = [[NSXMLParser alloc] initWithData:data];
            self.xmlParser.delegate = self;
            self.foundValue = [[NSMutableString alloc] init];
            [self.xmlParser parse];
        } else {
            NSLog(@"Error");
        }
    }];
}

EDIT: When I nslog array in DataParser class in parserDidEndDocument method, it contains data

Answer 1

When [self.parser downloadBooksXML] returns, self.parser.array has not been set yet, because the data has not been downloaded and parsed.

The data will be available once the document is done parsing, which you can check using the delegate method:

- (void)parserDidEndDocument:(NSXMLParser *)parser {
    NSLog(@"%@", self.parser.array);
}

You can therefore use a pattern like this:

- (void) viewDidLoad {
    [super viewDidLoad];
    self.parser = [[DataParser alloc] init];
    [self.parser downloadBooksXML];
}

- (void) dataFinished {
    NSLog(@"%@", self.parser.array);
    // update the user interface
}

- (void)parserDidEndDocument:(NSXMLParser *)parser {
   [self dataFinished];
}

---

Depending on the implementation of your other code, you might need to switch to the main queue to update the UI:

- (void) dataFinished {
    NSLog(@"%@", self.parser.array);

    dispatch_async(dispatch_get_main_queue(), ^(void){
        // update the user interface
    });
}