Question

I’m pulling data from an API and filling a UITableView with article titles (from Article objects). I have that part working, as far as I can tell. Where I’m having a hard time is trying to show the details of the article when a row is touched on the table view.

I’m correctly capturing the article ID, and then I make a request out to the API to pull it’s data and do a very similar thing to what I’m doing to load the articles UITableView in the step before. Here’s my parser for getting article details:

// ArticleDetailParser.h
#import <Foundation/Foundation.h>
#import "Article.h"

@class Article;
@class ArticleDetailParser;

@protocol ArticleDetailParserDelegate <NSObject>
 - (void)parserDidFinish:(ArticleDetailParser *)parser;
 - (void)parser:(ArticleDetailParser *)parser didFailWithError:(NSError *)error;
@end

@interface ArticleDetailParser : NSObject <NSXMLParserDelegate> {
 id<ArticleDetailParserDelegate> delegate;

 NSMutableString *currentCharacters;
 Article *currentArticle;
 NSMutableArray *articlesCollection;
 NSMutableData *xmlData;
 NSURLConnection *connectionInProgress;
}

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

- (void)parseUrl:(NSString *)url;
- (void)beginParsing:(NSURL *)xmlUrl;
- (Article *)detailedArticle;

@end

And here’s the implementation:

// ArticleDetailParser.m
#import "ArticleDetailParser.h"

@implementation ArticleDetailParser

@synthesize delegate;

#pragma mark -
#pragma mark Parsing methods

- (void)parseUrl:(NSString *)url
{
 NSURL *xmlUrl = [NSURL URLWithString:url];
 [self beginParsing:xmlUrl];
}

- (void)beginParsing:(NSURL *)xmlUrl
{
 [articlesCollection removeAllObjects];
 articlesCollection = [[NSMutableArray alloc] init];

 NSURLRequest *request = [NSURLRequest requestWithURL:xmlUrl cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:30];

 // clear existing connection if there is one
 if (connectionInProgress) {
  [connectionInProgress cancel];
  [connectionInProgress release];
 }

 [xmlData release];
 xmlData = [[NSMutableData alloc] init];

 // asynchronous connection
 connectionInProgress = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES];
}

- (Article *)detailedArticle
{
 return [articlesCollection objectAtIndex:0];
}

#pragma mark -
#pragma mark NSXMLParserDelegate methods

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
 [xmlData appendData:data];
}

- (void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName attributes:(NSDictionary *)attributeDict
{
 if ([elementName isEqual:@"article"]) {
  currentArticle = [[Article alloc] init];
  return;
 }
 if ([elementName isEqual:@"id"]) {
  currentCharacters = [[NSMutableString alloc] init];
  return;
 }
 if ([elementName isEqual:@"title"]) {
  currentCharacters = [[NSMutableString alloc] init];
  return;
 } 
 if ([elementName isEqual:@"alphabetical_title"]) {
  currentCharacters = [[NSMutableString alloc] init];
  return;
 }
 if ([elementName isEqual:@"body"]) {
  currentCharacters = [[NSMutableString alloc] init];
  return;
 }
 if ([elementName isEqual:@"body_html"]) {
  currentCharacters = [[NSMutableString alloc] init];
  return;
 }
 if ([elementName isEqual:@"category"]) {
  currentCharacters = [[NSMutableString alloc] init];
  return;
 }
 if ([elementName isEqual:@"authors"]) {
  currentCharacters = [[NSMutableString alloc] init];
  return;
 }
 if ([elementName isEqual:@"last_updated"]) {
  currentCharacters = [[NSMutableString alloc] init];
  return;
 }
}

- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string
{
 [currentCharacters appendString:string];
}

- (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName
{
 if ([elementName isEqual:@"article"]) {
  [articlesCollection addObject:currentArticle];
  [currentArticle release], currentArticle = nil;
  return;
 }
 if ([elementName isEqual:@"id"]) {
  [currentArticle setArticleID:currentCharacters];
  [currentCharacters release], currentCharacters = nil;
  return;
 }
 if ([elementName isEqual:@"title"]) {
  [currentArticle setTitle:currentCharacters];
  [currentCharacters release], currentCharacters = nil;
  return;
 }
 if ([elementName isEqual:@"alphabetical_title"]) {
  [currentArticle setAlphabeticalTitle:currentCharacters];
  [currentCharacters release], currentCharacters = nil;
  return;
 }
 if ([elementName isEqual:@"body"]) {
  [currentArticle setBody:currentCharacters];
  [currentCharacters release], currentCharacters = nil;
  return;
 }
 if ([elementName isEqual:@"body_html"]) {
  [currentArticle setBodyHtml:currentCharacters];
  [currentCharacters release], currentCharacters = nil;
  return;
 }
 if ([elementName isEqual:@"category"]) {
  [currentArticle setCategory:currentCharacters];
  [currentCharacters release], currentCharacters = nil;
  return;
 }
 if ([elementName isEqual:@"authors"]) {
  [currentArticle setAuthors:currentCharacters];
  [currentCharacters release], currentCharacters = nil;
  return;
 }
 if ([elementName isEqual:@"last_updated"]) {
  [currentArticle setLastModified:currentCharacters];
  [currentCharacters release], currentCharacters = nil;
  return;
 }
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
 NSXMLParser *parser = [[NSXMLParser alloc] initWithData:xmlData];
 [parser setDelegate:self];

 [parser parse];
 [parser release];

 [delegate parserDidFinish:self];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
 [currentArticle release];
 currentArticle = nil;

 [currentCharacters release];
 currentCharacters = nil;

 [articlesCollection release];
 articlesCollection = nil;

 [connectionInProgress release];
 connectionInProgress = nil;

 [xmlData release];
 xmlData = nil;

 NSLog(@"connection failed: %@", [error localizedDescription]);

 [delegate parser:self didFailWithError:error];
}

@end

Does anything stand out at you as being wrong? Here’s my controller where I’m making use of the ArticleDetailParser:

// ArticleDetailViewController.h
#import <UIKit/UIKit.h>
#import "ArticleDetailParser.h"

@class Article;

@interface ArticleDetailViewController : UIViewController <ArticleDetailParserDelegate> {
 Article *article;
 UILabel *aTitle;
 UILabel *aCategory;
 UILabel *aAuthors;
 UIActivityIndicatorView *activityView;
}

@property (nonatomic, assign) Article *article;
@property (nonatomic, assign) UIActivityIndicatorView *activityView;

- (void)loadArticleDetail;
- (void)showArticle;

@end

And the implementation of said controller:

// ArticleDetailViewController.m
#import "ArticleDetailViewController.h"

@implementation ArticleDetailViewController

@synthesize article, activityView;

#pragma mark -
#pragma mark init and dealloc

- (id)init
{
 [super initWithNibName:nil bundle:nil];

 [[self navigationItem] setTitle:@"Article Details"]; 

 article = [[Article alloc] init];

 aTitle = [[UILabel alloc] initWithFrame:CGRectMake(20, 10, 280, 25)];
 aCategory = [[UILabel alloc] initWithFrame:CGRectMake(20, 30, 280, 25)];
 aAuthors = [[UILabel alloc] initWithFrame:CGRectMake(20, 50, 280, 25)];

 activityView = [[UIActivityIndicatorView alloc] initWithFrame:CGRectMake(0, 0, 25, 25)];
 [[self activityView] sizeToFit];
 [[self activityView] setAutoresizingMask:(UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleBottomMargin)];

 return self;
}

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
 return [self init];
}

- (void)dealloc {
 [article release];
 [aTitle release];
 [aCategory release];
 [aAuthors release];
 [activityView release];
    [super dealloc];
}

#pragma mark -
#pragma mark Web Service methods

- (void)loadArticleDetail
{
 NSString *urlToRequest = [NSString stringWithFormat:@"http://wvencyclopedia.org/articles/%@.xml", [[self article] articleID]];
 ArticleDetailParser *aDetailParser = [[ArticleDetailParser alloc] init];
 [aDetailParser setDelegate:self];
 [aDetailParser parseUrl:urlToRequest]; 
 [aDetailParser release];
}

#pragma mark -
#pragma mark ArticleDetailParserDelegate methods

- (void)parserDidFinish:(ArticleDetailParser *)parser
{
 article = [parser detailedArticle]; 
 [[self activityView] stopAnimating];
}

- (void)parser:(ArticleDetailParser *)parser didFailWithError:(NSError *)error
{
 NSString *errorString = [NSString stringWithFormat:@"Fetch failed: %@", [error localizedDescription]];

 UIActionSheet *actionSheet = [[UIActionSheet alloc] initWithTitle:errorString delegate:nil cancelButtonTitle:@"OK" destructiveButtonTitle:nil otherButtonTitles:nil];
 [actionSheet showInView:[[self view] window]];
 [actionSheet autorelease]; 
}

#pragma mark -
#pragma mark UIView methods

- (void)showArticle
{
 [aTitle setText:[[self article] title]];
 [[self view] addSubview:aTitle];
 [aTitle release];

 [aCategory setText:[[self article] category]];
 [[self view] addSubview:aCategory];
 [aCategory release];

 [aAuthors setText:[[self article] authors]];
 [[self view] addSubview:aAuthors];
 [aAuthors release];
}

#pragma mark -
#pragma mark UIViewController methods

- (void)viewWillAppear:(BOOL)animated
{
 [super viewWillAppear:animated];
 [self loadArticleDetail];
 [[self activityView] startAnimating];
 [self showArticle];
}

- (void)viewDidLoad {
    [super viewDidLoad];
 [[self view] setBackgroundColor:[UIColor groupTableViewBackgroundColor]]; 
}

- (void)viewDidUnload {
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;
}

- (void)loadView
{
 [super loadView];

 UIBarButtonItem *loadingView = [[UIBarButtonItem alloc] initWithCustomView:[self activityView]];
 [[self navigationItem] setRightBarButtonItem:loadingView];
}

#pragma mark -
#pragma mark Memory Management

- (void)didReceiveMemoryWarning {
    // Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];

    // Release any cached data, images, etc that aren't in use.
}

@end

This is my first real iPhone application outside of tutorials in books, and I’m new to Objective-C as well, so I could easily be misunderstanding the memory management aspects, as well as getting labels on the view.

What’s happening is I click on the table view to load an article, and for the first one it will display the article title only. Then I go back, try to click another and it crashes.

I’m sorry to post this much code, but I figured if someone chooses to help it might be best to have the whole picture.

Thanks SO much in advance, if you should take on such a task!

Answer 1

The reason your app is crashing could be due to your UILabels getting deallocated when you release them in showArticle:.
You’ll want to use
@property (nonatomic, retain)
instead of
@property (nonatomic, assign)
when dealing with objects (variable types that require the star in front of the name). Use assign for primitive types like integers and doubles.