How do I parse in a background thread?

Question

I basically used this tutorial to make a RSS reader. I think this one is not multithreaded.

I also took a look at raywenderlich's tutorial which should be multithreaded, but I couldn't use this because it is out of date and the library doesn't work anymore.

The first tutorial loads the content pretty fast. But I try to load it in a pageview. So I have 5 pages each loading a different RSS.

In the tutorial (and so in my code) parsing happens in ViewDidLoad :

- (void)viewDidLoad
{
    [super viewDidLoad];

    //initialize
    images = [[NSMutableArray alloc] init];
    feeds = [[NSMutableArray alloc] init];
    currentRssUrl = [NSURL URLWithString:self.rssUrl];
    parser = [[NSXMLParser alloc] initWithContentsOfURL:currentRssUrl];
    [parser setDelegate:self];
    [parser setShouldResolveExternalEntities:NO];
    [parser parse];
}

Now it start with me not completely understanding the code but I think [parser parse] does the parsing.

here is the rest of my code.

#pragma mark - Table View

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return feeds.count;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"Cell" forIndexPath:indexPath];
    cell.textLabel.numberOfLines = 2;
    cell.textLabel.lineBreakMode = NSLineBreakByWordWrapping;
    cell.textLabel.text = [[feeds objectAtIndex:indexPath.row] objectForKey: @"title"];


    NSString *yt = @"videos";
    NSString *videoCheck = [currentRssUrl absoluteString];
    if(images.count > 0)
    {
        NSURL *imgUrl = [NSURL URLWithString:images[indexPath.row]];
        NSData *data = [[NSData alloc] initWithContentsOfURL:imgUrl];
        UIImage *tmpImage = [[UIImage alloc] initWithData:data];
        cell.imageView.image = tmpImage;
    }
    else if ([videoCheck rangeOfString:yt].location != NSNotFound)
    {
        NSString *ytLink = [feeds[indexPath.row] objectForKey: @"link"];
        NSArray *stringArray = [ytLink componentsSeparatedByString:@"/"];
        NSString *ytId = stringArray[stringArray.count-1];
        ytId = [ytId stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
        NSString *ytThumbUrl = [NSString stringWithFormat:@"http://img.youtube.com/vi/%@/default.jpg", ytId];
        NSURL *imgUrl = [NSURL URLWithString:ytThumbUrl];
        NSData *data = [[NSData alloc] initWithContentsOfURL:imgUrl];
        UIImage *tmpImage = [[UIImage alloc] initWithData:data];
        cell.imageView.image = tmpImage;
    }

    return cell;
}

- (void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName attributes:(NSDictionary *)attributeDict
{

    element = elementName;

    if ([element isEqualToString:@"item"])
    {
        item    = [[NSMutableDictionary alloc] init];
        title   = [[NSMutableString alloc] init];
        link    = [[NSMutableString alloc] init];
        enclosure = [[NSMutableString alloc] init];
    }
    if(![elementName isEqual:@"enclosure"])
        return;

    NSString *urlTitle = @"app_thumb";
    //als de title gelijk is aan app_thumb
    if([[attributeDict objectForKey:@"title"] isEqual:urlTitle])
    {
        //lees de url attribute uit
        NSString * name = [attributeDict objectForKey:@"url"];
        // voeg de url toe aan images
        [images addObject:name];
    }

}

- (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName
{

    if ([elementName isEqualToString:@"item"])
    {
        [item setObject:title forKey:@"title"];
        [item setObject:link forKey:@"link"];
        [item setObject:enclosure forKey:@"enclosure"];
        [feeds addObject:[item copy]];
    }
}

- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string {

    if ([element isEqualToString:@"title"])
    {
        [title appendString:string];
    }
    else if ([element isEqualToString:@"link"])
    {
        [link appendString:string];
    }
}

- (void)parserDidEndDocument:(NSXMLParser *)parser
{

    [self.tableView reloadData];

}

So how can I make the parsing in a background thread. Also when is it best to load. In my android version I started a AsyncTask when the page became visible.

I find this very hard but as far as this question goes. I want to know how to do the parsing in a background thread, any extra multithreading tips are appreciated.

Answer 1

Try with the GCD :

 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // Background running Code
    /* 
   //initialize
      images = [[NSMutableArray alloc] init];
      feeds = [[NSMutableArray alloc] init];
      currentRssUrl = [NSURL URLWithString:self.rssUrl];
      parser = [[NSXMLParser alloc] initWithContentsOfURL:currentRssUrl];
      [parser setDelegate:self];
      [parser setShouldResolveExternalEntities:NO];
      [parser parse];
   */

});

Then update the result in the Main Thread:

- (void)parserDidEndDocument:(NSXMLParser *)parser
{
    dispatch_sync(dispatch_get_main_queue(), ^{
      // [self.tableView reloadData];

   });

}

Here is the good tutorial.