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POImages Persistence and Lazy Loading Conflict With Dispatch_Async
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copied!<p>I am working on a feed reader and i am doing it by parsing rss feeds using nsxmlparser. I also have thumbnail objects that i am taking from the CDATA block.</p> <pre><code>-(void)parser:(NSXMLParser *)parser foundCDATA:(NSData *)CDATABlock { dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ NSString *someString = [[NSString alloc] initWithData:CDATABlock encoding:NSUTF8StringEncoding]; NSString *storyImageURL = [self getFirstImageUrl:someString]; NSURL *tempURL = [NSURL URLWithString:storyImageURL]; NSData *tempData = [NSData dataWithContentsOfURL:tempURL]; thumbnail = [UIImage imageWithData:tempData]; }); } </code></pre> <p>I am persisting the images and other tableview objects using archiving and encode and decode methods. </p> <p>The issue is when i use the above code as it is, it does not persist images while other tableview objects like title and published date do persist. But when i use the above code without dispatch_async around it, then it starts persisting the images but locks the user interface.</p> <p>How can i persist the images without locking the user interface? </p> <p>Kindly do not answer with some library to store and cache images or Apple's lazy loading example. Thanks</p> <p><strong>UPDATE:</strong></p> <p>Based on the answer provided and after watching a lecture by Stanford in iTunes on Multithreading, i have implemented the above code as follow:</p> <pre><code> NSString *someString = [[NSString alloc] initWithData:CDATABlock encoding:NSUTF8StringEncoding]; NSString *storyImageURL = [self getFirstImageUrl:someString]; NSURL *tempURL = [NSURL URLWithString:storyImageURL]; dispatch_queue_t downloadQueue = dispatch_queue_create("image downloader", NULL); dispatch_async(downloadQueue, ^{ NSData *tempData = [NSData dataWithContentsOfURL:tempURL]; dispatch_async(dispatch_get_main_queue(), ^{ thumbnail = [UIImage imageWithData:tempData]; }); }); } </code></pre> <p>Logically, everything is correct now but still it didn't work. I am all blocked how to solve this issue. </p> <p><strong>UPDATE:</strong></p> <p>I am using the Model View Controller Store approach and my store and connection code are defined in separate files. Below is the code where i am using NSKeyedArchiver.</p> <pre><code>- (FeedChannel *) FeedFetcherWithCompletion:(void (^)(FeedChannel *, NSError *))block { NSURL *url = [NSURL URLWithString:@"http://techcrunch.com/feed/"]; NSURLRequest *req = [NSURLRequest requestWithURL:url]; FeedChannel *channel = [[FeedChannel alloc] init]; TheConnection *connection = [[TheConnection alloc] initWithRequest:req]; NSString *pathOfCache = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0]; pathOfCache = [pathOfCache stringByAppendingPathComponent:@"check.archive"]; FeedChannel *channelCache = [NSKeyedUnarchiver unarchiveObjectWithFile:pathOfCache]; if (!channelCachel) { channelCache = [[FeedChannel alloc] init]; } FeedChannel *channelCopy = [channelCache copy]; [connection setCompletionBlock:^(FeedChannel *obj, NSError *err) { if (!err) { [channelCopy addItemsFromChannel:obj]; [NSKeyedArchiver archiveRootObject:channelCopy toFile:pathOfCache]; } block(channelCopy, err); }]; [connection setXmlObject:channel]; [connection start]; return channelCache; } </code></pre> <p>and below are my encode and decode methods. </p> <pre><code>- (void)encodeWithCoder:(NSCoder *)aCoder { [aCoder encodeObject:title forKey:@"title"]; [aCoder encodeObject:link forKey:@"link"]; [aCoder encodeObject:creator forKey:@"creator"]; [aCoder encodeObject:pubDate forKey:@"pubDate"]; [aCoder encodeObject:thumbnail forKey:@"thumbnail"]; //[aCoder encodeObject:UIImagePNGRepresentation(thumbnail) forKey:@"thumbnail"]; [aCoder encodeObject:publicationDate forKey:@"publicationDate"]; } - (id)initWithCoder:(NSCoder *)aDecoder { self = [super init]; if (self) { [self setTitle:[aDecoder decodeObjectForKey:@"title"]]; [self setLink:[aDecoder decodeObjectForKey:@"link"]]; [self setCreator:[aDecoder decodeObjectForKey:@"creator"]]; [self setPubDate:[aDecoder decodeObjectForKey:@"pubDate"]]; [self setThumbnail:[aDecoder decodeObjectForKey:@"thumbnail"]]; //[self setThumbnail:[UIImage imageWithData:[aDecoder decodeObjectForKey:@"thumbnail"]]]; [self setPublicationDate:[aDecoder decodeObjectForKey:@"publicationDate"]]; } return self; } </code></pre> <p>Based on the answer, i don't know whether i have to use some extra code to persist images. because other objects like title, creator etc. are persisting accurately.</p>

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