Databases
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    <p>First, don't queue operations while scrolling. Instead, load images for just the visible rows in <code>viewDidLoad</code> and when the user stops scrolling:</p> <pre><code>-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView { for (NSIndexPath *indexPath in [self.tableView indexPathsForVisibleRows]) { [self loadImageForCellAtPath:indexPath]; } } </code></pre> <p>If you still want to be able to cancel loading for invisible cells, you could use <code>NSBlockOperation</code> instead of GCD:</p> <pre><code>self.operationQueue = [[[NSOperationQueue alloc] init] autorelease]; [self.operationQueue setMaxConcurrentOperationCount:NSOperationQueueDefaultMaxConcurrentOperationCount]; // ... -(void)loadImageForCellAtPath:(NSIndexPath *)indexPath { __block NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{ if (![operation isCancelled]) { NSString *galleryTinyImageUrl = [[self.smapi getImageUrls:imageId imageKey:imageKey] objectForKey:@"TinyURL"]; NSData *imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:galleryTinyImageUrl]]; dispatch_async(dispatch_get_main_queue(), ^{ if (imageData != nil) { UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath]; cell.imageView.image = [UIImage imageWithData:imageData]; } }); } }]; NSValue *nonRetainedOperation = [NSValue valueWithNonretainedObjectValue:operation]; [self.operations addObject:nonRetainedOperation forKey:indexPath]; [self.operationQueue addOperation:operation]; } </code></pre> <p>Here <code>operations</code> is an <code>NSMutableDictionary</code>. When you want to cancel an operation, you retrieve it by the cell's <code>indexPath</code>, cancel it, and remove it from the dictionary:</p> <pre><code>NSValue *operationHolder = [self.operations objectForKey:indexPath]; NSOperation *operation = [operationHolder nonretainedObjectValue]; [operation cancel]; </code></pre>
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    1. POIn a UITableView, best method to cancel GCD operations for cells that have gone off screen?
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    1. USChristopher Pickslay
    1. USChristopher Pickslay
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    1. POIn a UITableView, best method to cancel GCD operations for cells that have gone off screen?
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    1. COI need to call the cancel part (your second code snippet) when a cell exits the view. The only method I know of that gets invoked on this event is the cell object's `prepreForReuse`, whose scope has no access to the object `[self.operations]` (or its `indexPath`). Am I right? Thanks, this is great.
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    2. COWhat about grabbing `[self.tableView indexPathsForVisibleRows]` and then iterate over `self.operations` and cancel all operations that aren't in the array of indexPaths? You could do that in `scrollViewDidEndDecelerating`, and perhaps `viewWillDisappear`.
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    3. COI tried this approach and noticed the app slowing down dramatically. When adding the operation, should we be adding to something other than mainQueue?
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Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

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