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    <p>To keep your scrolling smooth regardless of where your data comes from, you need to fetch your data on a separate thread and only update the UI when you have the data in memory. <em>Grand Central Despatch</em> is the way to go. Here's a skeleton which assume you have a <code>self.photos</code> dictionary with a text reference to an image file. The image thumbnail may or may not be loaded into a live dictionary; may or may not be in a filesystem cache; otherwise is fetched from an online store. It could use Core Data, but the key to smooth scrolling is that you don't wait around for the data <em>wherever</em> it comes from.</p> <pre><code>- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { static NSString *CellIdentifier = @"Photo Cell"; UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier]; //identify the data you are after id photo = [self.photos objectAtIndex:indexPath.row]; // Configure the cell based on photo id dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ //move to an asynchronous thread to fetch your image data UIImage* thumbnail = //get thumbnail from photo id dictionary (fastest) if (!thumbnail) { //if it's not in the dictionary thumbnail = //get it from the cache (slower) // update the dictionary if (!thumbnail) { //if it's not in the cache thumbnail = //fetch it from the (online?) database (slowest) // update cache and dictionary } } } if (thumbnail) { dispatch_async(dispatch_get_main_queue(), ^{ //return to the main thread to update the UI if ([[tableView indexPathsForVisibleRows] containsObject:indexPath]) { //check that the relevant data is still required UITableViewCell * correctCell = [self.tableView cellForRowAtIndexPath:indexPath]; //get the correct cell (it might have changed) [[correctCell imageView] setImage:thumbnail]; [correctCell setNeedsLayout]; } }); } }); return cell; } </code></pre> <p>If you are using some kind of singleton image store manager, you would expect the manager to deal with the details of cache / database access, which simplifies this example.</p> <p>This part</p> <pre><code> UIImage* thumbnail = //get thumbnail from photo id dictionary (fastest) if (!thumbnail) { //if it's not in the dictionary thumbnail = //get it from the cache (slower) // update the dictionary if (!thumbnail) { //if it's not in the cache thumbnail = //fetch it from the (online?) database (slowest) // update cache and dictionary } } </code></pre> <p>would be replaced with something like</p> <pre><code> UIImage* thumbnail = [[ImageManager singleton] getImage]; </code></pre> <p>(you wouldn't use a completion block as you are effectively providing one in GCD when you return to the main queue)</p>
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    1. POLoading image from CoreData at cellForRowAtIndexPath slows down scrolling
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    1. COthanks! Can I ask u one more question, is it the same if I use completion block? like [ImageManager singleton] getImageWithBlock:(void (^)(NSArray *images))block
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    2. CO@dunforget - Kind of - a completion block waits (asynchronously) for it's method to finish - the method triggers the block on completion (it's similar to the way delegates work). But the block itself will be queued on the main thread unless you instruct it otherwise. In general, you need to run stuff on the main thread if it affects the UI. But anything that is non-UI should be moved off the main thread if it is going to block the user experience.
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    3. CO@dunforget, if you are using some kind of Image manager singleton, you can expect that to deal with all of the dictionary/cache/url access details... so the async part of my example can be reduced to one line. I've updated the answer. Also there was an error where I called `dispatch_async(dispatch_get_main_queue()` right after the call to the async thread - corrected.
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