iOS: UITableView lagging with image data - ios

Question

I have a UITableview that has a list of images like so. But it is very laggy for some reason when I scroll up and down. Any way to stop this? The images dont need to be reloaded. It needs to stay static.
- (UITableViewCell *)tableView:(UITableView *)tableView
cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *MyIdentifier = #"MyIdentifier";
UITableViewCell *cell = [self.tableView dequeueReusableCellWithIdentifier:MyIdentifier];
if (cell == nil)
{
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
reuseIdentifier:MyIdentifier];
}
ContentModel *contentModel = [self.tableArray objectAtIndex:indexPath.row];
NSURL *url = [NSURL URLWithString:contentModel.txtImages];
NSData *data = [NSData dataWithContentsOfURL:url];
cell.imageView.image = [[UIImage alloc] initWithData:data];
return cell;
}

Answer 1

You may better use AFNetworking UIImageView Methods:
[imageView setImageWithURL:[NSURL URLWithString:#"http://image.com/image.png"]];
Even with a PlaceHolder:
[self.image setImageWithURL:[NSURL URLWithString:#"http://image.com/image.png"] placeholderImage: [UIImage imageNamed:#"logo"]];
https://github.com/AFNetworking/AFNetworking
This will dramatically reduce the lag on your tableview.

Answer 2

We have very similar requirements but my code works perfectly, I am guessing that
these commands slow down your routine:
NSURL *url = [NSURL URLWithString:contentModel.txtImages];
NSData *data = [NSData dataWithContentsOfURL:url];
cell.imageView.image = [[UIImage alloc] initWithData:data];
instead I use the following:
NSString *icon = CurrentQRCode.parsed_icon;
NSString *filePath = [[NSBundle mainBundle] pathForResource:icon ofType:#"png"];
UIImage *image_file = [UIImage imageWithContentsOfFile:filePath];
cell.imageView.image = image_file;
the whole routine is below:
QRTypeCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[QRTypeCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];}
CurrentQRCode = (QRCode *)[fetchedResultsController objectAtIndexPath:indexPath];
NSString *icon = CurrentQRCode.parsed_icon;
NSString *string = CurrentQRCode.parsed_string;
cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
NSString *filePath = [[NSBundle mainBundle] pathForResource:icon ofType:#"png"];
UIImage *image_file = [UIImage imageWithContentsOfFile:filePath];
cell.imageView.image = image_file;
cell.labelview.text = string;
I believe I originally tried handling UIImages using initWithData but found them a lot slower than the other methods.

Answer 3

Never ever ever ever load data from a network on the main thread. Use GCD or a framework like AFNetworking or restkit. The worst thing you could ever do to you app is lock up your main thread with things that can be computed off the main thread. A good rule of thumb is to ask is this something that is updating the UI, if not think about moving it to another thread.
I would also look at some of the WWDC videos about multithreading and anything related to moving off the main thread if you really want to understand the reasons why.
AFNetworking
RestKit
GCD