TableView with little less performance - objective-c

Question

I'll try to implement a TableView with 2 different kinds of cells, the first one is a higher Cell with a fullwidth image inside, all other cells got a little thumb image on the left side.
My problem it the performance of the TableView. If I scroll a list of maybe 20 items it jerks a bit. I've red something the performance and I'll hope the code is not so bad:
"Caching of images is not necessary to raise the performance" Is this right?
Does I reuse the cells in the right way.
Is that the normal way to use 2 different kinds of Cells?
Here is the important Part:
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
if (indexPath.section == 0 && indexPath.row == 0) {
return 160;
}
return 60;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
NSString *cellName = [[NSString alloc] init];
// [...]
if(indexPath.section == 0 && indexPath.row == 0){
cellName = #"FirstMainCell";
CellTemplateFirstNews *cell = [tableView dequeueReusableCellWithIdentifier:cellName];
if(cell == nil){
cell = [[[NSBundle mainBundle] loadNibNamed:#"CellTemplateFirstNewsView" owner:nil options:nil] objectAtIndex:0];
}
NSURL *urlRowImage = [NSURL URLWithString:[[NSString alloc]initWithFormat:#"http://webserver.de/inhalte/news/title/%#", detailDataNews.title_picture]];
NSData *dataRowImage = [NSData dataWithContentsOfURL:urlRowImage];
UIImageView *firstNewsImageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 320, 160)];
firstNewsImageView.backgroundColor = [UIColor clearColor];
firstNewsImageView.opaque = NO;
firstNewsImageView.image = [UIImage imageWithData:dataRowImage];
cell.backgroundView = firstNewsImageView;
// [...]
return cell;
}else{
cellName = #"MainCell";
CellTemplateNews *cell = [tableView dequeueReusableCellWithIdentifier:cellName];
if(cell == nil){
cell = [[[NSBundle mainBundle] loadNibNamed:#"CellTemplateNewsView" owner:nil options:nil] objectAtIndex:0];
}
NSURL *urlRowImage = [NSURL URLWithString:[[NSString alloc]initWithFormat:#"http://webserver.de/inhalte/news/cover/%#", detailDataNews.cover_picture]];
NSData *dataRowImage = [NSData dataWithContentsOfURL:urlRowImage];
UIImage *rowImage = [UIImage imageWithData:dataRowImage];
cell.thumbImage.image = rowImage;
// [...]
return cell;
}
}r

Answer 1

I think the main problem is the synchronous network request that's happening for every cell, whether it's reused or not. The culprit is dataWithContentsOfURL:.
Check out this apple sample code, or google the phrase "ios lazy table images". In this SO answer, I provide a fairly simple to implement method that takes care of the loading an image asynch, then finding and updating the correct table view row upon completion.

Answer 2

One posibility is that you've forgotten to enter the CellIdentifier in your Xib. In fact the reuseIdentifier method needs your cell to have the same identifier, but you never set it when you create your cell. Those cells in your Xib have to have their identifier entered.

Answer 3

Do not alloc-init any object in cellForRowAtIndexPath You are probably leaking massive memory. Remember, this method called every time a cell comes into view, even cells that have been previously shown.
Also, your assignments to string variables are redundant. It is sufficient to do it like this.
BOOL firstCell = (indexPath.row==0 && indexPath.section==0);
NSString *cellIdentifier = firstCell ? #"FirstMainCell" : #"MainCell";
UITableViewCell *genericCell = [tableView
dequeueReusableCellWithIdentifier:cellIdentifier];
if (firstCell) {
cell = (CellTemplateFirstNews*) genericCell;
// configure...
}
else {
cell = (CellTemplateNews*) genericCell;
// configure...
}
return cell;
The other possible reason for your lag is the synchronous call to the web service. You are doing this each time the cell becomes visible. You need to load the images lazily and update the UI when necessary.