UITableView scroll shows bad performance - ios

Question

I'm having a big performance issue on my tableviewcontroller. The scroll is very slow. I've made a NSLOG on the didSelectRowAtIndexPath method, and I realized that this is called on every scroll I do. It's supposed to be like that?
I've a search on this table, and I've some logic because the data depends of the json response. You can check this method here:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
//NSLog(#" scroll");
// Configure the cell...
static NSString *CellIdentifier = #"contactCell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
}
UILabel *nameLabel = (UILabel *)[cell viewWithTag:1];
UILabel *workPlaceLabel = (UILabel *)[cell viewWithTag:2];
if(searching)
{
//NSLog(#" copyListOfItems: %#",copyListOfItems);
NSString*lastName=[[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"]objectForKey:#"lastname"];
if(lastName==nil)
{
lastName=#" ";
}
NSString*firstName=[[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"]objectForKey:#"firstname"];
if(firstName==nil)
{
NSArray*phonesArray=[[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"]objectForKey:#"phone"];
NSLog(#"NUMERO TELEFONE %d",[phonesArray count]);
if([phonesArray count]>0)
{
NSString*phoneNumber=[[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"] objectForKey:#"Phone"];
nameLabel.text=phoneNumber;
}else{
nameLabel.text=[[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"] objectForKey:#"Current"];
workPlaceLabel.text=#"";
}
}else{
NSString *stringName= [NSString stringWithFormat:#"%# %#", firstName, lastName];
nameLabel.text=stringName;
workPlaceLabel.text=[[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"] objectForKey:#"Current"];
}
}
else {
//NSLog(#" _contactsArray: %#",_contactsArray);
NSString*lastName=[[[_contactsArray objectAtIndex:indexPath.row]objectForKey:#"Contact"] objectForKey:#"Lastname"];
if(lastName==nil)
{
lastName=#" ";
}
NSString*firstName=[[[_contactsArray objectAtIndex:indexPath.row]objectForKey:#"Contact"] objectForKey:#"Firstname"];
if(firstName==nil)
{
NSArray*phonesArray=[[[_contactsArray objectAtIndex:indexPath.row]objectForKey:#"Contact"] objectForKey:#"Phone"];
//NSLog(#"NUMERO TELEFONE %d",[phonesArray count]);
if([phonesArray count]>0)
{
NSString*phoneNumber=[[[[_contactsArray objectAtIndex:indexPath.row]objectForKey:#"phone"] objectAtIndex:0]objectForKey:#"phonenumber"];
nameLabel.text=phoneNumber;
}else{
nameLabel.text=[[[_contactsArray objectAtIndex:indexPath.row]objectForKey:#"Contact"] objectForKey:#"Current"];
workPlaceLabel.text=#"";
}
}else{
NSString *stringName= [NSString stringWithFormat:#"%# %#", firstName, lastName];
nameLabel.text=stringName;
if([[[_contactsArray objectAtIndex:indexPath.row]objectForKey:#"Contact"] objectForKey:#"Current"])
{
workPlaceLabel.text=[[[_contactsArray objectAtIndex:indexPath.row]objectForKey:#"Contact"] objectForKey:#"Current"];
}
}
}
// Configure the cell...
return cell;
}

Answer 1

There are a lot of needless calls in this that could be removed from your code. All those calls to get a contact that are repeated are taking time to perform when you could make them once. Like in your first branch of the if statement, you have calls like these:
NSString*lastName=[[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"]objectForKey:#"lastname"];
NSString*firstName=[[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"]objectForKey:#"firstname"];
NSArray*phonesArray=[[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"]objectForKey:#"phone"];
You could compact these calls by doing something like this:
id contact = [[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"];
NSString *lastName=[contact objectForKey:#"lastname"];
NSString *firstName=[contact objectForKey:#"firstname"];
NSArray *phonesArray=[contact objectForKey:#"phone"];
Ideally though, you would have a class of your own that has those items as properties so you could do something like this:
Contact *contact = (Contact *)[[copyListOfItems objectAtIndex:indexPath.row]objectForKey:#"Contact"];
NSString *lastName = contact.lastName;
NSString *firstName = contact.firstName;
NSArray *phonesArray = contact.phone;
Edit: How to do asynchronous image loading
Here's how I've done asynchronous loading of an image with a placeholder image in the past. The cell I was using was a custom class I wrote, but it should give you an idea of how to do it.
// Setup the image view
UIImageView* imageView = cell.imageView;
imageView.image = [UIImage imageNamed:#"Loading.png"];
// Load the image asynchronously
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(queue, ^{
// Here you will want to make a call to your web server to get the image
// and store it in a UIImage named image
UIImage *image = // your code to get the image from the server
// Only update if the cell is still on the screen
if ([[tableView indexPathsForVisibleRows] containsObject:indexPath]) {
// Have to update UI elements on the main thread
dispatch_sync(dispatch_get_main_queue(), ^{
[[cell imageView] setImage:image];
[cell setNeedsLayout];
});
}
});

Answer 2

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
is called for every cell that appears so it would be better if you kept your logic out of this loop since it will be called multiple amount of times as the user is scrolling you could set your array outside this loop and just replace the values in this method