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];
        });
    }
});