Slow Scrolling UITableView? - iphone

Question

My UITableView, after the messages (content) is loaded into the cells, experiences a very noticeable lag in scrolling and sometimes freezes up for a few seconds. This is weird because all the messages are loaded once the user scrolls. Any ideas on how to make this fast scrolling no problem?
Thank you!
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
static NSString *simpleTableIdentifier = #"MailCell";
MailCell *cell = (MailCell *)[tableView dequeueReusableCellWithIdentifier:simpleTableIdentifier];
if (cell == nil)
{
NSArray *nib = [[NSBundle mainBundle] loadNibNamed:#"MailCell" owner:self options:nil];
cell = [nib objectAtIndex:0];
// Anything that should be the same on EACH cell should be here.
UIView *myBackView = [[UIView alloc] initWithFrame:cell.frame];
myBackView.backgroundColor = [UIColor colorWithRed:40.0/255.0 green:148.0/255.0 blue:196.0/255.0 alpha:1];
cell.selectedBackgroundView = myBackView;
cell.messageText.textAlignment = NSTextAlignmentLeft;
cell.messageText.lineBreakMode = NSLineBreakByTruncatingTail;
}
NSUInteger row = [indexPath row];
// Extract Data
// Use the message object instead of the multiple arrays.
CTCoreMessage *message = [[self allMessages] objectAtIndex:row];
// Sender
CTCoreAddress *sender = [message sender];
NSString *senderName = [sender name];
// Subject
NSString *subject = [message subject];
if ([subject length] == 0)
{
subject = #"(No Subject)";
}
// Body
BOOL isPlain = YES;
NSString *body = [message bodyPreferringPlainText:&isPlain];
body = [[body componentsSeparatedByCharactersInSet:
[NSCharacterSet whitespaceAndNewlineCharacterSet]]
componentsJoinedByString:#" "];
body = [body stringByReplacingOccurrencesOfString:#" " withString:#" "];
// Populate Cell
[[cell nameText] setText:senderName];
[[cell subjectField] setText:subject];
[[cell messageText] setText:body];
if ([message isUnread])
{
cell.nameText.textColor = [UIColor colorWithRed:15.0/255.0 green:140.0/255.0 blue:198.0/255.0 alpha:1];
}
else
{
cell.nameText.textColor = [UIColor blackColor];
}
return cell;
}

Answer 1

xCode comes with a profiler called Instruments. It's CPU time profiler is perfect for figuring out which code is slowing things down. Run your app with the profiler and spend a few seconds just scrolling around. It will give you statistics.
Keep in mind, the code inside if (cell == nil) will run about 10 times (UITableView caches just enough cells to fill itself). But the code outside the if is expensive - it runs every time a cell becomes visible.
I would guess the most expensive operations in the code you posted are:
Giving iOS too many subviews to draw on a cell
Do your own drawing instead.
Replacing runs of whitespace in the entire body text with single spaces
The code you posted allocates new strings for each word, plus an array to hold them. Then it allocates two more copies (one with words rejoined and one with runs of spaces compacted). It processes the entire body text string, even if the majority will never be visible to the user in a tiny preview of the body!
Cache the resulting string so that this operation is performed only once per cell.
Also, you can create a new mutable string, reserve space in it, and copy characters from the original in a loop (except runs of whitespace). Instead of processing the entire body text, you could stop at 100 characters or so (enough to fill a table cell). Faster and saves memory.
Slow UITableView scrolling is a very very common question. See:
How to solve slow scrolling in UITableView
iPhone UITableView stutters with custom cells. How can I get it to scroll smoothly?

Answer 2

Nothing seems wrong with your code. I'd recommend using a table optimization framework such as the free Sensible TableView.