cellforRowAtIndexPath efficiency? - ios

Question

Whenever I scroll my tableview it is very laggy. I think it has to do with how I am loading up my cells. I use UINib (5.0+) whenever I can while still providing backwards compatibility. Then I load my custom cell's labels and images with items from a NSDictionary from a NSArray which is loaded from NSUserDefaults in the ViewDidLoad.
Is there any way to improve the efficiency of this cellForRowAtIndexPath?
- (UITableViewCell *)tableView:(UITableView *)aTableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
CustomCell *cell = (CustomCell *)[aTableView dequeueReusableCellWithIdentifier:#"Cell"];
if (cell == nil) {
if ([self labelCellNib]) {
[[self labelCellNib] instantiateWithOwner:self options:nil];
} else {
[[NSBundle mainBundle] loadNibNamed:#"CustomCell" owner:self options:nil];
}
cell = [self CustomTableCell];
[self setCustomTableCell:nil];
}
NSDictionary *dictionary = [myArray objectAtIndex:indexPath.row];
NSData *data = [dictionary objectForKey:#"OCRImage"];
cell.previewPicture.image = [self roundCorneredImage:[UIImage imageWithData:data] radius:60];
cell.titleLabel.text = [dictionary objectForKey:#"Title"];
cell.titleLabel.delegate = self;
cell.dateLabel.text = [dictionary objectForKey:#"Date"];
if (indexPath.row%2) {
cell.backgroundImage.image = firstImage;
}
else {
cell.backgroundImage.image = secondImage;
}
return cell;
}
Edit:
- (UIImage*)roundCorneredImage: (UIImage*)orig radius:(CGFloat) r {
UIGraphicsBeginImageContextWithOptions(orig.size, NO, 0);
[[UIBezierPath bezierPathWithRoundedRect:(CGRect){CGPointZero, orig.size}
cornerRadius:r] addClip];
[orig drawInRect:(CGRect){CGPointZero, orig.size}];
UIImage* result = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return result;
}
Edit2: These are the lines that are causing the lag:
NSData *data = [dictionary objectForKey:#"OCRImage"];
cell.previewPicture.image = [self roundCorneredImage:[UIImage imageWithData:data] radius:60];

Answer 1

As #Till said in a comment, you should launch your app in Instruments (Product -> Profile in Xcode), and select the CPU -> Time Profiler instrument.
Then, scroll around over the place for a few seconds, then hit the Record toolbar icon in instruments to close your app. You will be able to see the scrolling section because CPU usage will probably be pinned at 100% (unless it's slow because of network activity problem).
Click on the timeline after the start of the high CPU activity area, and click the "start inspection range" toolbar button, then click before the end of the high CPU activity area and click the "stop inspection range" toolbar button.
You can now drill down into the call tree view at the bottom of the window to figure out exactly where all your CPU usage is. In my experience it's usually easier to find the problem if you turn off "invert call tree" option on the left.
Performance bugs can be very hard to find, and sometimes a line of code that is obviously slow actually isn't causing any problems at all. The only way to fix performance issues without wasting time is to use Instruments.

Answer 2

Make sure that you've set the reuse identifier for your cell to the same thing that you've specified in your code, i.e. #"Cell". If they don't match, then you won't be reusing cells properly, and probably spending a lot more time creating cells than necessary.
If you are properly recycling cells, then you should take a look at the code after the if (cell == nil) {...} block. You'll be skipping that entire block once the table has created enough cells to fill the screen (and maybe one or two more), so most of the time attributable to this method while scrolling will be due to the following code. It'd be interesting to know what myArray is, and if it's actually an array, what the objectForKey: method does. Nothing else there looks like it should take a long time, but the best way to find out where the cycles are going is to profile your code in Instruments.

Answer 3

Some of my notes after looking at your code:
Is roundCorneredImage:radius: caching the result? If not, executing CG calls for every cell would surely present a bottleneck. Updated: Use instruments to be sure, but it might be faster (memory allowing) to store the processed UIImage in a collection so that you can pull it out again the next time that method is called with the same parameters.
All of your UIImages could be declared elsewhere and then presented in this method. Your current code instantiates a new UIImage for each cell which can also bottleneck your scrolling. Updated: Since Image1.png and Image2.png are basically static, you could declare them in your interface or as a static ivar and then just assign them to the background image rather than instantiating UIImage each time.
It may be faster to subclass UITableViewCell and instantiate that instead of reaching into UINib. Also, you'd then be able to separate your layout/data logic from the delegate method. Here's a gist of what I did in my UITableViewCell subclass. Basically, I store the entity with the cell and the cell knows about it's labels and such. This keeps the cell layout logic out of my data source code.
It looks like you're using an NSDictionary as your data source. If you have a lot of objects in that dictionary, it may be considerable faster to use CoreData and an NSFetchedResultsController. Here's a good post on the matter. Updated: Ok, that shouldn't be an issue.
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Edit
So if you removed all of this:
NSDictionary *dictionary = [myArray objectForKey:#"OCRImage"];
cell.previewPicture.image = [self roundCorneredImage:[UIImage imageWithData:data] radius:60];
if (indexPath.row%2) {
cell.backgroundImage.image = firstImage;
}
else {
cell.backgroundImage.image = secondImage;
}
and it still lags, let's look at your constructors...what do these lines do?
cell = [self CustomTableCell];
[self setCustomTableCell:nil];
Also, you're not using any transparent images or anything in your table cell are you? Those have been known to cause drawing lag...
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Edit #2
If you strip down to this, what happens?
CustomCell *cell = (CustomCell *)[aTableView dequeueReusableCellWithIdentifier:#"Cell"];
if (cell == nil) {
if ([self labelCellNib]) {
[[self labelCellNib] instantiateWithOwner:self options:nil];
} else {
[[NSBundle mainBundle] loadNibNamed:#"CustomCell" owner:self options:nil];
}
cell = [self CustomTableCell];
[self setCustomTableCell:nil];
}
cell.titleLabel.text = [dictionary objectForKey:#"Title"];