UITableView with complex cells is slow and laggy

Question

I've almost finished my app and everything seems to work but the main view. It's an `UIViewController` with an embedded `UITableView`. I'm using Parse as the backend, and I get an array of the objects I need in my `viewDidLoad` method. Each cell contains some data that I'm fetching in the `tableView:cellForRowAtIndexPath` and I'm afraid that this is the reason why my table view is so laggy, but I don't know how to fetch the data I need for each object in my array without having the `indexPath.row` number. I've already made each cell element "opaque" as suggested in other answers. This is my code, any help would be greatly appreciated: ``` - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { static NSString *CellIdentifier = @"cellHT"; CellHT *cell = (CellHT *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier]; if (!cell) { cell = [[CellHT alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier]; } //…

Accepted Answer

I would move as much of the logic out of `cellForRowAtIndexPath:` as you can, it needs to be very light-weight to get good scrolling performance. You're doing a lot of work on the main thread, and I would do a lot more of this work when you get your model objects back from Parse (if you could post `viewDidLoad` I can give you more specific help) and update the table view when these calls are done:

- `[UIImage imageWithData:data]`

- anything to do with `NSDateFormatter`

- `CLLocation`'s `initWithLatitude:longitude:`

- creating the rating stars image

None of these depend on the state of the table view, so they can be effectively precomputed and cached in a model object. If you simply scroll up and down the table, you're doing allo f the same work over and over, killing your performance.

Updated for the questioner's newest code:

I won't include all of your functionality here but this should give you an idea:

```
// create a single shared formatter instead of one per object
NSDateFormatter *dateFormatter = [NSDateFormatter dateFormatFromTemplate:@"dd/MM" options:0 locale:[NSLocale currentLocale]];
NSDateFormatter *timeFormatter = [NSDateFormatter dateFormatFromTemplate:@"j:mm" options:0 locale:[NSLocale currentLocale]];

[hours findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    if (!error) {
        self.objectsNumber = objects.count;

for (SomeObject *modelObj in objects) {
             // if you can add properties to your model object directly, do that
             // otherwise write a category on the Parse object to add the ones you need
             modelObj.dateString = [NSString stringWithFormat:@"%@ %@ %@ %@", modelObj.dal, [self.dateFormatter stringFromDate:[modelObj objectForKey:@"startDate"]], modelObj.ore, [self.timeFormatter stringFromDate:[modelObj objectForKey:@"endDate"]]];

// create your locations, images, etc in here too
        }

self.hH = [[NSArray alloc] initWithArray:objects];
    }
}];]
```

Then in `cellForRowAtIndexPath:`, take the precomputed properties and simply assign them to the appropriate labels, image views, etc.

It would be even better to do most of this processing off the main thread via GCD, but that is most likely out of scope for this question. See Using GCD and Blocks Effectively for more information. Just remember do only interact with UIKit from the main thread!

UITableView with complex cells is slow and laggy

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