Don't reload cell when scroll back - ios

Question

I have an issue that i didn't find anywhere on the web. I have to display some custom cell loaded from a Nib. I download the informations from my DB on a separated thread and allocate it on a new MutableArray.
I have also images that are allocated in a separate array and called when necessary, but not downloaded from the web.
My table view is "lagging" when scrolled down, that is (i guess) because it has to place the things on the correct cell, but when i scroll back it lags again and it reloads again all informations.
I see that Facebook app loads cells when scrolling down (but not so slowly) and when scrolled back it doesn't reload anything and cells are already loaded (no matter how many). How can i do something like this? My table is very slow and i have (at the moment) only 3 cells.. But when the application is finished these would be 100 or 200.
Can anyone help me?
This is my code: (this on viewDidLoad)
NSString *strURL = [NSString stringWithFormat:#"..(myurl)..];
// to execute php code
NSData *dataURL = [NSData dataWithContentsOfURL:[NSURL URLWithString:[strURL stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]]];
// to receive the returend value
NSString *strResult = [[NSString alloc] initWithData:dataURL encoding:NSUTF8StringEncoding];
if(![strResult isEqualToString:#"0"]) {
posts = [strResult componentsSeparatedByString:#"^"];
}
immProfilo = [NSMutableArray array];
for (int i = 0; i < [posts count]; i++) {
NSArray *datiPost = [[posts objectAtIndex:i] componentsSeparatedByString:#"/"];
FBProfilePictureView *fotoProfiloFB = [[FBProfilePictureView alloc] initWithFrame:CGRectMake(22, 22, 55, 55)];
fotoProfiloFB.profileID = [datiPost objectAtIndex:1];
[immProfilo addObject:fotoProfiloFB];
}
[self.postTab reloadData];
And that is my tableview code:
- (NSInteger)numberOfSectionsInTableView:(UITableView *)postTab {
return [posts count];
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
return 1;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
NSString *CellIdentifier = [NSString stringWithFormat:#"%ld_%ld",(long)indexPath.section,(long)indexPath.row];
PostTabCell *cell = (PostTabCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
NSArray *topLevelObjects = [[NSBundle mainBundle] loadNibNamed:#"PostTabCell" owner:self options:nil];
cell = [topLevelObjects objectAtIndex:0];
}
NSString *annuncio = [posts objectAtIndex:indexPath.section];
datiAnnuncio = [annuncio componentsSeparatedByString:#"/"];
[cell addSubview:[immProfilo objectAtIndex:indexPath.section]];
cell.nome.text = [datiAnnuncio objectAtIndex:0];
cell.location.text = [self getAddressFromLatLon:[[datiAnnuncio objectAtIndex:2] floatValue] withLongitude:[[datiAnnuncio objectAtIndex:3] floatValue]];
cell.testoPost.text = [datiAnnuncio objectAtIndex:4];
return cell;
}

Answer 1

The reason why your table view is so laggy is because each time the table view asks the delegate for a cell (your cellForRowAtIndexPath method), you perform a synchronous network request with getAddressFromLatLon, blocking the main thread.
An immediate fix would be to -at least- store these texts in some kind of array, so that next time the table view asks for the same cell you don't have to perform a network request again.
This would solve the problem of the tableview being laggy when you scroll back up, but not when you scroll down the first time. One general rule you can always consider true, is that you shouldn't ever block the main thread with network requests.
You've got two options now: load all of these texts at the very beginning on a secondary thread while showing a spinner (easy but presents several problems, such as it wouldn't scale up very well with the number of cells). Or you would have to design an asynchronous loader that will show a placeholder string, such as loading address..., until the address is actually loaded.
Also Totumus has a point in his answer, but that is not the main cause of your lag (although it will be a big problem once the number of your cells increase).

Answer 2

The lag you describe is caused by the imageview you keep adding to your reusable cell. Everytime the cell reloads an imageview is added to your cell again.
Prevent functions like addSubview: in your cellForRowAtIndexPath:.
Instead you should create a custom cell that already has this UIImageView you need for your profilepictures.
The reason you load information when you scroll is that you probably load information (getAddressFromLatLon: ?) each time a cell is being created/reused. This is fine but should be done in a seperate thread (and thus the response should be handled asynchronically).
The reason you see facebook not loading anymore when you scroll back up again is because they cache their data when it is loaded into the application. Probably with CoreData.