UITableView Data Inconsistency - objective-c

Question

I have a UITableView backed by a NSArray.
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
return 1;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
return self.data.count;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"CellIdentifier";
UITableViewCell *cell = [tv dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleValue1 reuseIdentifier:CellIdentifier] autorelease];
}
id item = [self.data objectAtIndex:indexPath.row];
cell.item = item;
return cell;
}
Very standard. Now the problem is that reloadData will ask for numberOfSections and numberOfRows synchronously, but will call cellForRow asynchronously. So sometimes, by the time cellForRowAtIndexPath gets called, the data array has changed, and so [self.data objectAtIndex:indexPath.row] gets an out of bounds exception and crashes the app. How do I avoid this?
Note that every time I set the data array, I also call [self.tableView reloadData].

Answer 1

cellForRowAtIndexPath gets called frequently, (on scroll etc), you could just add a simple line of code to check if the size of the data array is smaller than the cell being requested. Although this means you might end up with blank cells.
I'd set breakpoints on both methods, right click the breakpoints -> "edit breakpoint" and tick "automatically continue after evaluating". Then click "add action" -> "debugger command" and then type "po data" or "po [data count]".
This will print information about the array in the debug console every time the breakpoint is hit (without stopping). You should then be able to look through the debug output and see where it is falling out of sync. Add some NSLog statements to to tell you which method is is being called etc and work from there.

Answer 2

I think the best way to avoid such a situation is to avoid user interaction while data is updated.May be you can show a screen to user that "updating.." and an activity indicator.
Another way is that to have another array to populate new data, handling can be done in separate thread and at times only it is assigned back to the datasource array with reloading call after that.There also a screen with same can be used while datasource array gets changed

Answer 3

Quick hack I used, try this and see if it works for you:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"CellIdentifier";
UITableViewCell *cell = [tv dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleValue1 reuseIdentifier:CellIdentifier] autorelease];
}
// -----------------------------------------------------------------
// the magical line that prevents the table from fetching the data
// -----------------------------------------------------------------
if([indexPath row] < [self.data count])
{
id item = [self.data objectAtIndex:indexPath.row];
cell.item = item;
}
return cell;
}
:D

Answer 4

You should store a local array, that doesnt get modified. then, when your base array changes, you can update your storred array safely. Look into adding/removing cells from a table view using the built in api Add rows to existing UITableView section