iphone table view scroll aborts - unrecognized selector

Question

I know there have been a ton of questions about the same thing, but so far I haven't been able to apply any solutions to my problem. And I still haven't figured out how to use Instruments.

I'm taking a basic tutorial for an iPhone app and just trying to tweak it slightly (I am new to Objective C). I want it to read from a plist with an array of dicts instead of an array of strings. The table initially displays the data correctly. However whenever I scroll the table up (and off the screen), I am getting Unrecognized Selector exceptions. Just populating employees with NSStrings works fine. I am lost.

Relevant portions of the ViewController:

@interface RootViewController : UITableViewController {

NSMutableArray *employees_;
}

@property (nonatomic, retain) NSMutableArray *employees;
@end

and

@implementation RootViewController

@synthesize employees=employees_;


- (void)viewDidLoad
{
 [super viewDidLoad];

 NSString *path = [[NSBundle mainBundle] pathForResource:@"Employees" ofType:@"plist"];

 NSMutableArray *empArray = [[NSMutableArray alloc] initWithContentsOfFile:path];

 employees_ = [empArray valueForKey:@"name"];
 [empArray release];

}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"Cell";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
}

// Configure the cell.

cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
cell.textLabel.text = [self.employees objectAtIndex:indexPath.row];//this is where it errors

return cell;
}
- (void)dealloc
{
 [employees_ release];
 [super dealloc];
}

@end

and plist:

array
      dict
        key name /key
        string Employee One /string
        key id /key
        string T1234 /string 
      /dict
      dict
        key name /key
        string Employee Two /string
        key id /key
        string T5678 /string 
     /dict
/array

Error that I received:

2011-10-18 20:02:44.313 MyApp[65148:bc03] -[NSCFString objectAtIndex:]: unrecognized selector sent to instance 0x689a050
2011-10-18 20:02:44.316 MyApp[65148:bc03] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSCFString objectAtIndex:]: unrecognized selector sent to instance 0x689a050'
*** Call stack at first throw:
(
    0   CoreFoundation                      0x00dc25a9 __exceptionPreprocess + 185
    1   libobjc.A.dylib                     0x00f16313 objc_exception_throw + 44
    2   CoreFoundation                      0x00dc40bb -[NSObject(NSObject) doesNotRecognizeSelector:] + 187
    3   CoreFoundation                      0x00d33966 ___forwarding___ + 966
    4   CoreFoundation                      0x00d33522 _CF_forwarding_prep_0 + 50
    5   MyApp                    0x00002a96 -[RootViewController tableView:cellForRowAtIndexPath:] + 326
    6   UIKit                               0x00089b98 -[UITableView(UITableViewInternal) _createPreparedCellForGlobalRow:withIndexPath:] + 634
    7   UIKit                               0x0007f4cc -[UITableView(UITableViewInternal) _createPreparedCellForGlobalRow:] + 75
    8   UIKit                               0x000948cc -[UITableView(_UITableViewPrivate) _updateVisibleCellsNow:] + 1561
    9   UIKit                               0x0008c90c -[UITableView layoutSubviews] + 242
    10  QuartzCore                          0x016aca5a -[CALayer layoutSublayers] + 181
    11  QuartzCore                          0x016aeddc CALayerLayoutIfNeeded + 220
    12  QuartzCore                          0x016540b4 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 310
    13  QuartzCore                          0x01655294 _ZN2CA11Transaction6commitEv + 292
    14  QuartzCore                          0x0165546d _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 99
    15  CoreFoundation                      0x00da389b __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 27
    16  CoreFoundation                      0x00d386e7 __CFRunLoopDoObservers + 295
    17  CoreFoundation                      0x00d011d7 __CFRunLoopRun + 1575
    18  CoreFoundation                      0x00d00840 CFRunLoopRunSpecific + 208
    19  CoreFoundation                      0x00d00761 CFRunLoopRunInMode + 97
    20  GraphicsServices                    0x00ffa1c4 GSEventRunModal + 217
    21  GraphicsServices                    0x00ffa289 GSEventRun + 115
    22  UIKit                               0x00022c93 UIApplicationMain + 1160
    23  MyApp                    0x00002249 main + 121
    24  MyApp                    0x000021c5 start + 53
)
terminate called throwing an exceptionCurrent language:  auto; currently objective-c
(gdb)

Rog Rog · Accepted Answer · 2011-10-19T01:50:58

There are two potential problems:

You need to make sure the call to employees_ = [empArray valueForKey:@"name"] is actually returning a NSArray
Once one is ruled out, and assuming you are not using ARC, your employees_ ivar is getting released before the table view gets a chance to configure itself. Try

employees_ = [[empArray valueForKey:@"name"] retain];

And then release employees_ in your viewDidUnload & dealloc methods.

Hard to tell it from the stack as it does say your ivar is a NSCFString but it could be just because it is referencing an invalid/garbage memory address. Based on your plist description though, the likely cause on on point #1.

iphone table view scroll aborts - unrecognized selector

2 Answers