-[__NSArrayI addObject:]: unrecognized selector sent to instance - ios

Question

I have this code:
override func tableView(tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
var array: AnyObject = []
for obj in Category.allObjects() {
if var add = obj as? Category {
array.addObject(add.name)
println(add.name)
}
}
return String(array[section] as String)
}
Im using a Realm database and I am trying to get one of the columns of the database to print in the section headers. Im also using a the same process for all the other required UITableView methods eg/ numberOfSectionsInTable etc etc. The code is giving me this error:
2014-10-26 20:47:33.479 Project[14631:937721] -[__NSArrayI addObject:]: unrecognized selector sent to instance 0x7fdaf270ddb0
2014-10-26 20:47:33.481 Project[14631:937721] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSArrayI addObject:]: unrecognized selector sent to instance 0x7fdaf270ddb0'
*** First throw call stack:
(
0 CoreFoundation 0x0000000109960f35 __exceptionPreprocess + 165
1 libobjc.A.dylib 0x00000001095f9bb7 objc_exception_throw + 45
2 CoreFoundation 0x000000010996804d -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
3 CoreFoundation 0x00000001098c027c ___forwarding___ + 988
4 CoreFoundation 0x00000001098bfe18 _CF_forwarding_prep_0 + 120
5 BuildersUtility 0x0000000108d7f720 _TFC15BuildersUtility29ProductsDetailsViewController27numberOfSectionsInTableViewfS0_FCSo11UITableViewSi + 1408
6 BuildersUtility 0x0000000108d7f8ca _TToFC15BuildersUtility29ProductsDetailsViewController27numberOfSectionsInTableViewfS0_FCSo11UITableViewSi + 58
7 UIKit 0x000000010a478a7e -[UITableViewRowData _updateNumSections] + 84
8 UIKit 0x000000010a479474 -[UITableViewRowData invalidateAllSections] + 69
9 UIKit 0x000000010a2cdb03 -[UITableView _updateRowData] + 214
10 UIKit 0x000000010a2e300f -[UITableView numberOfSections] + 27
11 UIKit 0x000000010a4e0645 -[UITableViewController viewWillAppear:] + 97
12 UIKit 0x000000010a327821 -[UIViewController _setViewAppearState:isAnimating:] + 487
13 UIKit 0x000000010a352960 -[UINavigationController _startTransition:fromViewController:toViewController:] + 776
14 UIKit 0x000000010a353487 -[UINavigationController _startDeferredTransitionIfNeeded:] + 523
15 UIKit 0x000000010a353f47 -[UINavigationController __viewWillLayoutSubviews] + 43
16 UIKit 0x000000010a499509 -[UILayoutContainerView layoutSubviews] + 202
17 UIKit 0x000000010a277973 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 521
18 QuartzCore 0x000000010a089de8 -[CALayer layoutSublayers] + 150
19 QuartzCore 0x000000010a07ea0e _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 380
20 QuartzCore 0x000000010a07e87e _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24
21 QuartzCore 0x0000000109fec63e _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 242
22 QuartzCore 0x0000000109fed74a _ZN2CA11Transaction6commitEv + 390
23 QuartzCore 0x0000000109feddb5 _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 89
24 CoreFoundation 0x0000000109895dc7 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
25 CoreFoundation 0x0000000109895d20 __CFRunLoopDoObservers + 368
26 CoreFoundation 0x000000010988bb53 __CFRunLoopRun + 1123
27 CoreFoundation 0x000000010988b486 CFRunLoopRunSpecific + 470
28 GraphicsServices 0x000000010dec79f0 GSEventRunModal + 161
29 UIKit 0x000000010a1fe420 UIApplicationMain + 1282
30 BuildersUtility 0x0000000108d9589e top_level_code + 78
31 BuildersUtility 0x0000000108d958da main + 42
32 libdyld.dylib 0x000000010baaf145 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb)
BTW if you have a better way for me to extract the code from the database for a project like this please chirp in! :)
Thanks in Advance

Answer 1

allObjects will return you an instance of RLMResults, which is one of the two list types in Realms, the other being RLMArray. They are both made to be similar to NSArrays or swift arrays, and implement a protocol called RLMCollection. Among other things, that means you can access its elements with normal [i] notation. So you can simply write
return Category.allObjects()[section].name
although you might want to do some checking before you return. Also, it is generally not advisable to repeat the allObjects() query more than you need to, so you could cache in a lazy instance variable or similar. In this case there probably aren't that many categories/section headers, so it shouldn't be an issue.
More importantly, keep in mind that although the RLMResults list you get back is ordered in the sense that it is an ordered list, there is no inherent order among the Category instances in the Realm, so the next time you do Category.allObjects() you are not really guaranteed to receive the objects in the same order. So what you should do is really to create an RLMArray of Category objects and make that a property of another object. Then the order will be preserved.

Answer 2

It is because AnyObject does not support the method addObject
I think what you have in mind is that you want the variable array to be an array of type AnyObject.
You should declare it this way:
var array:[AnyObject] = []
And then when you want to add anything to the array, do this:
array.append(add.name)

Answer 3

In addition to what #AnthonyKong is pointing out, it is a bit unclear from the above code why you would want an array of AnyObject in the first place.
var array = [AnyObject]()
It seems what you actually want is an array of String?
var array = [String]()
then your return statement is simplified down to:
return array[section]
When feeling up to it you might also look into filter to rip out some of that almost-boiler-plate-logic in your code.

Answer 4

I had a similar problem but with an NSMutableArray. Thanks to #Gusutafu's comment above that setting an array to #[] converts it to an NSArray.
My incorrect code (selectedTags is an NSMutableArray):
selectedTags = ([theInputParameters valueForKey:kAcronymTagsSelectedTags] != nil)
? [theInputParameters valueForKey:kAcronymTagsSelectedTags]
: #[];
This gave the error -[__NSArrayI addObject:]: unrecognized selector sent to instance when I tried to add an object.
My corrected code
selectedTags = ([theInputParameters valueForKey:kAcronymTagsSelectedTags] != nil)
? [theInputParameters valueForKey:kAcronymTagsSelectedTags]
: [NSMutableArray arrayWithArray:#[]];