What does this assertion failure mean in a UITableViewDataSource? - objective-c

Question

Currently following a checklists tutorial built with io6 in mind. I'm using xcode 5, ios7 sdk. The tutorial hasn't been updated for IOS7 but I didn't want to stop my learning so decided to go ahead and work with the outdated tutorial and hopefully use it as a learning experience. Using reading of official Apple docs and extensive googling as my guide along the way.
Very early I've run into an issue already and not sure what is wrong. I noticed that the autocomplete had part of the methods below crossed out (a deprecation maybe?). The issue is definitely coming from the code below because once I remove it the simulator loads the app fine.
Code causing crash:
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
return 1;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:#"ChecklistItem"];
return cell;
}
Here is the stack trace:
2013-09-28 20:51:26.208 Checklists[47289:a0b] *** Assertion failure in -[UITableView _configureCellForDisplay:forIndexPath:], /SourceCache/UIKit_Sim/UIKit-2903.2/UITableView.m:6235
2013-09-28 20:51:26.218 Checklists[47289:a0b] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'UITableView dataSource must return a cell from tableView:cellForRowAtIndexPath:'
*** First throw call stack:
(
0 CoreFoundation 0x017335e4 __exceptionPreprocess + 180
1 libobjc.A.dylib 0x014b68b6 objc_exception_throw + 44
2 CoreFoundation 0x01733448 +[NSException raise:format:arguments:] + 136
3 Foundation 0x0109723e -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 116
4 UIKit 0x00311ae5 __53-[UITableView _configureCellForDisplay:forIndexPath:]_block_invoke + 426
5 UIKit 0x0028af5f +[UIView(Animation) performWithoutAnimation:] + 82
6 UIKit 0x0028afa8 +[UIView(Animation) _performWithoutAnimation:] + 40
7 UIKit 0x00311936 -[UITableView _configureCellForDisplay:forIndexPath:] + 108
8 UIKit 0x00317d4d -[UITableView _createPreparedCellForGlobalRow:withIndexPath:] + 442
9 UIKit 0x00317e03 -[UITableView _createPreparedCellForGlobalRow:] + 69
10 UIKit 0x002fc124 -[UITableView _updateVisibleCellsNow:] + 2378
11 UIKit 0x0030f5a5 -[UITableView layoutSubviews] + 213
12 UIKit 0x00293dd7 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 355
13 libobjc.A.dylib 0x014c881f -[NSObject performSelector:withObject:] + 70
14 QuartzCore 0x03aed72a -[CALayer layoutSublayers] + 148
15 QuartzCore 0x03ae1514 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 380
16 QuartzCore 0x03aed675 -[CALayer layoutIfNeeded] + 160
17 UIKit 0x0034eca3 -[UIViewController window:setupWithInterfaceOrientation:] + 304
18 UIKit 0x0026dd27 -[UIWindow _setRotatableClient:toOrientation:updateStatusBar:duration:force:isRotating:] + 5212
19 UIKit 0x0026c8c6 -[UIWindow _setRotatableClient:toOrientation:updateStatusBar:duration:force:] + 82
20 UIKit 0x0026c798 -[UIWindow _setRotatableViewOrientation:updateStatusBar:duration:force:] + 117
21 UIKit 0x0026c820 -[UIWindow _setRotatableViewOrientation:duration:force:] + 67
22 UIKit 0x0026b8ba __57-[UIWindow _updateToInterfaceOrientation:duration:force:]_block_invoke + 120
23 UIKit 0x0026b81c -[UIWindow _updateToInterfaceOrientation:duration:force:] + 400
24 UIKit 0x0026c573 -[UIWindow setAutorotates:forceUpdateInterfaceOrientation:] + 870
25 UIKit 0x0026fb66 -[UIWindow setDelegate:] + 449
26 UIKit 0x00340dc7 -[UIViewController _tryBecomeRootViewControllerInWindow:] + 180
27 UIKit 0x002657cc -[UIWindow addRootViewControllerViewIfPossible] + 609
28 UIKit 0x00265947 -[UIWindow _setHidden:forced:] + 312
29 UIKit 0x00265bdd -[UIWindow _orderFrontWithoutMakingKey] + 49
30 UIKit 0x0027044a -[UIWindow makeKeyAndVisible] + 65
31 UIKit 0x002238e0 -[UIApplication _callInitializationDelegatesForURL:payload:suspended:] + 1851
32 UIKit 0x00227fb8 -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 824
33 UIKit 0x0023c42c -[UIApplication handleEvent:withNewEvent:] + 3447
34 UIKit 0x0023c999 -[UIApplication sendEvent:] + 85
35 UIKit 0x00229c35 _UIApplicationHandleEvent + 736
36 GraphicsServices 0x036862eb _PurpleEventCallback + 776
37 GraphicsServices 0x03685df6 PurpleEventCallback + 46
38 CoreFoundation 0x016aedd5 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 53
39 CoreFoundation 0x016aeb0b __CFRunLoopDoSource1 + 523
40 CoreFoundation 0x016d97ec __CFRunLoopRun + 2156
41 CoreFoundation 0x016d8b33 CFRunLoopRunSpecific + 467
42 CoreFoundation 0x016d894b CFRunLoopRunInMode + 123
43 UIKit 0x002276ed -[UIApplication _run] + 840
44 UIKit 0x0022994b UIApplicationMain + 1225
45 Checklists 0x00001b7d main + 141
46 libdyld.dylib 0x01d6f725 start + 0
)
libc++abi.dylib: terminating with uncaught exception of type NSException

Answer 1

I guess you didn't set an identifier on you cell in the storyboard, and it crashes because it can't instantiate a cell with an identifier that doesn't exists

Answer 2

dequeueReusableCellWithIdentifier: is used to reuse table view cells. If there's not a cell to reuse, this method returns nil, and you have to manually create the table view cell and return it. When there is a table view cell which is identical to another one that was used previously, dequeueReusableCellWithIdentifier: may return a valid cell.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:#"ChecklistItem"];
if(!cell)
cell= [[UITableViewCell alloc]initWithStyle: UITableViewCellStyleDefault reuseIdentifier: #"ChecklistItem"];
return cell;
}

Answer 3

It means that
UITableView dataSource must return a cell from tableView:cellForRowAtIndexPath:
as the error message says.
dequeueReusableCellWithIdentifier: is not guaranteed to always return a cell since it may return nil.
If you are supporting iOS >= 6, change it with dequeueReusableCellWithIdentifier:forIndexPath: which will never return nil.
The documentation is pretty clear about it.
To wrap it up your code should be
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:#"ChecklistItem"
forIndexPath:indexPath];
/* Configure the cell here */
return cell;
}

Answer 4

You need to set the class as the datasource for the tableview. You can do this by control dragging from the tableview in the storyboard to the viewController icon on the bottom (far left) and click dataSource and delegate.