unrecognized selector sent to instance thrown in a strange way - ios

Question

I have a tableView which should load its cells with data from server.
When my data is loaded from server I call [self.tableView reloadData] inside completionhandler block.
So the method cellForRowAtIndexPath gets called again.
My implementation for cellForRowAtIndexPath is this:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
// Configure the cell...
NewsCell* cell = [self.tableView dequeueReusableCellWithIdentifier:#"NewsCell" forIndexPath:indexPath];
if(!cell){
cell = [[NewsCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:#"NewsCell"];
}
News* currentNews = (News*)[self.newsList objectAtIndex:indexPath.row];
cell.newsTitle.text = currentNews.title;
// cell.newsTitle.text = #"salam";
return cell;
}
Note: self.newsList is an array that is populated with data from server
The code that parses response json from server is this:
[GNetworkHelper getJsonDataFromURL:urlString
withCompletionHandler:^(NSDictionary * jsonResponse) {
// Parse retrieved json and pass it to completion handler;;
NSMutableArray* news = [[NSMutableArray alloc] init];
if (jsonResponse) {
for (NSDictionary* new in jsonResponse) {
News *currentNew = [[News alloc] initWithDictionary:new];
[news addObject:currentNew];
}
}else{
News* testNew = [[News alloc] init];
[testNew setTitle:#"salam"];
[news addObject:testNew];
}
completionHandler(news);
}];
My problem: When I debug this code the cell variable I create using dequeueReusableCellWithIdentifier:forIndexPath function is null in the debugger's variables section, but when I use lldb command po to print it out I get this
(lldb) po cell
<NewsCell: 0x7fc5fadb2ee0; baseClass = UITableViewCell; frame = (0 0; 375 143); autoresize = W; layer = <CALayer: 0x7fc5fad9fcd0>>
The same happens for currentNews variable. For it I get this from po command
(lldb) po currentNews
<News: 0x7fc5fd0480d0>
I even can get properties on it like this:
(lldb) po currentNews.title
<__NSCFArray 0x7fc5fd047300>(
"some title"
)
There is no exception when stepping through this method's statements. But when the method returns the Unrecognized Selector exception is thrown
Why this happens??
The Strange thing is that if I change this line:
cell.newsTitle.text = currentNews.title;
to this:
cell.newsTitle.text = #"Some test text";
everything goes well, specifically the cell and currentNews variables are not null anymore and tableView renders the cells without any problem
The full stack trace is :
[__NSCFArray length]: unrecognized selector sent to instance 0x7fc5fd047300
2015-09-16 11:30:40.630 Gallery[12013:952548] Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFArray length]: unrecognized selector sent to instance 0x7fc5fd047300'
--- First throw call stack:
(
0 CoreFoundation 0x0000000105af3f35 __exceptionPreprocess + 165
1 libobjc.A.dylib 0x000000010578cbb7 objc_exception_throw + 45
2 CoreFoundation 0x0000000105afb04d -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
3 CoreFoundation 0x0000000105a5327c ___forwarding___ + 988
4 CoreFoundation 0x0000000105a52e18 _CF_forwarding_prep_0 + 120
5 UIKit 0x00000001060a5f45 -[UILabel _textRectForBounds:limitedToNumberOfLines:includingShadow:] + 65
6 UIKit 0x00000001060a5da0 -[UILabel textRectForBounds:limitedToNumberOfLines:] + 76
7 UIKit 0x00000001060a9852 -[UILabel _intrinsicSizeWithinSize:] + 170
8 UIKit 0x00000001060a9932 -[UILabel intrinsicContentSize] + 76
9 UIKit 0x000000010655fea4 -[UIView(UIConstraintBasedLayout) _generateContentSizeConstraints] + 33
10 UIKit 0x000000010655fc64 -[UIView(UIConstraintBasedLayout) _updateContentSizeConstraints] + 422
11 UIKit 0x00000001065670d6 -[UIView(AdditionalLayoutSupport) updateConstraints] + 163
12 UIKit 0x00000001060a979d -[UILabel updateConstraints] + 272
13 UIKit 0x00000001065666fa -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 248
14 UIKit 0x00000001065668f2 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 124
15 CoreFoundation 0x00000001059fc194 CFArrayApplyFunction + 68
16 UIKit 0x000000010656669b -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 153
17 Foundation 0x0000000105332d6e -[NSISEngine withBehaviors:performModifications:] + 155
18 UIKit 0x00000001065668f2 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 124
19 CoreFoundation 0x00000001059fc194 CFArrayApplyFunction + 68
20 UIKit 0x000000010656669b -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 153
21 UIKit 0x00000001065668f2 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 124
22 UIKit 0x0000000106566dbe __60-[UIView(AdditionalLayoutSupport) updateConstraintsIfNeeded]_block_invoke + 96
23 UIKit 0x0000000106566a86 -[UIView(AdditionalLayoutSupport) updateConstraintsIfNeeded] + 231
24 UIKit 0x000000010635c8b8 -[UITableViewCellContentView updateConstraintsIfNeeded] + 95
25 UIKit 0x000000010656719e -[UIView(AdditionalLayoutSupport) _updateConstraintsAtEngineLevelIfNeeded] + 159
26 UIKit 0x0000000105f4db2d -[UIView(Hierarchy) _updateConstraintsAsNecessaryAndApplyLayoutFromEngine] + 114
27 UIKit 0x0000000105f59973 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 521
28 QuartzCore 0x000000010504ade8 -[CALayer layoutSublayers] + 150
29 QuartzCore 0x000000010503fa0e _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 380
30 QuartzCore 0x000000010503f87e _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24
31 QuartzCore 0x0000000104fad63e _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 242
32 QuartzCore 0x0000000104fae74a _ZN2CA11Transaction6commitEv + 390
33 QuartzCore 0x0000000104faedb5 _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 89
34 CoreFoundation 0x0000000105a28dc7 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
35 CoreFoundation 0x0000000105a28d20 __CFRunLoopDoObservers + 368
36 CoreFoundation 0x0000000105a1eb53 __CFRunLoopRun + 1123
37 CoreFoundation 0x0000000105a1e486 CFRunLoopRunSpecific + 470
38 GraphicsServices 0x00000001085d29f0 GSEventRunModal + 161
39 UIKit 0x0000000105ee0420 UIApplicationMain + 1282
40 Gallery 0x000000010346ded3 main + 115
41 libdyld.dylib 0x0000000107325145 start + 1
42 ??? 0x0000000000000001 0x0 + 1
)
Can anybody tell me the reason for this?

Answer 1

The server JSON is an array of a string, ["some title"], and you expect it to be just a string "some title".
Try adding an assertion so that your parsing of the server data is correct.
News* currentNews = (News*)[self.newsList objectAtIndex:indexPath.row];
NSAssert([currentNews.title isKindOfClass:[NSString class]],
#"Title is not a string but a %#.",
NSStringFromClass([currentNews.title class]));
cell.newsTitle.text = currentNews.title;

Answer 2

It simply indicates at some point when you try to access title value in currentNews object, it would be null. thats why your app throws exception. put a condition to check null value.
Use following method to check any value/object exist or not.
Usage
write following code in your cellForRowAtIndexPath method.
if([self isEmpty:currentNews.title])
{
cell.newsTitle.text = #"default value";
}
else
{
cell.newsTitle.text = currentNews.title[0];
}
// Method to check empty value/object
- (BOOL)isEmpty:(id)object
{
return object == nil
|| [object isKindOfClass:[NSNull class]]
|| ([object respondsToSelector:#selector(length)]
&& [(NSData *)object length] == 0)
|| ([object respondsToSelector:#selector(count)]
&& [(NSArray *)object count] == 0);
}