Screen doesn't update when I change a UILabel's text - ios

Question

My Environment is Xcode 6.3, target is iOS8.3 for iPad.
I'm learning the Master-Detail view by using Xcode's template. Some strange problem appears when I try to set a label's text in the detail view:
If I put the statement which changes the label's text in P1 position, it works.
But if I put the statement in P2 position, it doesn't work.
Under both circumstances, the detailDescriptionLabel.text was changed, but only the P1 statement actually update the screen.(I have checked that function configureView() was called both times)
Can anybody helps me? Thanks a lot.
Below are my code snippets:
MasterController.m:
#import "MasterViewController.h"
#import "DetailViewController.h"
#interface MasterViewController ()
#property (copy, nonatomic) NSMutableArray *objects;
#end
#implementation MasterViewController
- (void)awakeFromNib {
[super awakeFromNib];
self.clearsSelectionOnViewWillAppear = NO;
self.preferredContentSize = CGSizeMake(320.0, 600.0);
}
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
NSString *path = [[NSBundle mainBundle] pathForResource:#"PresidentList" ofType:#"plist"];
NSDictionary *presidentInfo = [NSDictionary dictionaryWithContentsOfFile:path];
self.objects = [NSMutableArray arrayWithArray:[presidentInfo objectForKey:#"presidents"]];
self.detailViewController = (DetailViewController *)[[self.splitViewController.viewControllers lastObject] topViewController];
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#pragma mark - Segues
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
if ([[segue identifier] isEqualToString:#"showDetail"]) {
DetailViewController *controller = (DetailViewController *)[[segue destinationViewController] topViewController];
controller.navigationItem.leftBarButtonItem = self.splitViewController.displayModeButtonItem;
controller.navigationItem.leftItemsSupplementBackButton = YES;
//P1
//controller.detailItem = (self.objects[indexPath.row])[#"url"];
}
}
#pragma mark - Table View
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
return 1;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
return self.objects.count;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:#"Cell" forIndexPath:indexPath];
cell.textLabel.text = (self.objects[indexPath.row])[#"name"];
return cell;
}
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
//P2
//self.detailViewController.detailItem = (self.objects[indexPath.row])[#"url"];
}
#end
DetailController.m:
#import "DetailViewController.h"
#interface DetailViewController ()
#property (strong, nonatomic) UIPopoverController *masterPopoverController;
#end
#implementation DetailViewController
#pragma mark - Managing the detail item
- (void)setDetailItem:(id)newDetailItem {
if (_detailItem != newDetailItem) {
_detailItem = newDetailItem;
// Update the view.
[self configureView];
}
if (self.masterPopoverController != nil) {
[self.masterPopoverController dismissPopoverAnimated:YES];
}
}
- (void)configureView {
// Update the user interface for the detail item.
if (self.detailItem) {
self.detailDescriptionLabel.text = self.detailItem;
}
}
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
[self configureView];
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#pragma mark - Split view
- (void)splitViewController:(UISplitViewController *)svc willHideViewController:(UIViewController *)aViewController withBarButtonItem:(UIBarButtonItem *)barButtonItem forPopoverController:(UIPopoverController *)pc {
barButtonItem.title = NSLocalizedString(#"Master", #"Master");
[self.navigationItem setLeftBarButtonItem:barButtonItem animated:YES];
self.masterPopoverController = pc;
}
- (void)splitViewController:(UISplitViewController *)svc willShowViewController:(UIViewController *)aViewController invalidatingBarButtonItem:(UIBarButtonItem *)barButtonItem {
[self.navigationItem setLeftBarButtonItem:nil animated:YES];
self.masterPopoverController = nil;
}
#end

Answer 1

I guess you are using segues and that's the point of using the dedicated function (prepareForSegue) designated appositely to initialize values AFTER the view destination view has been anyway created.
Unless you instantiate and show the view by yourself in didSelectRowAtIndexPath there is no point in setting something to such a viewcontroller.

Answer 2

I'm going to guess that self.detailViewController is nil. Hence P2 will not work.
Difficult to say without seeing all the code but in prepareForSegue, you might want to assign it there.