Label update is null

Question

I'm having an issue with a label not updating correctly in my UI. I have an Mac OS X app which uses a outline view to switch views. I want to simply have the date displayed to the user in a label on the view which is switched to (FirstViewController). When implemented alone in a new project I have no issue. But when implemented where the view changes, the value of the label does not update, in fact the console output indicates that _dateLabel is (null) even after being set prior. Any suggestions? I must be missing something quite fundamental!

Console output:

2014-08-30 19:54:22.719 OutlineView[10420:1613022] StringedText is 30 August 2014

2014-08-30 19:54:22.720 OutlineView[10420:1613022] label value is (null)

I include the following code:

//
//  FirstViewContorller.h
//  OutlineView


#import <Cocoa/Cocoa.h>

@interface FirstViewContorller : NSViewController

@property (weak) IBOutlet NSTextField *dateLabel;

-(void)updateDateScreen;

@end


//
//  FirstViewContorller.m
//  OutlineView


#import "FirstViewContorller.h"

@implementation FirstViewContorller
@synthesize dateLabel = _dateLabel;

-(void)updateDateScreen{
    //date calculation for main window
    NSDate *now = [NSDate date];
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateStyle:NSDateFormatterLongStyle];
    NSString *stringedText = [formatter stringFromDate:now];
    _dateLabel.stringValue = stringedText;
    NSLog(@"StringedText is %@", stringedText);
    NSLog(@"label value is %@", _dateLabel.value);
}

@end




//
//  AppDelegate.m
//  OutlineView


#import "AppDelegate.h"
#import "Book.h"
#import "FirstViewContorller.h"

@interface AppDelegate()

@property (weak) IBOutlet NSOutlineView *outlineView;
@property (weak) IBOutlet NSTreeController *booksController;

@end


@implementation AppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    //other code here 

    //Call the update method in FirstViewController to load date label
    FirstViewContorller *instance = [[FirstViewContorller alloc]init];
    [instance updateDateScreen];

}

//further unrelated code

@end

Answer 1

You have:

NSLog(@"label value is %@", _dateLabel.value);

And because it is outputting "null" you think that your value is "null" when in all likelihood it is _dateLabel that is null.

You are creating the instance object, but then calling a method that updates a UI object, which may not have been unarchived from the xib file by the time you are calling it. So although your date formatter is correctly creating a string, it is trying to set it on a nil object.

You can see this for yourself by examining the output of of:

NSLog(@"label is %@", _dateLabel);

which will probably return a "null" as well.