NSLogging UILabel's text outputs null - objective-c

Question

I have a custom view controller called TimerViewController and subclasses of this called FirstViewController and FourthViewController.
I declared an instance of FirstViewController in FirstViewController's .h named controller.
In the viewDidLoad method of FourthViewController's .m, I have:
controller = [self.storyboard instantiateViewControllerWithIdentifier:#"mainController"];
In the storyboard, I've declared the view controller ID as mainController and declared a custom class of FourthViewController. Then, in FourthViewController's .m I have:
controller.mainLab.text = [NSMutableString stringWithFormat:#"This is a string"];
NSLog(#"%#", controller.mainLab.text);
This, however, outputs (null).
Why does this happen?

Answer 1

mainLab must be nil. So your outlet probably isn't connected to your XIB.
As an aside, using stringWithFormat: with a string that isn't a format is wasteful.

Answer 2

Your neglecting to tell us something about the rest of your project, im just not sure what it is.
I fired up Xcode just to run through this real quick and the process is simple.
Drag UI Label to your XIB
Control click from label to your .h
For testing I did
#import "SOViewController.h"
- (void)viewDidLoad
{
[super viewDidLoad];
self.mainLabel.text = #"This is my label";
NSLog(#"%#", self.mainLabel.text);
}
My .h looks like this:
#import <UIKit/UIKit.h>
#interface SOViewController : UIViewController
#property (weak, nonatomic) IBOutlet UILabel *mainLabel;
#end
Is this part of a custom class? Is there something else going on? If its a vanilla label it should work without issue using the aforementioned code.

Answer 3

it looks like your mainLab has not yet been created. When you call methods on nil objects, the method automatically returns nil. Make sure you actually create the label before running this line of code.

Answer 4

You can't access the label (or any other UI element) of another controller right after you instantiate it, because its viewDidLoad method has not yet run. If you want to set the text of a label in another controller, you have to pass the text to that controller, and have it set the text on the label in its viewDidLoad method. So instead of the this:
controller = [self.storyboard instantiateViewControllerWithIdentifier:#"mainController"];
controller.mainLab.text = [NSMutableString stringWithFormat:#"This is a string"];
You need to do this:
controller = [self.storyboard instantiateViewControllerWithIdentifier:#"mainController"];
controller.mainLabText = #"This is a string";
where mainLabText is a string property you create in FourthViewController. Then populate the label in FourthViewController's viewDidLoad:
self.mainLab.text = self.mainLabText;