Discrepancy in passing data between classes - iphone

Question

Part 1 of the code works fine. Part 2 shows minor changes in the code which causes the code to stop working (without errors/warnings) as expected.
Part 1: (Works)
#import "ClassA.h"
#import "ClassB.h"
#implementation ClassA
- (void) sendData
{
NSString *temp = [NSString stringWithFormat:#"HI!"];
ClassB *classBObject = [[ClassB alloc] init];
classBObject.dataToDisplay = temp;
self.view = classBObject.view;
}
#end
Interface of ClassB:
#import <UIKit/UIKit.h>
#interface ClassB : UIViewController
#property (weak, nonatomic) IBOutlet UILabel *textLabel;
#property NSString * dataToDisplay;
#end
Implementation of ClassB:
#import "ClassB.h"
#implementation ClassB
#synthesize dataToDisplay, textLabel;
- (void)viewDidLoad
{
[super viewDidLoad];
textLabel.text = dataToDisplay;
}
#end
Part 2:
But if I change - (void)sendData of ClassA to the following:
- (void) sendData
{
NSString *temp = [NSString stringWithFormat:#"HI!"];
ClassB *classBObject = [[ClassB alloc] init];
classBObject.textLabel.text = temp; // statement changed from Part 1.
self.view = classBObject.view;
}
and remove textLabel.text = dataToDisplay; from implementation of ClassB, the textLabel on view controller of ClassB does not get updated. Can you please suggest, why is it so?
Thanks!
Edit 1:
In the statement: classBObject.textLabel.text = temp; // statement changed from Part 1., I had missed .text while copy pasting. Please excuse me for that.

Answer 1

The reason that the second technique is incorrect (besides the missing .text at the end of textLabel) is that when you return from the class B initializer, the underlying UILabel corresponding to textLabel undoubtedly has not been created yet. If you look at this diagram you'll see that the view configuration is not completed at the end of the the initialization methods, but rather upon access. So you must defer any access of the user interface controls until viewDidLoad.
Update:
When I run the following code, I get "0x0" in my log, proving that the UILabel on my second view is still nil and has not been initialized yet, as I would have expected. When the viewDidLoad in my second controller sets self.textLabel.text = self.dataToDisplay in viewDidLoad, it works like a champ (as it does for you). But the UILabel IBOutlet property is just not reliable until viewDidLoad.
SecondViewController *controller = [[SecondViewController alloc] init];
NSLog(#"%p", controller.textLabel);
controller.dataToDisplay = #"from first view";
[self presentViewController:controller animated:YES completion:nil];

Answer 2

You are setting a UITextLabel to NSString. Try
classBObject.textLabel.text = temp;

Answer 3

Change this line...
classBObject.textLabel = temp; // statement changed from Part 1.
to
classBObject.textLabel.text = temp; // statement changed from Part 1.
Also, you should do
[self.view addSubView:classBObject.view]; //using navigation controller or presenting modal viewcontroller would be recommended.
instead of
self.view = classBObject.view;
after this line, update the label's text with your value.
classBObject.textLabel.text = temp; // statement changed from Part 1.

Answer 4

I noticed that you're using a weak reference to the UILabel in your classB interface. Any reason you're not using a strong reference? The only time you want to use weak references is to avoid retain cycles. Most likely, your UILabel isn't being retained.
Where do you initialize your UILabel?