Setting the initial value of a UILABEL

I'm trying to create a simple Quiz app (I'm a beginner), when I launch the app I want a UILabel to show the first question (of an array of questions). I'm having some trouble with setting the initial value.

I've done a couple of attempts, whiteout success. I my QuizAppDelegate.h file I declare my UILabel like this:

 IBOutlet UILabel * questionField;

In my main .m file I have tried the following:

- (id)init {

    [super init];

    questions = [[NSMutableArray alloc] init];

    // Not working
    questionField = [[UILabel alloc] init];
    [questionField setText:@"Hello"];

    // Working
    NSLog(@"Hello");

    [self defaultQuestions];
    // [self showQuestion];

    return self;
}

Another thing I have tried is the following in QuizAppDelegate:

@property (nonatomic, retain) IBOutlet UILabel *questionField;

- (void)changeTitle:(NSString *)toName;

And in the .m file:

@synthesize questionField;

- (id)init {

    [super init开发者_运维知识库];

    questions = [[NSMutableArray alloc] init];

    // Not working
    [self changeTitle:@"Hello"];

    // Working
    NSLog(@"Hello");

    [self defaultQuestions];
    // [self showQuestion];

    return self;
}

-(void)changeTitle:(NSString *)toName {
    [questionField setText:toName];
}

Any tips on how to solve this would be great!

// Anders

Hopefully you're not actually putting code into main.m. On iOS, you rarely modify that file.

Since you're doing everything in the AppDelegate, let's keep it there (as opposed to creating a new UIViewController). Let's start with the basics.

Adding the Label as an instance variable

You're doing this correctly—inside the curly braces of the .h file, put the line

IBOutlet UILabel * questionField;

Then, declare the corresponding property, and make sure to synthesize it in the .m file.

@property (nonatomic, retain) IBOutlet UILabel *questionField;
@synthesize questionField    // in the .m file

Adding the UILabel in Interface Builder

Open up MainWindow.xib. Drag a UILabel from the Library to the Window that represents your app's window. Then Control-Drag from the AppDelegate object (the third icon on the left in Xcode 4; it'll be labelled in the Document window in IB 3). You'll see a little black window come up—select the option called questionField to make the connection.

See this link for screenshots and how to make connections in IB. The same applies in Xcode 4.

Changing the text

You don't need a separate method to change the text—just modify the label's text property.

Pick a method that'll be called when the app launches (applicationDidFinishLaunching:WithOptions: is a good place to do it in), and put the following code:

questionField.text = @"Hello";

And that's it!

Code

QuizAppDelegate.h

#import <UIKit/UIKit.h>

@interface QuizAppDelegate : NSObject <UIApplicationDelegate> {
    IBOutlet UILabel *questionField;
}

@property (nonatomic, retain) IBOutlet UIWindow *window;
@property (nonatomic, retain) IBOutlet UILabel *questionField;

@end

QuizAppDelegate.m

#import "QuizAppDelegate.h"

@implementation QuizAppDelegate

@synthesize window=_window;
@synthesize questionField;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    // Add the tab bar controller's current view as a subview of the window
    [self.window addSubview:self.questionField];
    [self.window makeKeyAndVisible];
    self.questionField.text = @"Hello";
    return YES;
}

- (void)dealloc
{
    [_window release];
    [questionField release];
    [super dealloc];
}

@end

If you're creating the label programmatically, then you have to add the label to the view:

[self.view addSubview:questionField];

This assumes that you have a ViewController. If not, and you're doing this directly in the AppDelegate (a very bad idea, by the way), then do

[self.window addSubview:questionField];

If you're creating it in the IB, make sure you set up the connections.

You should not both add the UILabel in the IB and instantiate it programmatically. Only call alloc if you are creating it programmatically. Otherwise, if using the IB, skip that part. You created it already with the xib.

I suspect that you have either not created your Interface Builder layout properly - either you have missed the control out all together or more likely you have not connected that control to the questionField outlet in yout header file.

You need to drag a UILabel view into the main view and then connect it to the correct line in your header file.

You shouldn't be using your main.m like that at all. In fact, you should almost certainly never do anything with it. Try creating a UIViewController subclass and practicing your quiz with that. (Add the UILabel to the IB file and then connect the outlet.) Perhaps use the View-Based Application template while you are practicing.

This is a good answer: "You're doing this correctly—inside the curly braces of the .h file, put the line

IBOutlet UILabel * questionField;" I was trying to change the value of mylabel.text and the screen didn't update the label with this.value. I included the {IBOutlet UILabel * mylabel} and it works like a charm! So this answer is valid to change the text of a label programmatically! Thanks