IBOutlet instances are (null) after loading from NIB

问题

I am working on an iPhone app and am getting (null) references to IBOutlet fields in my controller. I have a UIViewController subclass that is set as the File's Owner in my XIB. I have a set of UI elements that are wired into the controller. After loading from NIB and attempting to set properties on those UI elements, I find that they are (null). To clarify, some code:

ExpandSearchPageController.h:

@interface ExpandSearchPageController : UIViewController
{
  IBOutlet UITextView * completeMessageView;
}

-(void)checkTextField;

@property (nonatomic, retain) IBOutlet UITextView * completeMessageView;

ExpandSearchPageController.m:

@implementation ExpandSearchPageController

@synthesize completeMessageView;

-(void)checkTextField
{
  NSLog(@"text field: %@",completeMessageView);
}

ExpandSearchPageController is set as the File's Owner for ExpandSearchPage.xib. ExpandSearchPage.xib's UITextView is wired to the completeMessageView.

When I call

ExpandSearchPageController * searchExpanderPage = [[ExpandSearchPageController alloc] initWithNibName:@"ExpandSearchPage" bundle:[NSBundle mainBundle]];

[searchExpanderPage checkTextField];

the result is

"text field: (null)"

回答1:

I guess asking the question after looking at the problem for over an hour led me to figure it out:

I just changed my code to check the text box AFTER displaying the view... now everything is instantiated.

Imagine that: the UI elements aren't instantiated until you display them!

回答2:

This is the solution.

The IBOutlets aren't ready to use until the view controller finishes loading.

For example, if you want to set a UILabels text property, you would need to set a NSString on your controller first, and then set the labels text property in somewhere like viewDidLoad.

So in your firstViewController.m : (This is for storyboards, but same idea applies)

- (void)buttonPress:(id)sender {
   [self performSegueWithIdentifier:@"mySegue" sender:self];
}

- (void)prepareForSegue(UIStoryboardSegue *)segue sender:(id)sender {
   SecondViewController *secondViewController = [segue destinationViewController];
   secondViewController.stringForLabel = @"My Label String";
}

Then in the .h of your secondViewController:

@property (strong, nonatomic) IBOutlet UILabel *label;
@property (strong, nonatomic) NSString *stringForLabel;

Then we set the text property on the UILabel, in the viewDidLoad of secondViewController.m. By this stage, the IBOutlet has been created and is ready to go.

- (void)viewDidLoad {
   [super viewDidLoad];
   self.label.text = self.stringForLabel;
}

回答3:

Another potential cause of a null IBOutlet pointer is forgetting to save the xib file in Interface Builder after creating the outlet connection.

回答4:

Make sure the view property of your view controller (ie File's Owner in this case) is wired to the view in your xib. As your textField is almost certainly a subview of that, it's important to get that wired in too (and I don't think the nib will load properly without that being set).

回答5:

This is how I do it:

//in your view controller
-(void)setUp
{
    [self.cardView layoutIfNeeded];
    LearnCardPlainText *cardNib = [[[UINib nibWithNibName:@"LearnCardPlainText" bundle:nil] instantiateWithOwner:self options:nil] objectAtIndex:0];
    cardNib.frame = self.cardView.frame;
    [cardNib setUpWithPart:learnModel];
    [self.view addSubview:cardNib];
}

来源：https://stackoverflow.com/questions/1258021/iboutlet-instances-are-null-after-loading-from-nib

标签

ios

objective-c

null

iboutlet

retain