为什么不初始化UILabel? [英] Why UILabel is not initialized?

问题描述

代码来自斯坦福CS193p.我添加了一个NSLog进行检查.标签似乎未初始化.有什么主意吗?

The code is from Stanford CS193p. I added a NSLog to check it out. The label seems not being initialized. Any idea?

@interface AskerViewController() <UITextFieldDelegate>
@property (weak, nonatomic) IBOutlet UILabel *questionLabel;
@property (weak, nonatomic) NSString *question;
@end


@implementation AskerViewController
@synthesize questionLabel = _questionLabel;
@synthesize question = _question;


 - (void)setQuestion:(NSString *)question
{
    _question = question;
    self.questionLabel.text = question;
    NSLog(@"label is %@", self.questionLabel);
}

@end

NSLog结果为:

2012-07-31 01:56:45.177 Kitchen Sink[23931:f803] label is (null)

推荐答案

您可能在显示视图控制器之前设置了问题字符串属性，大概是在prepareForSegue:中?此时，视图尚未加载，label属性仍为nil.

You are probably setting the question string property before you are displaying the view controller, presumably in prepareForSegue:? At this point, the view has not loaded and the label property will still be nil.

通过使用单独的字符串属性，您正在做正确的事情，您只是错过了必须同时在viewDidLoad中设置标签文本的步骤-此时，您的标签已从情节提要中实例化并可以使用.

You're doing the right thing by having a separate string property, you're just missing the step that you must also set the label's text in viewDidLoad - at this point your label has been instantiated from the storyboard and is ready to use.

如果要在调用viewDidLoad之前设置属性，则标签应该为nil.而且，如果您要从prepareForSegue设置属性，则该视图尚未加载.在需要将其显示在屏幕上之前，视图控制器不会加载其视图和子视图，并且只有在执行segue时才会发生这种情况-您可以猜测，prepareForSegue在segue执行之前就完成了.

If you're setting properties before viewDidLoad is called, the label being nil is expected. And if you're setting properties from prepareForSegue, the view won't have been loaded yet. A view controller won't load it's view and subviews until it needs to display them on screen, and this doesn't happen until the segue is being performed - and as you can guess, prepareForSegue is done before the segue is performed.

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