Detect Change in UILabel Text

问题

Is it possible to set a Notification for when a UILabel's text property is changed? I tried the one used for UITextFields when I couldn't find one for a UILabel, but it didn't work.

 [[NSNotificationCenter defaultCenter]
 addObserver:self
 selector:@selector(posttosocial)
 name:UITextFieldTextDidChangeNotification
 object:nowplaying];

回答1:

You can use key-value observing (KVO):

[label addObserver:self
        forKeyPath:@"text"
           options:NSKeyValueObservingOptionNew
                 | NSKeyValueObservingOptionOld
           context:NULL];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if ([keyPath isEqualToString:@"text"]) {
        /* etc. */
    }
}

回答2:

For Swift 3.2+

let observation = label.observe(\UILabel.text, options: [.new, .old]) { label, change in
        print("\(change.newValue as? String ?? "" )")
    }

Call observation.invalidate() when done observing.

来源：https://stackoverflow.com/questions/14326566/detect-change-in-uilabel-text