What does it mean? Let’s look at the view hierarchy

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JSView
 |---- titleLabel
 |---- ...

In this case, I have a viewController that contain a property UILabel titleLabel. I want to perform some action upon the text changed.

i.e.

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jsView.titleLabel.text = @"new text";

Now, add an event listener (in iOS called observer) to it’s text changed

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[self addObserver:self forKeyPath:@"_titleLabel.text" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];

you can add the line above to init method

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- (void)dealloc
{
    [self removeObserver:self forKeyPath:@"_titleLabel.text"];
}

Don’t forget to remove it after the main view deallocated, otherwise the app will crash.

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#pragma mark - observer
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if ([keyPath isEqualToString:@"_titleLabel.text"])
    {
        NSLog(@"the title text has been changed");
    }
}

Once the text changed, it will output the console

Update Jan 6, 2015

The more proper way would be

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[_titleLabel addObserver:self forKeyPath:@"text" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
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- (void)dealloc
{
    [_titleLabel removeObserver:self forKeyPath:@"text"];
}
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#pragma mark - observer
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (object == _titleLabel) {
        if ([keyPath isEqualToString:@"text"]) {
            NSLog(@"the title text has been changed");
        }
    }
}

References: