In a pet project I have started working on, I have decided to use ReactiveCocoa (RAC) together with the pattern Model View ViewModel (MVVM).
If you are not familiar with RAC or MVVM for that matter, I can recommend Ash Furrow’s book Functional Reactive Programming on iOS which is a great book on both topics.

In my app, the user can create an entity and add a description text with a maximum length of 256 characters. The number of characters remaining is displayed to the user with a label.
This task can fairly easy be done without using RAC like the following.

#pragma mark - UITextViewDelegate

- (void)textViewDidChange:(UITextView *)textView {
    _charactersRemainingLabel.text = [@(EnityDescriptionMaxLength - (NSInteger)textView.text.length) stringValue];
}

But this is not much functional or reactive and since I am trying to learn and think functional and reactive I needed to find another way around this.
My first approach was using rac_signalForSelector:fromProtocol: to wrap the UITextViewDelegate method textViewDidChange: like Ash Furrow gave an example of in his book.

[[self rac_signalForSelector:@selector(textViewDidChange:) fromProtocol:@protocol(UITextViewDelegate)] subscribeNext:^(RACTuple *arguments) {
    UITextView *textView = arguments.first;
    _charactersRemainingLabel.text = [@(EnityDescriptionMaxLength - (NSInteger)textView.text.length) stringValue];
}];

So each time the UITextView calls textViewDidChange: on its delegate the signal fires and I get notified in the subscribeNext block.
This certainly looks better and the code can be place near the _charactersRemainingLabel.

Then the other day, I stumbled upon RACChannel which made me think I could do better.
After a little reserach (#1069) I ended up with the following code.

RACChannelTerminal *characterRemainingTerminal = RACChannelTo(_charactersRemainingLabel, text);
[[_textView.rac_textSignal map:^id(NSString *text) {
    return [@(EnityDescriptionMaxLength - (NSInteger)text.length) stringValue];
}] subscribe:characterRemainingTerminal];

A RACChannelTerminal is created for _charactersRemainingLabel and the property text, which is subscribed to the rac_textSignal signal on UITextView. So each time the text changes the rac_textSignal fires and the value is first mapped and then passed to the channel terminal.

RACChannelTo in action

I find the final form kinda beautiful :)

Catch me on Twitter @mbogh if you have any comments or want to discuss.