Better way to observe in ReactiveCocoa

Question

I'm still new to ReactiveCocoa. I wanted to simply add an observer to a field, so did it like this:

[_countryPicker rac_observeKeyPath:@"value" options:nil observer:self block:^(VBCountry* value, NSDictionary* change)
{
    if ([_mobileField.textField.text length] == 0)
    {
        [_mobileField.textField setText:[NSString stringWithFormat:@"+%i", value.dialCode]];
    }
}];

With block callback, and no need to explicitly detach the observer, this is already better than old-style KVO.

However, is this a low-level method to which there's a higher level of abstraction? If so, is it OK to call this method directly? And what's the better/higher way to do it?

Answer 1

I'd advise against depending on the direct KVO methods. They're really an implementation detail.

Let's progressively re-write that with idiomatic RAC operators.

First we'll just replace the direct KVO code with RACObserve:

[RACObserve(_countryPicker, value) subscribeNext:^(VBCountry *value) {
    if ([_mobileField.textField.text length] == 0)
    {
        [_mobileField.textField setText:[NSString stringWithFormat:@"+%i", value.dialCode]];
    }
}];

Then we'll replace the if and string formatting with -filter: and -map::

[[[RACObserve(_countryPicker, value) filter:^(id _) {
    return [_mobileField.textField.text length] > 0;
}] map:^(VBCountry *value) {
    return [NSString stringWithFormat:@"+%i", value.dialCode];
}] subscribeNext:^(NSString *text) {
    [_mobileField.textField setText:text];
}];

Finally we'll use the RAC macro to make the assignment over time explicit:

RAC(_mobileField.textField, text) = [[RACObserve(_countryPicker, value) filter:^(id _) {
    return [_mobileField.textField.text length] > 0;
}] map:^(VBCountry *value) {
    return [NSString stringWithFormat:@"+%i", value.dialCode];
}];