title: Enable/disable UIButton according to number of characters in UITextField in ReactiveCocoa 4
body: <p>I want to set the <code>enabled</code> property of a <code>UIButton</code> to <code>true</code> iff there is at least one character in a <code>UITextField</code>. How can this be achieved in <a href="https://github.com/ReactiveCocoa/ReactiveCocoa" rel="nofollow">Reactive Cocoa 4</a>? </p>