then.m

self.currentlyLoggingIn = NO;
self.didLoginSubject    = [RACSubject subject];
self.loginCommand       = [RACCommand commandWithCanExecuteSignal:[RACSignal
                                                                   combineLatest:@[self.usernameTextField.rac_textSignal,
                                                                                   self.passwordTextField.rac_textSignal,
                                                                                   RACObserve(self, currentlyLoggingIn)]
                                                                   reduce:^(NSString *username, NSString *password, NSNumber *currentlyLoggingIn) {
                                                                       return @(username.length > 0 && password.length > 0 && NO == currentlyLoggingIn.boolValue );
                                                                   }]];

@weakify(self);

// Every time loginCommand is triggered…
RACSignal *loginResult = [[self.loginCommand then:^{
    @strongify(self);

    // Try logging in, and return the result.
//
// -materialize means that we wrap each next value or error in
// a RACEvent. This means that even if the API hits an error, the
// loginResult signal will still be valid.
    return [[[TCBCADNController sharedInstance] loginAccountWithUserName:self.usernameTextField.stringValue password:self.passwordTextField.stringValue] materialize];
}] replayLazily];

Made a fork to show how to replace this with KVO blocks instead.