Reactive Cocoa Validation - swift

Question

How can I validate a form after a RAC Command or a button click? I have this somewhere in my View Model
self.executeLoginRacCommand = RACCommand(enabled: activateButtonSignal, signalBlock: { (any : AnyObject!) -> RACSignal! in
return self.executeLoginAPI()
})
//My question is here, how can I display an alert to my view controller saying may email is invalid.
//MARK: - Execute Login API
private func executeLoginAPI() -> RACSignal {
if self.isEmailValid(self.email) {
return self.signInServices.signInAPIService.signInAPISignal(self.email, password: self.password)
} else {
self.errorMessage = "Please insert a valid email"
return RACSignal.empty()
}
}
//MARK: - Is Email Valid
func isEmailValid(email: String) -> Bool {
let emailRegEx = "^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+#[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$"
let emailTest = NSPredicate(format:"SELF MATCHES %#", emailRegEx)
return emailTest.evaluateWithObject(email)
}
I access it in my View Controller like this:
self.signInButton.rac_command = self.signInViewModel.executeLoginRacCommand
Can you please suggest a good way of validating an invalid email and display alert after a button click? Thanks in advance!

Answer 1

In your else branch, you are returning an empty RACSignal.
Instead, return the a RACSignal that sends the error (RACSignal.error(<the error>)).
In your ViewController, you can subscribe to the errors property of self.executeLoginRacCommand and display the error to the user.