Extract variable

private func performLogin() {
    // email: self.textFieldEmail.text ?? will be extracted
    // to the email variable
    let isEmailValid = self.emailValidator.isEmailValid(email: self.textFieldEmail.text ?? "")
    if (isEmailValid == true) {
        self.loginModel.login(email: self.textFieldEmail.text ?? "")
    }
}

private func performLogin() {
    // Extracted variable
    let email = self.textFieldEmail.text ?? ""
    let isEmailValid = self.emailValidator.isEmailValid(email: email)
    if (isEmailValid == true) {
        self.loginModel.login(email: email)
    }
}

- (void)performLogin {
    // self.textFieldEmail.text will be extracted
    // to the email variable
    BOOL isEmailValid = [self.emailValidator isEmailValid:self.textFieldEmail.text];
    if (isEmailValid == NO) {
        [self.loginModel loginWithEmail:self.textFieldEmail.text];
    }
}

- (void)performLogin {
    // Extracted variable
    NSString *email = self.textFieldEmail.text;
    BOOL isEmailValid = [self.emailValidator isEmailValid:email];
    if (isEmailValid == NO) {
        [self.loginModel loginWithEmail:email];
    }
}