Extract instance variable

@implementation LoginViewController {
}
- (void)performLogin {
     // ...

    [self.loginModel loginWithEmail:email password:password completion:^(BOOL success) {
        // success will be extracted to
        // an instance variable
        if (success) {
            [self showAlertLoginSuccess];
        } else {
            [self showAlertLoginFailed];
        }
    }];
}

@implementation LoginViewController {
    // Extracted instance variable
    BOOL _loginSuccessful;
}
- (void)performLogin {
     // ...

    [self.loginModel loginWithEmail:email password:password completion:^(BOOL success) {
        _loginSuccessful = success;
        if (_loginSuccessful) {
            [self showAlertLoginSuccess];
        } else {
            [self showAlertLoginFailed];
        }
    }];
}