Extract property

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

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

@interface LoginViewController ()
// ...
// Extracted property
@property(nonatomic) BOOL loginSuccessful;
@end
@implementation LoginViewController {
}
- (void)performLogin {
     // ...

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