Question

So I am trying to transition from one view controller to another with a segue programmatically so that I can only initialize it when a user needs to login. However, xcode keeps giving me the error that it is not in the view hierarchy and refuses to load to it.

ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

@end

ViewController.m

#import "ViewController.h"
#import "API.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    if (![[API sharedInstance] isAuthorized]) {
        [self performSegueWithIdentifier:@"ShowLogin" sender:nil];
    }// Do any additional setup after loading the view, typically from a nib.
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

LoginScreen.h (the controller not in the hierarchy)

#import <UIKit/UIKit.h>

@interface LoginScreen : UIViewController
{
    //the login form fields
    IBOutlet UITextField *fldUsername;
    IBOutlet UITextField *fldPassword;
}

//action for when either button is pressed
-(IBAction)btnLoginRegisterTapped:(id)sender;

@end

LoginScreen.m (the controller apparently not in the hierarchy)

#import "LoginScreen.h"
#import "UIAlertView+error.h"
#import "API.h"
#include <CommonCrypto/CommonDigest.h>

#define kSalt @"(protected value)"

@implementation LoginScreen

-(void)viewDidLoad {
    [super viewDidLoad];

}

#pragma mark - View lifecycle

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    // Return YES for supported orientations
    return (interfaceOrientation == UIInterfaceOrientationPortrait);
}

-(IBAction)btnLoginRegisterTapped:(UIButton*)sender {
    //form fields validation
    if (fldUsername.text.length < 4 || fldPassword.text.length < 4) {
        [UIAlertView error:@"Enter username and password over 4 chars each."];
        return;
    }
    //salt the password
    NSString* saltedPassword = [NSString stringWithFormat:@"%@%@", fldPassword.text, kSalt];
    //prepare the hashed storage
    NSString* hashedPassword = nil;
    unsigned char hashedPasswordData[CC_SHA1_DIGEST_LENGTH];
    //hash the pass
    NSData *data = [saltedPassword dataUsingEncoding: NSUTF8StringEncoding];
    if (CC_SHA1([data bytes], [data length], hashedPasswordData)) {
        hashedPassword = [[NSString alloc] initWithBytes:hashedPasswordData length:sizeof(hashedPasswordData) encoding:NSASCIIStringEncoding];
    } else {
        [UIAlertView error:@"Password can't be sent"];
        return;
    }
    //check whether it's a login or register
    NSString* command = (sender.tag==1)?@"register":@"login";
    NSMutableDictionary* params =[NSMutableDictionary dictionaryWithObjectsAndKeys:command, @"command", fldUsername.text, @"username", hashedPassword, @"password", nil];
    //make the call to the web API
    [[API sharedInstance] commandWithParams:params onCompletion:^(NSDictionary *json) {
        //result returned
        NSDictionary* res = [[json objectForKey:@"result"] objectAtIndex:0];
        if ([json objectForKey:@"error"]==nil && [[res objectForKey:@"IdUser"] intValue]>0) {
            [[API sharedInstance] setUser: res];
            [self.presentingViewController dismissViewControllerAnimated:YES completion:nil];
            //show message to the user
            [[[UIAlertView alloc] initWithTitle:@"Logged in" message:[NSString stringWithFormat:@"Welcome %@",[res objectForKey:@"username"]] delegate:nil cancelButtonTitle:@"Close" otherButtonTitles: nil] show];
        } else {
            //error
            [UIAlertView error:[json objectForKey:@"error"]];
        }
    }];

}

@end

API.h (calling my web service)

#import "AFHTTPClient.h"
#import "AFNetworking.h"

typedef void (^JSONResponseBlock)(NSDictionary* json);

@interface API : AFHTTPClient

@property (strong, nonatomic) NSDictionary* user;

+(API*)sharedInstance;
//check whether there's an authorized user
-(BOOL)isAuthorized;
//send an API command to the server
-(void)commandWithParams:(NSMutableDictionary*)params onCompletion:(JSONResponseBlock)completionBlock;
-(NSURL*)urlForImageWithId:(NSNumber*)IdPhoto isThumb:(BOOL)isThumb;

@end

API.m (calling my web service)

#import "API.h"

//the web location of the service
#define kAPIHost @"(protected web address)"
#define kAPIPath @""

@implementation API

@synthesize user;

#pragma mark - Singleton methods
/**
 * Singleton methods
 */
+(API*)sharedInstance {
    static API *sharedInstance = nil;
    static dispatch_once_t oncePredicate;
    dispatch_once(&oncePredicate, ^{
        sharedInstance = [[self alloc] initWithBaseURL:[NSURL URLWithString:kAPIHost]];
    });

    return sharedInstance;
}

#pragma mark - init
//intialize the API class with the deistination host name

-(API*)init {
    //call super init
    self = [super init];
    if (self != nil) {
        //initialize the object
        user = nil;
        [self registerHTTPOperationClass:[AFJSONRequestOperation class]];
        // Accept HTTP Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1
        [self setDefaultHeader:@"Accept" value:@"application/json"];
    }
    return self;
}

-(BOOL)isAuthorized {
    return [[user objectForKey:@"IdUser"] intValue]>0;
}

-(void)commandWithParams:(NSMutableDictionary*)params onCompletion:(JSONResponseBlock)completionBlock {
    NSData* uploadFile = nil;
    if ([params objectForKey:@"file"]) {
        uploadFile = (NSData*)[params objectForKey:@"file"];
        [params removeObjectForKey:@"file"];
    }

    NSMutableURLRequest *apiRequest = [self multipartFormRequestWithMethod:@"POST" path:kAPIPath parameters:params constructingBodyWithBlock: ^(id <AFMultipartFormData>formData) {
        if (uploadFile) {
            [formData appendPartWithFileData:uploadFile name:@"file" fileName:@"photo.jpg" mimeType:@"image/jpeg"];
        }
    }];
    AFJSONRequestOperation* operation = [[AFJSONRequestOperation alloc] initWithRequest: apiRequest];
    [operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
        //success!
        completionBlock(responseObject);
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        //failure :(
        completionBlock([NSDictionary dictionaryWithObject:[error localizedDescription] forKey:@"error"]);
    }];
    [operation start];
}

-(NSURL*)urlForImageWithId:(NSNumber*)IdPhoto isThumb:(BOOL)isThumb {
    NSString* urlString = [NSString stringWithFormat:@"%@/%@upload/%@%@.jpg", kAPIHost, kAPIPath, IdPhoto, (isThumb)?@"-thumb":@""];
    return [NSURL URLWithString:urlString];
}

@end

Answer 1

The issue is that the view of your initial view controller has not been added to the window by the time viewDidLoad is called.

Move that call to viewDidAppear:

-(void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    if (![[API sharedInstance] isAuthorized]) {
        [self performSegueWithIdentifier:@"ShowLogin" sender:self];
    }
}