How to present a splash/login view controller using storyboards

I've seen this question in various forms without a clear answer. I'm going to ask and answer here. My app needs to do work on startup... inits, a few network calls, login, etc. I don't want my main view controller working until that's done. What's a good pattern for doing this?

Requirements:

• iOS5+, storyboards, ARC.
• the main vc's view cannot appear until the startup work is done.
• want a choice of transition styles to the main vc when the startup work is done.
• want to do as much layout as possible in storyboard.
• the code should be cleanly contained somewhere obvious.

EDIT, July 2017 Since the first writing, I've changed my practice to one where I give the start up tasks to their own view controller. In that VC, I check startup conditions, present a "busy" UI if needed, etc. Based on the state at startup, I set the window's root VC.

In addition to solving the OP problem, this approach has the additional benefits of giving better control of the startup UI and UI transitions. Here's how to do it:

In your main storyboard, add a new VC, called LaunchViewController and make it the app's initial vc. Give your app's "real" initial vc an identifier like "AppUI" (identifiers are on the Identity tab in IB).

Identify other vcs that are the starts of main UI flows (e.g. Signup/Login, Tutorial, and so on) and give these descriptive identifiers, too. (Some prefer to keep each flow in it's own storyboard. That's a good practice, IMO).

One other nice optional idea: give your app's launch storyboard's vc an identifier, too (like "LaunchVC"), so that you can grab it and use it's view during startup. This will provide a seamless experience for the user during launch and while you do your startup tasks.

Here's what my LaunchViewController looks like....

@implementation LaunchViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    // optional, but I really like this:
    // cover my view with my launch screen's view for a seamless start
    UIStoryboard *storyboard = [self.class storyboardWithKey:@"UILaunchStoryboardName"];
    UINavigationController *vc = [storyboard instantiateViewControllerWithIdentifier:@"LaunchVC"];
    [self.view addSubview:vc.view];
}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    [self hideBusyUI];
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    [self showBusyUI];

    // start your startup logic here:
    // let's say you need to do a network transaction... 
    //
    [someNetworkCallingObject doSomeNetworkCallCompletion:^(id result, NSError *error) {
        if (/* some condition */) [self.class presentUI:@"AppUI"];
        else if (/* some condition */) [self.class presentUI:@"LoginUI"];
        // etc.
    }];
}

#pragma mark - Busy UI

// optional, but maybe you want a spinner or something while getting started
- (void)showBusyUI {
    // in one app, I add a spinner on my launch storyboard vc
    // give it a tag, and give the logo image a tag, too
    // here in animation, I fade out the logo and fade in a spinner
    UIImageView *logo = (UIImageView *)[self.view viewWithTag:32];
    UIActivityIndicatorView *aiv = (UIActivityIndicatorView *)[self.view viewWithTag:33];
    [UIView animateWithDuration:0.5 animations:^{
        logo.alpha = 0.0;
        aiv.alpha = 1.0;
    }];
}

- (void)hideBusyUI {
    // an animation that reverses the showBusyUI
}

#pragma mark - Present UI

+ (void)presentUI:(NSString *)identifier {
    UIStoryboard *storyboard = [self storyboardWithKey:@"UIMainStoryboardFile"];
    UINavigationController *vc = [storyboard instantiateViewControllerWithIdentifier:identifier];

    UIWindow *window = [UIApplication sharedApplication].delegate.window;
    window.rootViewController = vc;

    // another bonus of this approach: any VC transition you like to
    // any of the app's main flows
    [UIView transitionWithView:window
                      duration:0.3
                       options:UIViewAnimationOptionTransitionCrossDissolve
                    animations:nil
                    completion:nil];
}

+ (UIStoryboard *)storyboardWithKey:(NSString *)key {
    NSBundle *bundle = [NSBundle mainBundle];
    NSString *storyboardName = [bundle objectForInfoDictionaryKey:key];
    return [UIStoryboard storyboardWithName:storyboardName bundle:bundle];
}

@end

The original answer below, though I prefer my current approach

Let's express the app's readiness to run the main vc with a boolean, something like:

BOOL readyToRun = startupWorkIsDone && userIsLoggedIn;

1. Create an AppStartupViewController and lay it out in app storyboard.
2. Don't drag any segue's to it, and don't make it the staring vc, just leave it floating somewhere.
3. In the vc's attributes inspector in storyboard, set it's identifier as "AppStartupViewController".

In the AppStartupViewController.m, when the readyToRun conditions have been met, it can dismiss itself:

self.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;   // your choice here from UIModalTransitionStyle
[self dismissViewControllerAnimated:YES completion:nil];

Now, whenever the app becomes active, it can check for readiness to run, and present the AppStartupViewController if it's needed. In AppDelegate.h

- (void)applicationDidBecomeActive:(UIApplication *)application {

    BOOL readyToRun = startupWorkIsDone && userIsLoggedIn;

    if (!readyToRun) {
        UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];
        AppStartupViewController *startupVC = [storyboard instantiateViewControllerWithIdentifier:@"AppStartupViewController"];

        [self.window.rootViewController presentViewController:startupVC animated:NO completion:nil];
        // animate = NO because we don't want to see the mainVC's view
    }
}

That's mostly the answer, but there is one hitch. Unfortunately the main vc gets loaded (that's okay) and gets a viewWillAppear: message (not okay) before the AppStartupViewController is presented. It means we have to spread a little extra startup logic, like this, in MainViewController.m:

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];

    if (readyToRun) {
        // the view will appear stuff i would have done unconditionally before
    }
}

I hope this is helpful.