在制作没有ViewController的应用程序时如何使用RootViewController？

Question

I am trying to make a simple app from a tutorial that does not have a viewController at all. All the code is in the AppDelegate. I am on xcode 4.2 and I am getting this error:

Applications are expected to have a root view controller at the end of application launch

I'm not sure how to deal with this. There are some blogs out there with fixes but none of them are working for me and I really would like to understand what is going on here. And, how to fix it.

I do have a view that contains some buttons and labels. But I have no "ViewController". The files contained in my project are: AppDelegate.h, AppDelegate.m, and Window.xib only. There is no ViewController.h, ViewController.m

** edit **

I ended up making the app from a 'view based application' instead and just moving all the logic from the app delegate to the view controller. So, I didn't really solve the problem per se. The app works now though. Thanks for the help

Answer 1

It's not possible to have an iOS app that doesn't have a view controller. You can always create a trivial view controller, ie,

[[UIWindow alloc] initWithFrame:UIScreen.mainScreen.bounds].rootViewController =
  [[[UIViewController alloc] init] autorelease];

Answer 2

While I agree that there may be workarounds, another question to address is: why do you want an app without a view? Even if it's designed to run in the background and present no visual interface, at least make a simple view showing the application name and version, a largeish icon and perhaps a status. This kind of idle screen uses very little system resources, especially when the app is backgrounded, but improves the overall experience of the app.

Answer 3

It sounds like you're looking at an old tutorial. UIWindow got a rooViewController property in iOS4. I believe it became required in iOS5 to help keep controller hierarchies and view hierarchies in sync with the addition of custom container controllers (and to fix a corner case where replacing the "root controller" of a UIWindow could stop orientation changes from propagating). There was a WWDC presentation in 2011 that explained this in some detail. I think it was Session 102, Implementing UIViewController Containment .

At then end of the day, there's no good reason not to have a root view controller. Apple wants to be able to assume it's there in their APIs going forward. If the tutorial you're looking at doesn't account for that, it's broken.

Answer 4

If you set your deployment target to 4.3 and run on the iPhone 4.3 simulator, you won't get the warning.

To install the iOS 4.3 simulator, go to Xcode > Preferences > Downloads.