Swift/iOS: IBOutlet nil after loading view control

问题:

I'm building an app (in XCode 8.2.1) where some objects are displayed on a 2D board, and when the user taps one of these objects some info should be displayed about it as a styled modal info box. My design is to have the info written in a separate view controller, which I would display when needed.

I've designed a basic stub for the second view controller and added a single label to it in the interface builder. Then I've ctrl-linked this label to my custom VC class:

class InfoViewController: UIViewController {
    @IBOutlet weak var info: UILabel!
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    func displayInfo() {
        info.attributedText = NSAttributedString(string: "abc")
    }
}

However, when I test my app and tap the object, the info field is nil even in the viewDidLoad() method of my custom VC class. The way I'm displaying my VC is as follows:

let infoViewController = InfoViewController()
infoViewController.modalPresentationStyle = .overCurrentContext
self.present(infoViewController, animated: true, completion: nil)
infoViewController.displayInfo()

(Note: In the end I will have only one single instance of InfoViewController but this is just for testing. I don't expect having a global instance would make any difference?)

As I said, be it inside the viewDidLoad() method or in the displayInfo() method, info is always nil, such that setting its attributedString attribute crashes the app. Thinking the present method might be called asynchronously, I've tried calling displayInfo() from inside viewDidLoad(), but that didn't make any difference.

Can anyone tell my what I've forgotten that would allow my IBOutlet from being properly initialized properly?

Thanks!

David

回答1:

The problem is the reference to InfoViewController(), which instantiates the view controller independent of any storyboard scene. You want to use instantiateViewController:

let infoViewController = storyboard?.instantiateViewController(withIdentifier: "Info") as! InfoViewController
infoViewController.modalPresentationStyle = .overCurrentContext
present(infoViewController, animated: true) {
    infoViewController.displayInfo()
}

A couple of notes:

• This assumes that (a) you've given the scene in the storyboard a "storyboard id"; (b) you've set the base class for that scene to InfoViewController.

• Note, I called displayInfo in the completion handler of present because you probably don't want that called until the scene has been presented and the outlets have been hooked up.

---

Alternatively, you can update non-outlet properties of the InfoViewController immediately after instantiating it and then have its viewDidLoad take those properties and update the outlets, e.g.:

class InfoViewController: UIViewController {
    var info: String!
    @IBOutlet weak var infoLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()
        infoLabel.attributedText = NSAttributedString(string: info)
    }
}

Note, I changed the @IBOutlet name to be infoLabel and added the String property called info. That tends to be the convention, that outlets bear some suffix indicating the type of control, and model objects, like the String property, are without the suffix. (You'll just want to make sure you remove that old outlet in the connections inspector in IB so that you don't have problems with these property name changes.)

Anyway, you can then do:

let infoViewController = storyboard?.instantiateViewController(withIdentifier: "Info") as! InfoViewController
infoViewController.info = "abc"
infoViewController.modalPresentationStyle = .overCurrentContext
present(infoViewController, animated: true, completion: nil)

The key point is don't try to update outlets of the scene immediately after instantiating it, but make sure that this is deferred until after viewDidLoad was called.

回答2:

I Replaced

let vc = CCDetailViewController()

With

let vc = self.storyboard?.instantiateViewController(withIdentifier: "CCDetailViewController")

Finally

self.present(vc!, animated: true, completion: nil)

Now It Works...

回答3:

In my case, I had created new view controller for the same class. Which had ended up with two view controllers in storyboard, but referring to the same class. After deleting old view controller, everything worked fine.

Hope it helps to someone.