swift 3 中的单例计算属性

Question

I am trying to achieve a singleton UIView instance in my code, so whenever I create an instance of 'MyView' the nib file will load for only once and then reuse it again:

class MyView : UIView {

    @IBOutlet weak var someLabel: UILabel!
    
    static var customeView : UIView = {

        let view = Bundle.main.loadNibNamed(String(describing: MyView.self), owner: self, options: nil)?[0] as! UIView
        return view
    }()
    
    convenience init() {
        
        self.init(frame: CGRect(x: 0, y: 0, width: 576, height: 30))
        
        let viewForOnce = MyView.customeView
        viewForOnce.frame = bounds
    }
} 

The problem is that whenever I call the MyView.customeView it get crashed saying that 'this class is not key value coding-compliant', I think this is happening because of the 'owner: self' inside the computed property.

Any help will be appreciated.

Answer 1

The issue is most probably with your ReceiptView.xib . You should review it first. Look for an IBOutlet , which is there in a xib, but not in your ReceiptView class and delete it.

Edit:

I meant with the singleton is loading the nib only once whenever I create and instance of that class.

1. No, this is not going to work like that. Every time init is called, your computed property is also called and a new instance is created from xib every time.

2. As Carien van Zyl already mentioned, you are using self in a class var which corresponds to MyView class itself (or it's subclass if its called for a subclass), not an instance . Try passing nil as owner instead.

3. The whole technique is looking wrong to me. You should not use singleton pattern with UIView subclasses. There is nothing wrong in calling loadNibNamed multiple times and create exactly the same instances. If you want to use the same instance multiple times in a view hierarchy, it's not possible since every view can have only one superview. In this case you should follow MVC pattern: create multiple MyView instances -> update model whenever you change something in a view and want those changes to be reflected elsewhere -> update another view using updated model.

Answer 2

customeView is a type property. Therefor, self inside of it, will reference MyView.self which is a class type. The class type do not hold the instance variables, which includes someLabel .

See Apple's documentation on Types