Swift：具有全局变量的属性观察器

Question

I have two files , and in one of them, a variable is set ' on button click '.

class Home: UIViewController {

    var swipeNumber = 99

    @IBAction func pressedThreeSwipes(sender: AnyObject) {
        swipeNumber = 3
    }
}

Then in a second file ( which acts as a second view ), I have a didSet property observer that is checking for a change in the variable swipeNumber .

func didMoveToView(view: UIView) {
    /* Setup your scene here */

    var menu = Home()

    var swipeNumber: Int = 0 {
        didSet {
            println("\(menu.swipeNumber) should be three.")
        }
    }
}

However, when you run the program , the println line does not operate , suggesting that the variable swipeNumber hasn't been changed .

So I thought that this would be a problem with the = 0 in the property observer, but if I remove that, I receive an error about having no initialisers.

However, after double checking , I realised what (I think) the problem is. As the didSet code doesn't actually 'do anything', I knew that it was not recognising a change in the variable . I think this is because the button is clicked and the variable 'changed' before the view loads , ie the variable has to be changed with the second view is open . I think the problem is where I am putting the property observer . Perhaps I should put it in AppDelegate or somewhere else, and link that to my second view?

Any advice you could give me would be greatly appreciated.

Thank you in advance.

EDIT 1: Following @utahwithak's advice, I put a property observer inside the AppDelegate file:

class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    var swipeNumber: Int = 0 {
        didSet {
            println("\(swipeNumber)")
        }
    }
}

I also changed menu to be appDelegate . Now I get the message '3' when I press pressedThreeSwipes . However, when I go to the next view and play, a different message tells me that swipeNumber = 0. I think this is to do with the first line of the property observer in my AppDelegate file . Thank you.

Answer 1

In the first file it is setting the local member variable that has no didSet This is a different swipeNumber than in your second file and setting that will not fire the didSet in the second file.

In file 2 is it a copy paste error or are you declaring menu and swipeNumber inside your function? If they are in the function that is very likely the wrong spot, they should be at the same level of your function for a class member or outside the class declaration for a global variable.

I'm not sure I understand what you're trying to accomplish but if you want file 2 to be aware of file 1 information there are many ways without using a global variable. Such as a delegate that file 1 calls on a didSet or posting an NSNotification through NSNotificationCenter

Edit for Delegate example

in file 1 you could have something like this outside your class declaration

protocol SwipeNumberDelegate{
    func swipeNumberChanged(newVal:Int)
}

then inside your file 1 class would have something like this

class Home: UIViewController {
    var swipeDelegate:SwipeNumberDelegate? = nil
    var swipeNumber = 99{
         didSet{
            if let delegate = self.swipeDelegate{
                delegate.swipeNumberChanged(self.swipeNumber)
            }
         }
    }
 ...
}

somehow from file 1 or file 2 you have to then set swipeDelegate

and in File 2 in your class declaration you'll have to say it conforms to the protocol by making it look something like:

class mySecondClass:SuperClass, SwipeNumberDelegate

and implement the protocol

func swipeNumberchanged(newVal:Int){
    println("Number Changed:\(newVal)")
}