如何在不崩溃的情况下同时引用其他类的实例？

Question

I have a simple question that I have trouble finding the answer two. The code below crash. From debug it seems to come from a loop, I think it is because I reference the other class in both classes. Can someone please explain what is going on and if possible show how I can have both classes talk to each other (both ways)? Any help would be very much appreciated ! Thank you.

View Controller One

import UIKit

class ViewControllerOne: UIViewController{

    let doSomething = DoSomething()
    var one = "test"

    @IBAction func buttonTapped(sender: AnyObject) {
        doSomething.functionTwo()
   }
}

Do Something This Crash

import UIKit

class DoSomething: NSObject {

    let viewControllerOne = ViewControllerOne()

    func functionTwo() -> String{
        var two = viewControllerOne.one
    return two
    }
}

Do Something This works

import UIKit

class DoSomething: NSObject {


    func functionTwo() -> String{
        let viewControllerOne = ViewControllerOne()

        var two = viewControllerOne.one
    return two
    }
}

Answer 1

In the first instance the initializer of DoSomething has to create a ViewControllerOne . Since the initializer of ViewControllerOne creates DoSomething they loop infinitely. In the second instance DoSomething doesn't create until functionTwo() is called, so there is no loop. You can have them talk to each other for example this way:

import UIKit

class ViewControllerOne : UIViewController 
{        
    let doSomething = DoSomething()
    var one = "test"

    override func viewDidLoad()
    {
        super.viewDidLoad()
        doSomething.viewControllerOne = self      
    }

    @IBAction func buttonTapped(sender: AnyObject)
    {
        doSomething.functionTwo()
    }
}

class DoSomething: NSObject
{    
    weak var viewControllerOne : ViewControllerOne?

    func functionTwo() -> String
    {
        return viewControllerOne?.one ?? String()
    }
} 

--

So this is how it works in steps:

1. ViewControllerOne is created, the default initializer is called. It calls the initializers for doSomething = DoSomething() and var one = "test" (this is equal to String("test") ).
2. The DoSomething default initializer sets the optional viewControllerOne to nil
3. When the instance of ViewControllerOne is finished loading the viewDidLoad is called. It sets the weak doSomething.viewControllerOne property to itself. This means that:
   • the reference count (used for ARC) is not incremented, it is important because if you ever decided to remove the instance of ViewControllerOne, and the doSomething would hold a strong reference to it, they would never get removed by ARC and you would loose memory
   • the property is referencing the view instance that had the viewDidLoad called - self in Swift is similar to this in other languages
4. When the functionTwo is called, it either returns the value of one or if the viewControllerOne is nil it returns an empty String

Answer 2

Classes never talk. They are blueprints.

You mean instances of a class that need to communicate. Two ways: either each instance has a reference to the other instance or instances send messages to a third object that act as postmaster. See NSNotification for details how to communicate within apps without knowing the other objects by reference.