Home » Swift » Swift: how to change a property's value without calling its didSet function

Swift: how to change a property's value without calling its didSet function

Posted by: admin November 30, 2017 Leave a comment

Questions:

How can you set a property’s value in Swift, without calling its didSet() function outside of an initialization context? The code below was a failed experiment to achieve this within the classes’ noside() function

class Test
{
    var toggle : Bool = 0
    var hoodwink : Int = 0 {
        didSet(hoodwink)
        {
            toggle = !toggle
        }
    }

// failed attempt to set without a side effect

    func noside(newValue : Int)
    {
        hoodwink = newValue
        println("hoodwink: \(hoodwink) state: \(toggle)")
    }

    func withside(newValue : Int)
    {
        self.hoodwink = newValue
        println("hoodwink: \(hoodwink) state: \(toggle)")
    }
}

It is quite trivial to do in Objective-C with auto-synthesized properties:

With side effect (if present in setter):

self.hoodwink = newValue;

Without side effect:

_hoodwink = newValue;
Answers:

What you do in Objective-C to “avoid side effects” is accessing the backing store of the property – its instance variable, which is prefixed with underscore by default (you can change this using the @synthesize directive).

However, it looks like Swift language designers took specific care to make it impossible to access the backing variables for properties: according to the book,

If you have experience with Objective-C, you may know that it provides two ways to store values and references as part of a class instance. In addition to properties, you can use instance variables as a backing store for the values stored in a property.

Swift unifies these concepts into a single property declaration. A Swift property does not have a corresponding instance variable, and the backing store for a property is not accessed directly. (emphasis is mine)

Of course this applies only to using the “regular language” means, as opposed to using reflection: it might provide a way around this restriction, at the expense of readability.

Questions:
Answers:

A possible hack around this is to provide a setter which bypasses your didSet

 var dontTriggerObservers:Bool = false
    var selectedIndexPath:NSIndexPath? {
        didSet {
            if(dontTriggerObservers == false){
                //blah blah things to do
            }
        }
    }
    var primitiveSetSelectedIndexPath:NSIndexPath? {
        didSet(indexPath) {
            dontTriggerObservers = true
            selectedIndexPath = indexPath
            dontTriggerObservers = false
        }
    }

Ugly but workable

Questions:
Answers:

If you exactly know when you want to apply side effects just make it explicitly:

1 Solution:

func noside(newValue : Int) {
    hoodwink = newValue
}

func withside(newValue : Int) {
    self.hoodwink = newValue
    toggle = !toggle
}

2 Solution:

var toggle : Bool = false
var hoodwink : Int = 0 
var hoodwinkToggle: Int {
    get { return hoodwink }
    set(newValue) {
        hoodwink = newValue
        toggle = !toggle
    }
}
  1. func setHoodwinkWithToggle(hoodwink: Int) {…}
  2. ….

I think these solutions will be more clear and readable, then using one variable which at some calls should have side effects and shouldn’t at others.