Swift syntax: index of array element accessed in setter?

Question

Say I have an array of Strings with a didSet:

var bar: [String] = [] {
    didSet {
        println("Set to \(bar)")
    }
}

Setting some elements gives us:

bar = ["Hello", "world"] // Set to [Hello, world]
bar[0] = "Howdy" // Set to [Howdy, world]

Question: in my didSet, how do I get the index of the element which was set?

Nate Cook · Accepted Answer · 2015-02-09 14:39:42Z

1

You don't have direct access to the index of the changed element, in part because setting a new value at a particular index is only one action that will trigger the didSet handler. Any mutating method will result in a call:

bar = ["Hello", "world"]                       // Set to [Hello, world]
bar[0] = "Howdy"                               // Set to [Howdy, world]
bar.insert("cruel", atIndex: 1)                // Set to [Howdy, cruel, world]
bar.replaceRange(0..<1, with: ["So", "long"])  // Set to [So, long, cruel, world]
bar.removeRange(2..<3)                         // Set to [So, long, world]
bar.append("!")                                // Set to [So, long, world, !]
bar.removeAll()                                // Set to []

Inside the didSet handler, you do have access to a special variable named oldValue, which contains the previous value of the observed variable. If you need more than that you'd need to implement a struct or class that uses an Array for storage but provides its own true accessor methods.

answered Feb 9, 2015 at 14:39

Nate Cook

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4 Comments

Anna Dickinson Over a year ago

Ok... that makes sense. So, maybe the question is actually: if you were to override the subscript() method for the array, what would that look like? I'm lost on the syntax at that point...

Nate Cook Over a year ago

The problem there is that Array is a struct, so you can't subclass and override the subscript. (This is the part where Objective-C stalwarts begin to gnash their teeth.) You'd need to create a whole new type, with an Array as a property and the logic you want in its subscript and get/set methods.

Nate Cook Over a year ago

What's your use case? Is this an array you'll be creating and working with completely, or are you trying to monitor the changes in an array from somewhere else?

Anna Dickinson Over a year ago

It's the later -- the array is a property, and I want to know when it changes. (Yes, there are other ways to implement that functionality, but I wanted to know if there was some clever way of doing it in this spiffy new language!). I wonder if it's possible to implement this an extension to Array?

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