How to initialize a variable when IBOutlet is initialized?

Question

It happened for me to write following code snippet in one of my UIViewControllers in my new iOS Swift Application.

var starButtonsCount = starButtons.count
@IBOutlet var starButtons: [UIButton]!

Then straight next to the starButtonsCount variable declaration following error appeared in red.

Error: Cannot use instance member ‘starButtons’ within property initializer; property initializers run before ‘self’ is available.

So I found out that by declaring the starButtonCount variable as lazy we can resolve this error (temporary in the long run of the iOS App development process).

I'm curious to find out what are the other methods to solve this?

Is there a way to trigger the initialization for starButtonCount when the starButtons IBOutlets get initialized?

I thought so while I was searching on the Internet. But couldn't find a specific answer right to the point! Could you elaborate further in an answer? — Randika Vishman, Dec 16 '18 at 16:00
What is the purpose not to declare a constant? The buttons are connected at design/compile time so you are supposed to know the number of buttons. — vadian, Dec 16 '18 at 16:20
@vadian even though I had declared the `starButtonsCount` as a constant, still the same error appears. So the focus should be how the `BOutlets` get initialized and how to catch that and initialize my variable. — Randika Vishman, Dec 16 '18 at 16:24
The same error cannot occur if `starButtonsCount` is declared as `let` with a constant `Int` — vadian, Dec 16 '18 at 16:30

Shehata Gamal · Answer 1 · 2018-12-16T16:15:01.990

3

Another way

var starButtonsCount:Int! 
@IBOutlet var starButtons: [UIButton]! { 
    didSet { 
         starButtonsCount = starButtons.count
    }
}

edited Dec 16 '18 at 16:15

answered Dec 16 '18 at 15:59

Shehata Gamal

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Works well in that case, but this doesn't let you control multiple/complex settings of IBOutlets... – Jean-Baptiste Yunès Dec 16 '18 at 16:01
1

@Jean-BaptisteYunès don't get what you mean by multiple complex IBS , but handling in awakeFromNib has same effect as viewDidLoad ? the op is asking for a new way on the fly instead of the usual way – Shehata Gamal Dec 16 '18 at 16:05
Personally I don't like the `propertyObserver` way since it lengthens the number of code lines within a particular `UIViewController` class and it can be not that much readable too! – Randika Vishman Dec 16 '18 at 16:06
@Sh_Khan Basically you can't control order of outlet initializations, then in case of complex dependancies it is not manageable with didSet – Jean-Baptiste Yunès Dec 16 '18 at 16:10
And why i care with that if eventually the var will be set correctly plus will get an easy observer – Shehata Gamal Dec 16 '18 at 16:12
@Sh_Khan I did not mean that your answer is wrong, just that it can be problematic in more general case. A better answer would have then been to implement it as a `get` to have the right value every time... – Jean-Baptiste Yunès Dec 16 '18 at 16:15
can be `problematic` how ? – Shehata Gamal Dec 16 '18 at 16:16
@Sh_Khan If a value depend on another, order is not guaranteed! – Jean-Baptiste Yunès Dec 16 '18 at 16:18

Jean-Baptiste Yunès · Accepted Answer · 2018-12-16T16:08:48.160

3

awakeFromNib is a method called when every object in a .xib have been deserialized and the graph of all outlets connected. Documentation says:

Declaration

func awakeFromNib()

Discussion

message to each object recreated from a nib archive, but only after all the objects in the archive have been loaded and initialized. When an object receives an awakeFromNib message, it is guaranteed to have all its outlet and action connections already established.The nib-loading infrastructure sends an awakeFromNib

You must call the super implementation of awakeFromNib to give parent classes the opportunity to perform any additional initialization they require. Although the default implementation of this method does nothing, many UIKit classes provide non-empty implementations. You may call the super implementation at any point during your own awakeFromNib method.

As in:

class MyUIViewController : UIViewController {
    var starButtonsCount = 0
    @IBOutlet var starButtons: [UIButton]!
    override func awakeFromNib() {
        super.awakeFromNib()
        starButtonsCount = startButtons.count
    }
}

edited Dec 16 '18 at 16:08

answered Dec 16 '18 at 16:04

Jean-Baptiste Yunès

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Great, It would be nicer if you could provide the answer with a code example too! :-) – Randika Vishman Dec 16 '18 at 16:05
@RandikaVishman this code can be set in `viewDidLoad` also so there is no new thing to learn here – Shehata Gamal Dec 16 '18 at 16:14
1

@sh_Khan I think that's bit incorrect since more than `viewDidLoad` method, `awakeFromNib` deals directly with connections and initialization made related to the given `Nib` file (or the `StoryBoard` in this case). Please read this answer's document excerpt! – Randika Vishman Dec 16 '18 at 16:17
2

@Sh_Khan Of course there is many places where to put such a code, but OP exactly wanted to know how to capture the fact that outlets have been correctly set, and that is exactly the purpose of `awakeFromNib`, `viewDidLoad` is another thing. – Jean-Baptiste Yunès Dec 16 '18 at 16:17

score 2 · Answer 3 · answered Dec 16 '18 at 16:16

Also note there is no need for starButtonsCount to be a stored variable:

var starButtonsCount: Int {
   return starButtons.count
}

In this case it would be probably better to use starButtons.count directly since the variable does not improve the code in no way.

In a general case, there is no need to store derived state unless storing it provides some performance boost (e.g. caching calculations).

score 1 · Answer 4 · answered Dec 16 '18 at 16:29

1

Simple solution is a constant

let starButtonsCount = 8 // or whatever the number of buttons is

IBOutlets are connected at build time so the number of buttons won't change at runtime.

You could add an assert line in viewDidLoad to get informed if the number of buttons changed in a future version of the app

assert(starButtons.count == starButtonsCount, "number of buttons has changed, adjust starButtonsCount")

answered Dec 16 '18 at 16:29

vadian

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Sorry, the information was insufficient for you it seems! Actually the number of buttons may change over the time if I decide to add more buttons. – Randika Vishman Dec 16 '18 at 16:30
Also, by adding an `assert` line, isn't it make the program more complex even though it might secure the app from the options to fail? (thinking) – Randika Vishman Dec 16 '18 at 16:32
1

In this case you get a fatal error in the `assert` line to change the constant value. – vadian Dec 16 '18 at 16:32
But thanks for the effort! This is also a simplification for some programs. – Randika Vishman Dec 16 '18 at 16:33
`Asserts` are only considered in `debug` scheme. Any runtime check or calculation of a value which will never change is unnecessary. – vadian Dec 16 '18 at 16:35

How to initialize a variable when IBOutlet is initialized?

4 Answers4