How to execute code in init block after property y is initialized

Question

I would like to call some code inside my class init block, after a property y is initialized. I've tried it with this::y.isInitialized ..., but the problem here is that at the creation of the class, the property is not initialized (which it gets during runtime).

ExampleClass

class Example {
     lateinit var y: String // this gets initialized after the creation of the fragment at a later time

     init {
         if(this::y.isInitialized) {
            doStuff()
         }
     }

}

Szenario

val testObject = Example()
testObject.y = "START INIT CODE" // here it should execute the init block

Is this even possible? Or should I change the init block with a function call after I ensure that y is initialized?

you could use [observer pattern](https://kotlin-code.com/design-patterns/behavioral-patterns/observer-pattern/). In the comment yuo mention fragment - are you writing for Android? if yes you could use [LiveData](https://developer.android.com/topic/libraries/architecture/livedata) — Stachu, Jan 11 '21 at 18:34
Yes in this example I am writing for android. But moreover, I am interested in a general solution — Andrew, Jan 11 '21 at 18:44

score 1 · Answer 1 · answered Jan 11 '21 at 18:45

The init block is exclusively for code called at instantiation time, so it is not applicable to something that is lateinit.

I also have to mention, a public lateinit var in my opinion is a huge code smell. There's no way to force outside classes to treat it appropriately at compile time. lateinit is intended for properties that are only usable (visible) after some post-construction event.

Assuming we don't care about the design issue, one way you could solve it is with a second nullable backing property.

class Example {
    private var _y: String? = null
        set(value) {
            val needInitialization = field == null
            field = value
            if (needInitialization) {
                // do initialization work here
            }
        }
    
    var y: String
        get() = _y ?: throw UninitializedPropertyAccessException("y must be initialized before getting.")
        set(value) {
            _y = value
        }
}

I don't think that it is a design issue in my case. My ExampleClass is only used inside on other class and I know that I have to assign the lateinit property inside this other class. — Andrew, Jan 11 '21 at 18:47

Sinner of the System · Answer 2 · 2021-01-11T19:32:44.693

0

you can do something like this:

class Example {

    var y: String = "UNINITIALIZED"
        set(value) {
            if (field == "UNINITIALIZED") doStuff(value)
            field = value
        }

    fun doStuff(y: String) {...}
}

or if you need to modify the initial value:

class Example {

    var y: String = "UNINITIALIZED"
        set(value) {
            field = if (field == "UNINITIALIZED") doStuff(value) else value
        }

    fun doStuff(y: String): String {...}

}

edited Jan 11 '21 at 19:32

answered Jan 11 '21 at 18:42

Sinner of the System

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the doStuff() method uses the property y. Is this still possible? – Andrew Jan 11 '21 at 18:43
it uses the getter or setter? – Sinner of the System Jan 11 '21 at 18:45
the getter. DoStuff() needs the value of y in order to execute properly – Andrew Jan 11 '21 at 18:46
you can use "value" instead of the getter – Sinner of the System Jan 11 '21 at 18:48
I gave you a more complete answer now – Sinner of the System Jan 11 '21 at 19:06

score 0 · Answer 3 · answered Jan 11 '21 at 20:04

You can use an observable property delegate for this purpose. Here's an example:

import kotlin.properties.Delegates

class Example {
    var y: Int? by Delegates.observable(initialValue = null) { _, oldValue, _ ->
        if (oldValue == null) {
            doStuff()
        }
    }

    private fun doStuff() {}
}

If you need more sophisticated logic, like to allow assigning property only once, you may want to implement your own property delegate similar to the proposed.

How to execute code in init block after property y is initialized

ExampleClass

Szenario

3 Answers3