Kotlin: lateinit to val, or, alternatively, a var

Question

Just curious: In Kotlin, I would love to get some val that can be initialized by lazy, but with a parameter. That's because I need something that's created very late in order to initialize it.

Specifically, I wish I had:

private lateinit val controlObj:SomeView

or:

private val controlObj:SomeView by lazy { view:View->view.findViewById(...)}

and then:

override fun onCreateView(....) {
    val view = inflate(....)


    controlObj = view.findViewById(...)

or in the 2nd case controlObj.initWith(view) or something like that:

return view

I cannot use by lazy because by lazy won't accept external parameters to be used when initialising. In this example - the containing view.

Of course I have lateinit var but it would be nice if I could make sure it becomes read only after setting and I could do it in one line.

Is there a pretty clean way to create a read only variable that initializes only once but only when some other variables are born? Any init once keyword? That after init the compiler knows it's immutable?

I am aware of the potential concurrency issues here but if I dare to access it before init, I surely deserve to be thrown.

Answer 1

In this solution you implement a custom delegate and it becomes a separate property on your class. The delegate has a var inside, but the controlObj property has the guarantees you want.

class X {
    private val initOnce = InitOnce<View>()
    private val controlObj: View by initOnce

    fun readWithoutInit() {
        println(controlObj)
    }

    fun readWithInit() {
        initOnce.initWith(createView())
        println(controlObj)
    }

    fun doubleInit() {
        initOnce.initWith(createView())
        initOnce.initWith(createView())
        println(controlObj)
    }
}

fun createView(): View = TODO()

class InitOnce<T : Any> {

    private var value: T? = null

    fun initWith(value: T) {
        if (this.value != null) {
            throw IllegalStateException("Already initialized")
        }
        this.value = value
    }

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T =
            value ?: throw IllegalStateException("Not initialized")
}

BTW if you need thread safety, the solution is just slightly different:

class InitOnceThreadSafe<T : Any> {

    private val viewRef = AtomicReference<T>()

    fun initWith(value: T) {
        if (!viewRef.compareAndSet(null, value)) {
            throw IllegalStateException("Already initialized")
        }
    }

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T =
            viewRef.get() ?: throw IllegalStateException("Not initialized")
}

Answer 2

For Activity it is OK to do follow:

private val textView: TextView by lazy { findViewById<TextView>(R.id.textView) }

For Fragment it is not makes sense to create final variable with any View type, because you will lost connection with that view after onDestroyView.

P.S. Use Kotlin synthetic properties to access views in Activity and Fragment.

Answer 3

You can implement own delegate like this:

private val maps = WeakHashMap<Any, MutableMap<String, Any>>()

object LateVal {
    fun bindValue(any: Any, propertyName: String, value: Any) {
        val map = maps.getOrPut(any) { mutableMapOf<String, Any>() }

        if (map[propertyName] != null) {
            throw RuntimeException("Value is initialized")
        }

        map[propertyName] = value
    }

    fun <T> lateValDelegate(): MyProperty<T> {
        return MyProperty<T>(maps)
    }

    class MyProperty<T>(private val maps: WeakHashMap<Any, MutableMap<String, Any>>) : ReadOnlyProperty<Any?, T> {

        override fun getValue(thisRef: Any?, property: KProperty<*>): T {
            val ret = maps[thisRef]?.get(property.name)
            return (ret as? T) ?: throw RuntimeException("Value isn't initialized")
        }
    }
}

fun <T> lateValDelegate(): LateVal.MyProperty<T> {
    return LateVal.MyProperty<T>(maps)
}

fun Any.bindValue(propertyName: String, value: Any) {
    LateVal.bindValue(this, propertyName, value)
}

After that you can use it as following:

class Hat(val name: String = "casquette") {
    override fun toString(): String {
        return name
    }
}

class Human {
    private val hat by lateValDelegate<Hat>()

    fun setHat(h: Hat) {
        this.bindValue(::hat.name, h)
    }

    fun printHat() {
        println(hat)
    }

}

fun main(args: Array<String>) {
    val human = Human()
    human.setHat(Hat())
    human.printHat()
}

In case when you try to access value before it initialized you will get exception as well as when you try to reassign new value.

also,you can write DSL to make it readable.

object to

infix fun Any.assigned(t: to) = this

infix fun Any.property(property: KProperty<*>) = Pair<Any, KProperty<*>>(this, property)

infix fun Pair<Any, KProperty<*>>.of(any: Any) = LateVal.bindValue(any, this.second.name, this.first)

and then call it like this:

fun setHat(h: Hat) {
    h assigned to property ::hat of this
}

Answer 4

I believe there is no such thing "init once". Variables are either final or not. lateinit variables are not final. Because, well, you reassign it some other value after initialization phase.

If someone finds the solution, I'm starring this question

Answer 5

You can use lazy. For example with TextView

    val text by lazy<TextView?>{view?.findViewById(R.id.text_view)}

where view is getView(). And after onCreateView() you can use text as read only variable

Answer 6

You can implement own delegate like this:

class LateInitVal {
    private val map: MutableMap<String, Any> = mutableMapOf()

    fun initValue(property: KProperty<*>, value: Any) {
        if (map.containsKey(property.name)) throw IllegalStateException("Value is initialized")

        map[property.name] = value
    }

    fun <T> delegate(): ReadOnlyProperty<Any, T> = MyDelegate()

    private inner class MyDelegate<T> : ReadOnlyProperty<Any, T> {

        override fun getValue(thisRef: Any, property: KProperty<*>): T {
            val any = map[property.name]
            return any as? T ?: throw IllegalStateException("Value isn't initialized")
        }

    }
}

After that you can use it as following:

class LateInitValTest {
    @Test
    fun testLateInit() {
        val myClass = MyClass()

        myClass.init("hello", 100)

        assertEquals("hello", myClass.text)
        assertEquals(100, myClass.num)
    }
}

class MyClass {
    private val lateInitVal = LateInitVal()
    val text: String by lateInitVal.delegate<String>()
    val num: Int by lateInitVal.delegate<Int>()

    fun init(argStr: String, argNum: Int) {
        (::text) init argStr
        (::num) init argNum
    }

    private infix fun KProperty<*>.init(value: Any) {
        lateInitVal.initValue(this, value)
    }
}

In case when you try to access value before it initialized you will get exception as well as when you try to reassign new value.