Kotlin: lateinit to val, or, alternatively, a var that can set once

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:

controlObj.initWith(view)

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.

by lazy

by lazy

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.

lateinit var

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?

init once

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

8 Answers
8

You can implement own delegate like this:

class InitOnceProperty<T> : ReadWriteProperty<Any, T>

private object EMPTY

private var value: Any? = EMPTY

override fun getValue(thisRef: Any, property: KProperty<*>): T
if (value == EMPTY)
throw IllegalStateException("Value isn't initialized")
else
return value as T



override fun setValue(thisRef: Any, property: KProperty<*>, value: T)
if (value != EMPTY)
throw IllegalStateException("Value is initialized")

this.value = value




After that you can use it as following:

inline fun <reified T> initOnce(): ReadWriteProperty<Any, T> = InitOnceProperty()

class Test

var property: String by initOnce()

fun readValueFailure()
val data = property //Value isn't initialized, exception is thrown


fun writeValueTwice()
property = "Test1"
property = "Test2" //Exception is thrown, value already initalized


fun readWriteCorrect()
property = "Test"
val data1 = property
val data2 = property //Exception isn't thrown, everything is correct





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.

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.

var

controlObj

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")



null

IllegalStateException

var

findViewById

null

view ?: throw IllegatStateException

null

InitOnce

View

val

You can use lazy. For example with TextView

lazy

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

view

getView()

onCreateView()

text

onCreateView(...) myTextView.text = "Boo!"

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)
var map = maps[any]
if (map == null)
map = mutableMapOf<String, Any>()
maps[any] = map


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



If you really want a variable to be set only once you can use a singleton pattern:

companion object
@Volatile private var INSTANCE: SomeViewSingleton? = null

fun getInstance(context: Context): SomeViewSingleton =
INSTANCE ?: synchronized(this)
INSTANCE ?: buildSomeViewSingleton(context).also INSTANCE = it


private fun buildSomeViewSingleton(context: Context) =
SomeViewSingleton(context)



Then all you have to do is call getInstance(...) and you will always get the same object.

getInstance(...)

If you want to bind the lifetime of the object to the surrounding object just drop the companion object and put the initializer in your class.

Also the synchronized block takes care of the concurrency issues.

getInstance

For Activity it is OK to do follow:

Activity

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.

Fragment

final

View

onDestroyView

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

Activity

Fragment

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.

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

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