Kotlin: assignable "it"?

Question

I'm trying to simplify Parcelable code in Kotlin:

public class C() : Parcelable {
    var b1: Boolean = false
    var b2: Boolean = false
    var b3: Boolean = false

    var i1: Int = 0
    var i2: Int = 0
    val parcelBooleans = listOf(b1, b2, b3)
    val parcelInts = listOf(i1, i2)

    override fun writeToParcel(p: Parcel, flags: Int) {
        parcelBooleans.forEach { p.writeBoolean(it) }
        parcelInts.forEach { p.writeInt(it) }
    }

    private fun readFromParcel(p: Parcel) {
         parcelBooleans.forEach{ it = p.readBoolean() } // does not compile as "it" is a "val"
    }
    // ... parcel creator bla bla
}

writeBoolean() and readBoolean() are extension functions.

Is there a way to have a forEach on list with an assignable "it"?

Update: in a comment to one of the answers the author clarifies this question as:

My point is that my list is not mutable, the elements inside are. Elements are my properties. But I'm realizing that maybe, listOf is making a value copy of my properties… so either I must rely on reflection, or wrapping basic type into an class to allow change to their value.

The intent is via the readonly list parcelBooleans which holds references to b1, b2, and b3 modify the values of b1, b2, and b3; but not mutating the list itself.

Answer 1

Update: KT-7033 has been fixed, this answer now works without requiring the kotlin-reflect.jar

I think simple way to achieve your wants is the using property references, but note that than you should provide kotlin-reflect.jar in classpath (until KT-7033 not fixed). Additionally it may be simplified after KT-6947 will be fixed.

public class C_by_Reflection() : Parcelable {
    var b1: Boolean = false
    var b2: Boolean = false
    var b3: Boolean = false

    var i1: Int = 0
    var i2: Int = 0
    val parcelBooleans = listOf(::b1, ::b2, ::b3)
    val parcelInts = listOf(::i1, ::i2)

    override fun writeToParcel(p: Parcel, flags: Int) {
        parcelBooleans.forEach { p.writeBoolean(it.get(this)) }
        parcelInts.forEach { p.writeInt(it.get(this)) }
    }

    private fun readFromParcel(p: Parcel) {
         parcelBooleans.forEach{ it.set(this, p.readBoolean()) }
    }
    // ... parcel creator bla bla
}

Another simple solution is using delegated properties:

public class C_by_Delgates() : Parcelable {
    val mapBooleans = hashMapOf<String, Any?>()
    var b1: Boolean by mapBooleans 
    var b2: Boolean by mapBooleans 
    var b3: Boolean by mapBooleans 

    val mapInts = hashMapOf<String, Boolean>()
    var i1: Int by mapInts 
    var i2: Int by mapInts 

    override fun writeToParcel(p: Parcel, flags: Int) {
        mapBooleans.forEach { p.writeBoolean(it.value as Boolean) }
        mapInts.forEach { p.writeInt(it.value as Int) }
    }

    private fun readFromParcel(p: Parcel) {
        mapBooleans.forEach { mapBooleans[it.key] = p.readBoolean() }
    }
    // ... parcel creator bla bla
}

Answer 2

NOTE: The question is not clear if the author is trying to mutate properties b1, b2, b3 via a reference within the list. Or if he is trying to modify the contents of list parcelBooleans which is a copy of the values from the properties. If the first, see answer from @Bashor (or just make the values of the list something who's sub-contents is mutable), if second, this answer is correct.

Given the extension functions defined in this other Stack Overflow question "how do you modify the contents of a list while iterating" you can change your code to simply:

public class C() : Parcelable {
    var b1: Boolean = false
    var b2: Boolean = false
    var b3: Boolean = false

    var i1: Int = 0
    var i2: Int = 0

    // CHANGED:  use mutable list, array, or primitive array
    val parcelBooleans = arrayListOf(b1, b2, b3) 

    val parcelInts = listOf(i1, i2)

    override fun writeToParcel(p: Parcel, flags: Int) {
        parcelBooleans.forEach { p.writeBoolean(it) }
        parcelInts.forEach { p.writeInt(it) }
    }

    private fun readFromParcel(p: Parcel) {
        // CHANGED:  using the extension function
        parcelBooleans.mapInPlace { p.readBoolean() }
    }
    // ... parcel creator bla bla
}

Answer 3

A way to do it is to map each value:

parcelBooleans = parcelBooleans.map{ p.readBoolean() }

You have to make parcelBooleans a mutable variable, though.

Answer 4

Creating your own Iterator would be the OOP way to do it.

If you want to go the more functional way, you can use map(), e.g.

private fun readFromParcel(p: Parcel) {
     val mappedBooleans = parcelBooleans.map{ p.readBoolean() }
}

map() is similar to forEach, it uses the return value of each lambda execution to create a new List<>

Answer 5

Found out that my code at first can't work as the listOf makes a copy of the properties' value.

So I wrap the basic types in a simple class with get/set method:

public class MBoolean() {
var internal: Boolean = false

constructor(b: Boolean) : this() {
    internal = b
}

fun set(b: Boolean) {
    internal = b
}

fun get(): Boolean {
    return internal
}
}

public class MInt() {
private var internal: Int = 0

constructor(i: Int) : this() {
    set(i)
}

public fun set(i: Int) {
    internal = i
}

public fun get(): Int {
    return internal
}
}

override fun writeToParcel(p: Parcel, flags: Int) {
    parcelBooleans.forEach { p.writeBoolean(it.get()) }
    parcelInts.forEach { p.writeInt(it.get()) }
}

private fun readFromParcel(p: Parcel) {
    parcelBooleans.forEach { it.set(p.readBoolean()) }
    parcelInts.forEach { it.set(p.readInt()) }
}