CODEWITHSUNDEEP
kotlincollections
Elye
Elye

Reputation: 60321

mutableMap.setValue(...) function no longer exist?

In the documentation, I see we have setValue for a mutableMap. https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/set-value.html

operator fun <V> MutableMap<in String, in V>.setValue(
    thisRef: Any?,
    property: KProperty<*>,
    value: V)

However when try using it, it doesn't seem to exist. (I'm using Kotlin 1.3)

   val a = mutableMapOf(1 to 1, 2 to 3)
   a.setValue(...) // error out

Is it not available anymore?

Upvotes: 2

Views: 3009

Answers (2)

Elye
Elye

Reputation: 60321

To provide further context of my finding on how to use setValue directly, created an example below.

    data class Something(val boo: String? = null)

    @Test
    fun testSetValue() {
        // Showing auto delegate value of foo, which uses
        // setValue under the hood to extract 1000 for foo
        val foo: Int by mutableMapOf("foo" to 1000)
        println(foo)  // 1000

        // Demonstrating setValue ussage directly setting boo value
        val x = mutableMapOf("abc" to 1)
        x.setValue(null, Something::boo, 200)
        println(x)    // {abc=1, boo=200}

        // Similarly for `getValue`
        println(x.getValue(null, Something::boo) // 200
    }

Upvotes: 0

zsmb13
zsmb13

Reputation: 89668

This is an extension that implements the setValue method required on a type for it to be used as a proprety delegate. The official documentation covers how Maps can be used as delegates here: Storing Properties in a Map.

TLDR, you get to use this kind of syntax, delegating properties into a Map instance:

class User(val map: Map<String, Any?>) {
    val name: String by map
    val age: Int     by map
}

If you want to simply set values of a MutableMap, use the set method, also available as the [] operator:

val map = mutableMapOf(1 to 1, 2 to 3)
map.set(2, 2)
map[2] = 2

Or the put method, which also returns the previous value stored by the key you modified:

val oldValue: Int = map.put(2, 5)

Upvotes: 5

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