Modify Local Variables Inside Kotlin Lambdas

From what I already know: When function call completes all the local variables inside it get destroyed. Question is: How come in the below code snippet a lambda can change the valu

Solution 1:

In Android Studio you can use Tools -> Kotlin -> Show Kotlin Bytecode, and then Decompile to see what the corresponding Java code might look like.

You'll then see that color actually turns into a final IntRef color = new IntRef();, where IntRef is a class in the Kotlin runtime that has a public int element member holding the actual value.

Since color is a final reference to an object, your OnCheckedChangeListener can capture it just fine. The member int is not final however, which is why you can assign to it (the compiler turns your color = index into color.element = index;).

Solution 2:

It's changed internally to a single element array reference.

Think of it like this:

funinit() {
  var color = arrayOf()
  color[0] = 0
  rg.setOnCheckedChangeListener { group, checkedId ->
    val childCount = group.childCount
    for (index in0 until childCount) {
      val button = group.getChildAt(index)
      if (button.id == checkedId) {
        color[0] = index
      }
    }
  }
}

That's how it can be accessed after the fact.