Kotlin: Accessing parameter of when-statement

Question

Is there a way to get the value of the expression I passed into the when statement?

In my application I have a KeyListener like that

_content.addKeyListener(object : KeyAdapter() {
    override fun keyPressed(e: KeyEvent?) = when(e?.keyCode) {
        KeyEvent.VK_T -> mainWindow.enterTrainingState()
        KeyEvent.VK_P -> mainWindow.enterPlayState()
        KeyEvent.VK_E -> mainWindow.close()
        else -> println(e?.keyCode)
    }
})

Has Kotlin a neat syntax to access e?.keyCode? I don't really want to repeat the expression.

Answer 1

I had this problem myself a couple of days ago. I think it would have been nice to be able to access the value as it inside the when-expression.

I solved it by assigning the expression to a val before the when expression:

val keyCode = e?.keyCode
when(keyCode) {
    KeyEvent.VK_T -> mainWindow.enterTrainingState()
    KeyEvent.VK_P -> mainWindow.enterPlayState()
    KeyEvent.VK_E -> mainWindow.close()
    else -> println(keyCode)
}

Unfortunately, this would require you to add extra braces and lines. The upside though, is that e?.keyCode would only be evaluated once. It may not matter in this exact case, but if the expression was bigger, this approach would be suitable.

Edit:

Another possibility it to wrap the when expression in a call to let. It lets you access the parameter with it. Like this:

e?.keyCode.let {
    when(it) {
        KeyEvent.VK_T -> mainWindow.enterTrainingState()
        KeyEvent.VK_P -> mainWindow.enterPlayState()
        KeyEvent.VK_E -> mainWindow.close()
        else -> println(it)
  }
}

Edit2:

Kotlin 1.3 has support for capturing the subject expression of a when in a variable. This is the syntax:

when(val keyCode = e?.keyCode) {
    KeyEvent.VK_T -> mainWindow.enterTrainingState()
    KeyEvent.VK_P -> mainWindow.enterPlayState()
    KeyEvent.VK_E -> mainWindow.close()
    else -> println(keyCode)
}

Answer 2

With Kotlin 1.3, you can set the parameter as a scoped variable and access it.

_content.addKeyListener(object : KeyAdapter() {
    override fun keyPressed(e: KeyEvent?) = when(val keycode = e?.keyCode) {
        KeyEvent.VK_T -> mainWindow.enterTrainingState()
        KeyEvent.VK_P -> mainWindow.enterPlayState()
        KeyEvent.VK_E -> mainWindow.close()
        else -> println(keycode)
    }
})