Kotlin：访问 when 语句的参数

发布于02月01日

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)
    }
})

Kotlin有一个简洁的语法来访问e?.keyCode吗？我真的不想重复这个表达.

推荐答案

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)
}

不幸的是，这将需要您添加额外的花括号和线条.不过，好的一面是，e?.keyCode只会被判断一次.在这种情况下，这可能无关紧要，但如果表达式更大，则此方法将是合适的.

Edit:

另一种可能是将when表达式包装在对let的调用中.它允许您使用it访问参数.如下所示:

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)
}