Unresolved reference inside anonymous Kotlin liste

问题:

I have the code below. It is Kotlin. Any idea why textToSpeech from textToSpeech.setLanguage(Locale.UK) is telling that there is no reference resolved for textToSpeech?

val textToSpeech = TextToSpeech(
            applicationContext,
            object : TextToSpeech.OnInitListener {
                override fun onInit(status: Int) {
                    if (status == TextToSpeech.SUCCESS) {
                        textToSpeech.setLanguage(Locale.UK)
                    }
                }

            })

At first I assumed it is an Idea kotlin plugin bug, but it seems that it actually can't be compiled

回答1:

Kotlin has hardened the variables initialization policy, and it's now prohibited to reference the variable inside its initializer, even in lambdas and object expressions, which seems reasonable: imagine that a lambda is called immediately before the variable assignment.

For your case, I can suggest as a workaround using an object expression in this quite cumbersome construct:

val textToSpeech = object {
    val value: TextToSpeech get() = inner
    private val inner = TextToSpeech(
            applicationContext,
            { value.setLanguage(Locale.UK) }
    )
}.value

This will initialize an anonymous object with inner inside that is acceptable through value property. Note that the inner initializer uses value property. Then the value is extracted and can be used.

But please keep in mind that this trick is unsafe: in runtime, using value before inner is assigned (e.g. in TextToSpeech constructor) will throw NullPointerException.

Also, I've replaced the OnInitListener with a lambda using SAM conversion to be short, but object expression can still be used there.

---

UPD: check this question for my effort to generalize this approach. Using it, you can write

val textToSpeech = selfReference {
    TextToSpeech(
        applicationContext,
        { self.setLanguage(Locale.UK) }
    )
}

See the sources on Github.

回答2:

This is a very readable and clear way to face that problem. First you should define this:

fun <T> selfReferenced(initializer: () -> T) = initializer.invoke()
operator fun<T> T.getValue(any: Any?, property: KProperty<*>) = this

and later use

val valueName: ValueType by selfReferenced{
    //here you can create and use the valueName object
}

Using your problem as example you can do:

val textToSpeech:TextToSpeech by selfReferenced {
TextToSpeech(
        applicationContext,
        TextToSpeech.OnInitListener { status ->
            if (status == TextToSpeech.SUCCESS) {
                textToSpeech.setLanguage(Locale.UK)
            }
        })
    }

Inside the selfReferenced block you can use the outer object with no restrictions. The only thing you should take care of, is declaring the type explicitly to avoid recursive type checking issues.