AbstractExternalResource

class MyCustomExternalResource: AbstractExternalResource() {
     override fun inputStream0(): InputStream = FileInputStream("/test.txt")
     override val size: Long get() = File("/test.txt").length()
}

class MyRes(
     cleanup: ResourceCleanCallback,
     val delegate: Closable,
): AbstractExternalResource(cleanup) {
}

// 错误, 该写法会导致 Resource 永远也不会被自动释放
lateinit var myRes: MyRes
val cleanup = ResourceCleanCallback {
     myRes.delegate.close()
}
myRes = MyRes(cleanup, fetchDelegate())

// 正确
val delegate: Closable
val cleanup = ResourceCleanCallback {
     delegate.close()
}
val myRes = MyRes(cleanup, delegate)