由于val属性，无法从init块调用函数

Question

我想初始化类的属性。因为我大量使用Kotlin的函数元素，所以我想将这些初始化放在命名良好的函数上，以提高代码的可读性。问题是，如果代码不在init块中，而是在从init块调用的函数中，我无法分配val属性。

如果属性是VAL，是否可以将类的初始化分解为不同的函数？

代码如下：

    val socket: DatagramSocket = DatagramSocket()
    val data: ByteArray = "Cassiopeiae server discovery packet".toByteArray()
    val broadcastAddresses: List<InetAddress>

    init {
        socket.broadcast = true
        val interfaceAddresses = ArrayList<InterfaceAddress>()
        collectValidNetworkInterfaces(interfaceAddresses)
        collectBroadcastAddresses(interfaceAddresses)
    }

    private fun collectValidNetworkInterfaces(interfaceAddresses: ArrayList<InterfaceAddress>) {
        NetworkInterface.getNetworkInterfaces().toList()
                .filter { validInterface(it) }
                .forEach { nInterface -> nInterface.interfaceAddresses.toCollection(interfaceAddresses) }
    }

    private fun collectBroadcastAddresses(interfaceAddresses: ArrayList<InterfaceAddress>) {
        broadcastAddresses = interfaceAddresses
                .filter { address -> address.broadcast != null }
                .map { it.broadcast }
    }

当然它不是编译，因为collectBroadcastAddresses函数尝试重新分配broadcastAddresses val。虽然我不想将此函数的代码放入init块，因为代码的作用并不明显，函数名很好地说明了这一点。

在这种情况下我能做什么？我想保持我的代码干净，这是最重要的一点！

Answer 1

解决问题的一种方法是使用纯函数初始化字段：

class Operation {
    val socket = DatagramSocket().apply { broadcast = true }
    val data: ByteArray = "Cassiopeiae server discovery packet".toByteArray()
    val broadcastAddresses = collectBroadcastAddresses(collectValidNetworkInterfaces()) 

    private fun collectValidNetworkInterfaces() =
        NetworkInterface.getNetworkInterfaces().toList()
            .filter { validInterface(it) }
            .flatMap { nInterface -> nInterface.interfaceAddresses }

    private fun validInterface(it: NetworkInterface?) = true

    private fun collectBroadcastAddresses(interfaceAddresses: List<InterfaceAddress>) {
        interfaceAddresses
            .filter { address -> address.broadcast != null }
            .map { it.broadcast }
    }
}

请注意套接字字段初始化如何使用扩展名。

我经常发现将集合操作例程提取到扩展方法中很有用：

class Operation {
    val socket = DatagramSocket().apply { broadcast = true }
    val data: ByteArray = "Cassiopeiae server discovery packet".toByteArray()
    val broadcastAddresses = NetworkInterface.getNetworkInterfaces()
        .collectValidNetworkInterfaces { validInterface(it) }
        .collectBroadcastAddresses()

    private fun validInterface(it: NetworkInterface?) = true
}

fun Iterable<InterfaceAddress>.collectBroadcastAddresses(): List<InetAddress> =
    filter { address -> address.broadcast != null }.map { it.broadcast }

fun Enumeration<NetworkInterface>.collectValidNetworkInterfaces(isValid: (NetworkInterface) -> Boolean = { true }) =
    toList()
        .filter { isValid(it) }
        .flatMap { nInterface -> nInterface.interfaceAddresses }