由于 val 属性，无法从 init 块调用函数

Question

我想初始化 class 的属性。因为我大量使用 Kotlin 的功能元素，所以我想将这些初始化放在命名良好的函数中，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 vals，是否可以将 class 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 }
    }

当然它没有编译，因为 collectBroadcastAddresses 函数试图重新分配 broadcastAddresses val。虽然我不想把这个函数的代码放在init块中，因为代码在做什么并不明显，函数名就很好听了。

遇到这种情况我该怎么办？我想保持我的代码干净，这是最重要的一点！

Answer 1

解决该问题的一种方法是使用 pure functions 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 }
    }
}

注意 socket 字段初始化如何使用 apply 扩展。

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 }

由于 val 属性，无法从 init 块调用函数

Cannot call a function from init block because of val property

coding-style

kotlin