kotlin - 从类中获取可为空的变量

Question

首先，我说我的Kotlin经验有限。我试图从一个类中获取可为空的var，但是每执行一次它都获取一个空指针。

类(class)代码:

class MultiSpinner : Spinner, OnMultiChoiceClickListener, OnCancelListener {

    private var items: List<String>? = null
    private var selected: BooleanArray? = null
    private var selectedNew: BooleanArray? = null
    private var defaultText: String? = null
    private var listener: MultiSpinnerListener? = null
    var category: String = "010"

    constructor(context: Context) : super(context) {}

    constructor(arg0: Context, arg1: AttributeSet) : super(arg0, arg1) {}

    constructor(arg0: Context, arg1: AttributeSet, arg2: Int) : super(arg0, arg1, arg2) {}

    override fun onClick(dialog: DialogInterface, which: Int, isChecked: Boolean) {
        selectedNew!![which] = isChecked
    }

    override fun onCancel(dialog: DialogInterface) {
        // refresh text on spinner
        val spinnerBuffer = StringBuffer()
        var allUnselected = true
        for (i in items!!.indices) {
            if (selected!![i]) {
                spinnerBuffer.append(items!![i])
                spinnerBuffer.append(", ")
                allUnselected = false
            }
            selectedNew!![i] = selected!![i]
        }

        var spinnerText: String?
        if (allUnselected) {
            spinnerText = defaultText
        } else {
            spinnerText = spinnerBuffer.toString()

            /** Remove trailing comma*/
            spinnerText = spinnerText.substring(0, spinnerText.length - 2)
        }
        val adapter = ArrayAdapter(
            context,
            R.layout.simple_spinner_item,
            arrayOf(spinnerText)
        )
        setAdapter(adapter)

        category = selected!!.joinToString(limit = selected!!.size, separator = "") {it.toInt().toString()}
        listener!!.onItemsSelected(selected)
    }

    override fun performClick(): Boolean {
        val builder = AlertDialog.Builder(context)
        builder.setMultiChoiceItems(
            items!!.toTypedArray(), selectedNew, this
        )
        builder.setPositiveButton(R.string.ok
        ) { dialog, _ -> selected = selectedNew!!.copyOf(); dialog.cancel() }
        builder.setNegativeButton(R.string.cancel
        ) { dialog, _ -> dialog.cancel() }
        builder.setOnCancelListener(this)
        builder.show()
        return true
    }

    fun setItems(
        items: List<String>, allText: String,
        listener: MultiSpinnerListener
    ) {
        this.items = items
        this.defaultText = allText
        this.listener = listener

        // one selected by default
        selected = BooleanArray(items.size) {false}
        selectedNew = BooleanArray(items.size) {false}
        for (i in selected!!.indices) {
            selected!![i] = false
            selectedNew!![i] = false
        }

        selected!![0] = true
        selectedNew!![0] = true
        category = selected!!.joinToString(limit = selected!!.size, separator = "") {it.toInt().toString()}

        // all text on the spinner
        val adapter = ArrayAdapter(
            context,
            R.layout.simple_spinner_item, arrayOf(allText)
        )
        setAdapter(adapter)
    }

    fun getSelected(): BooleanArray? {
        return selected
    }

    interface MultiSpinnerListener {
        fun onItemsSelected(selected: BooleanArray?)
    }

    private fun Boolean.toInt() = if (this) 1 else 0
}

初始化中

override fun onCreate(savedInstanceState: Bundle?) {
    ...
    val spinnerCategory: MultiSpinner = findViewById(R.id.category_spinner)
    val categoryList: List<String> = resources.getStringArray(R.array.category).toList()

    spinnerCategory.setItems(categoryList, getString(R.string.default_category), this)
    ...
}

从 selected访问 MultiSpinner

val selected: BooleanArray? = MultiSpinner(this).getSelected()
println(selected)

println读取 null。

我尝试返回 BooleanArray而不是 BooleanArray?的 getSelected，但这只是给了我一个空指针异常(如println阅读所期望的那样)。

我现在通过在类中使用 category并在类内部执行操作来解决该问题。但是，我想在我的主要 Activity 中获取 selected数组，并使用其数据在类之外创建所需的String。

为什么不获取 selected数据，而是获取空指针？

Answer 1

您必须使用从selected获得的实例( View 层次结构中已经存在的实例)访问findViewById，而不是自己创建一个全新的对象:

val selected: BooleanArray? = spinnerCategory.getSelected()

编辑:

如果您需要在 selected方法之外访问 onCreate，只需将 spinnerCategory存储为属性而不是局部变量即可:

private lateinit var spinnerCategory: MultiSpinner

override fun onCreate(savedInstanceState: Bundle?) {
    spinnerCategory = findViewById(R.id.category_spinner)
    ...
}

Kotlin还为Android提供了一个不错的功能，称为 View Binding，它可以让您忽略调用 findViewById并直接使用其ID引用 View :

override fun onCreate(savedInstanceState: Bundle?) {
    ...
    category_spinner.setItems(categoryList, getString(R.string.default_category), this)
    ...
}

private fun doSomethingWithSelected() {
    val selected: BooleanArray? = category_spinner.getSelected()
}

但是在这种情况下 我建议您更改 View 的ID，使其与Kotlin变量命名准则一致，例如到 spinnerCategory，因为它已经被使用了。