使用 Kotlin 属性委托时出现 NullPointerException (NPE)

Question

我有一个 class，它接受用户在文本字段中的输入，并使用提供的函数 class 将它们转换为任何 class

class GenericTextFieldDelegate<T>(
    private val initBlock: () -> TextView,
    private val getConversion: (String?) -> T?,
    private val setConversion: (T?) -> String? = { it?.toString() }
    ) {
    private val textView: TextView by lazy { initBlock() }

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T? =
        getConversion(textView.text?.toString())

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T?) {
        textView.text = setConversion(value)
    }
}

我已经这样做了，所以当我有 TextViews 时我可以这样做

class IntegerInputView @JvmOverloads constructor(
    context: Context,
    attributeSet: AttributeSet? = null,
    defStyleAttr: Int = 0
) : LinearLayout(context, attributeSet, defStyleAttr), DataInput<Int> {

override var value: Int? by GenericTextFieldDelegate(
    { inputET },
    getConversion = { it?.toIntOrNull() },
    setConversion = { it.toString() }
)
...

我有一个具有上述自定义视图的片段，当我有

override var tareWeight: Kg?
    get() = tareWeightInput.value
    set(value) {
        tareWeightInput.value = value
    }

一切正常，我真正想做的是

override var tareWeight: Kg? by tareWeightInput

将这些行添加到 IntegerInputView

...

operator fun getValue(thisRef: Any?, property: KProperty<*>): Int? = value

operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int?) {
    this.value = value
}

override var value: Int? by GenericTextFieldDelegate(
...

当我构建时，运行并加载片段，我得到下面的堆栈跟踪。我哪里错了？

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Integer com.gameforeverything.storekeeper.customViews.IntegerInputView.getValue(java.lang.Object, kotlin.reflect.KProperty)' on a null object reference
        at com.gameforeverything.storekeeper.fragments.weighInFragment.WeighInFragment.getGrossWeight(Unknown Source:7)
        at com.gameforeverything.storekeeper.fragments.weighInFragment.WeighInPresenter.getNetWeight(WeighInPresenter.kt:40)

Answer 1

您的属性委托本身（本例中的 tareWeightInput）为空。

您可以通过检查堆栈跟踪来判断是这种情况：它指出由于 NPE 而失败的方法调用是 IntegerInputView.getValue。由于这是为了检索属性的值而在委托上调用的属性委托方法调用，我们知道委托必须为 null。

您的属性委托是 View，所以我怀疑它正在以某种方式动态检索，可能是 findViewById 调用的结果。您需要确保包含委托的变量在检索时是非空的。考虑这个类似的例子：

import kotlin.reflect.KProperty

class FooDelegate {
    private var value: Int = 0
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int = value
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) { this.value = value }
}

class Foo {
    private lateinit var delegate: FooDelegate
    val bar by delegate
    
    init {
        delegate = FooDelegate()
    }
}

fun main() {
    println(Foo().bar)
}

这会崩溃，因为它试图在初始化变量之前设置委托。在这种情况下，Kotlin 会更早地抛出错误，但由于 Kotlin/Java 互操作性，这可能在您的示例中被 platform types 掩盖了。

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