当表达式不适用于 onCreateDrawableState - android 中的枚举 class 时，kotlin

Question

我不明白为什么这会在 when (colorState) 行抛出 NPE。如果我删除枚举 class 并将值替换为整数，一切正常。

class ColorChangerButton(context: Context, attrs: AttributeSet) : AppCompatImageButton(context, attrs) {

    enum class ColorState {ACCENT, STRONG, WEAK}

    private val stateAccent = intArrayOf(R.attr.state_accent)
    private val stateStrong = intArrayOf(R.attr.state_strong)
    private val stateWeak = intArrayOf(R.attr.state_weak)

    var colorState = ColorState.ACCENT
        set(value) {
            if (field != value) {
                field = value
                refreshDrawableState()
            }
        }

    override fun onCreateDrawableState(extraSpace: Int): IntArray {
        val state = super.onCreateDrawableState(extraSpace + 1)

        when (colorState) {
            ColorState.ACCENT -> View.mergeDrawableStates(state, stateAccent)
            ColorState.STRONG -> View.mergeDrawableStates(state, stateStrong)
            ColorState.WEAK -> View.mergeDrawableStates(state, stateWeak)
        }

        return state
    }
}

堆栈跟踪：

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int com.test.uiplayground.buttons.ColorChangerButton$ColorState.ordinal()' on a null object reference
    at com.test.uiplayground.buttons.ColorChangerButton.onCreateDrawableState(ColorChangerButton.kt:28)
    at android.view.View.getDrawableState(View.java:15953)
    at android.view.View.setBackgroundDrawable(View.java:16188)
    at androidx.appcompat.widget.AppCompatImageButton.setBackgroundDrawable(AppCompatImageButton.java:122)
    at android.view.View.setBackground(View.java:16125)
    at android.view.View.<init>(View.java:4090)
    at android.widget.ImageView.<init>(ImageView.java:139)
    at android.widget.ImageButton.<init>(ImageButton.java:86)
    at android.widget.ImageButton.<init>(ImageButton.java:82)
    at androidx.appcompat.widget.AppCompatImageButton.<init>(AppCompatImageButton.java:73)
    at androidx.appcompat.widget.AppCompatImageButton.<init>(AppCompatImageButton.java:69)
    at com.test.uiplayground.buttons.ColorChangerButton.<init>(ColorChangerButton.kt:9)

正如我上面所说，如果我将枚举 class 替换为整数，那么它工作得很好：

class ColorChangerButton(context: Context, attrs: AttributeSet) : AppCompatImageButton(context, attrs) {

    private val stateAccent = intArrayOf(R.attr.state_accent)
    private val stateStrong = intArrayOf(R.attr.state_strong)
    private val stateWeak = intArrayOf(R.attr.state_weak)

    var colorState = 1
        set(value) {
            if (field != value) {
                field = value
                refreshDrawableState()
            }
        }

    override fun onCreateDrawableState(extraSpace: Int): IntArray {
        val state = super.onCreateDrawableState(extraSpace + 1)

        when (colorState) {
            1 -> View.mergeDrawableStates(state, stateAccent)
            2 -> View.mergeDrawableStates(state, stateStrong)
            3 -> View.mergeDrawableStates(state, stateWeak)
        }

        return state
    }
}

已解决，补充说明：

关于抛出 NPE 的原因，请参阅已接受的答案。
整数部分具有误导性。 Java 将未初始化的变量设置为默认值（除非是局部变量或块变量，否则您会收到错误消息说它未初始化）。对于整数，该值为 0。因此它在第一次调用时通过了所有 3 个条件。如果我添加 0 -> View.mergeDrawableStates(state, stateAccent) 案例，那么我会得到另一个 NPE，这次是因为 stateAccent 尚未初始化 - 现在这很有意义。

Answer 1

这是为 Kotlin 源文件生成的 Java 代码：

  public ColorChangerButton(@NotNull Context context, @NotNull AttributeSet attrs) {
      Intrinsics.checkParameterIsNotNull(context, "context");
      Intrinsics.checkParameterIsNotNull(attrs, "attrs");
      super(context, attrs);                  <---------- super will be called first
      this.stateAccent = new int[0];
      this.stateStrong = new int[0];
      this.stateWeak = new int[0];
      this.colorState = ColorChangerButton.ColorState.ACCENT;
   }

在调用 super() 之前，实例变量甚至不会被初始化。当超级构造函数调用 onCreateDrawableState 时发生 NPE。

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