Android XML 中的顶级布局总是 FrameLayout？

Question

我试图以编程方式设置 ConstraintLayout 的高度，我打算使用 ConstraintLayout.LayoutParams。 ConstraintLayout 是这个 XML 中的顶级布局。但是当我用 ConstraintLayout.LayoutParams 和 ClassCastException 发生了。

java.lang.ClassCastException: android.widget.FrameLayout$LayoutParams cannot be cast to androidx.constraintlayout.widget.ConstraintLayout$LayoutParams

详细代码如下

对话框

private fun initLayoutSize() = with(binding) {
    dialog?.let {
        val height = getDisplayHeight(it.window!!.windowManager)
        val params = homeBottomSheetContainer.layoutParams as ConstraintLayout.LayoutParams // excpeiton here!!
        params.height = (height * 0.3).toInt()
        homeBottomSheetContainer.layoutParams = params
    }
}

XML

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/homeBottomSheetContainer"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:paddingHorizontal="@dimen/basic16"
app:layout_behavior="com.google.android.material.bottomsheet.BottomSheetBehavior">

<View
    android:id="@+id/homeBottomSheetTopView"
    android:layout_width="@dimen/basic30"
    android:layout_height="@dimen/basic03"
    android:layout_marginTop="@dimen/basic12"
    android:background="@drawable/bottomsheet_top_radius_view"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<TextView
    android:id="@+id/homeBottomLottoSearchNotice"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="@dimen/basic12"
    android:background="?attr/selectableItemBackground"
    android:clickable="true"
    android:focusable="true"
    android:gravity="end"
    android:padding="@dimen/basic04"
    android:text="@string/ask_search"
    android:textColor="@color/black"
    android:textSize="@dimen/txt12"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintTop_toBottomOf="@id/homeBottomSheetTopView"
    tools:visibility="invisible" />

<EditText
    android:id="@+id/homeLottoSearch"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:hint="@string/input_lotto_round"
    android:imeOptions="actionDone"
    android:inputType="number"
    android:maxLines="1"
    android:textColor="@color/black"
    android:textCursorDrawable="@drawable/shape_edittext_cursor_black"
    android:textSize="@dimen/txt12"
    android:visibility="invisible"
    app:backgroundTint="@color/black"
    app:layout_constraintBottom_toBottomOf="@id/homeBottomLottoSearchNotice"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="@id/homeBottomLottoSearchNotice" />

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/homeBottomSheetLottoRV"
    android:layout_width="0dp"
    android:layout_height="0dp"
    android:layout_marginTop="@dimen/basic12"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@id/homeBottomLottoSearchNotice"
    tools:listitem="@layout/item_lotto_all_round" />

</androidx.constraintlayout.widget.ConstraintLayout>

发生了什么事？

我没有在我的 XML 中使用任何 FrameLayout，而且我在任何地方都找不到它。但为什么 android 系统告诉我有关 FrameLayout 的信息？ FrameLayout 是 Android View XML 中的默认顶级布局吗？？？

所以，我用下面的代码更改了代码。它奏效了！

private fun initLayoutSize() = with(binding) {
    dialog?.let {
        val height = getDisplayHeight(it.window!!.windowManager)
        val params = homeBottomSheetContainer.layoutParams as FrameLayout.LayoutParams // change here!!
        params.height = (height * 0.3).toInt()
        homeBottomSheetContainer.layoutParams = params
    }
}

但我现在很困惑。有人帮忙吗？并提前谢谢你。 :)

Answer 1

如果您只设置高度，那么您可以使用更通用的 ViewGroup.LayoutParams。所有布局参数都在扩展它，所以 class 没有机会进行转换，无论父级如何

val params = homeBottomSheetContainer.layoutParams as ViewGroup.LayoutParams

除此之外，您还可以使用 is 关键字来检查实例类型

Android XML 中的顶级布局总是 FrameLayout？

Top-level Layout in Android XML is ALWAYS FrameLayout?

android

android-layout

android-view

android-layoutparams

android-constraintlayout