简单的解决方案:

class TopRoundedCornersFragment : BottomSheetDialogFragment() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setStyle(STYLE_NORMAL, R.style.AppBottomSheetDialogTheme)
    }
}

在styles.xml

<style name="BottomSheetStyle" parent="Widget.Design.BottomSheet.Modal">
    <item name="android:background">@drawable/bottom_sheet_dialog_bg</item>
</style>

<style name="AppBottomSheetDialogTheme" parent="Theme.Design.Light.BottomSheetDialog">
    <item name="bottomSheetStyle">@style/BottomSheetStyle</item>
</style>

最后，创建一个顶部圆角可绘制资源(bottom_sheet_dialog_bg.xml)

<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">

    <solid android:color="@android:color/white" />
    <corners
        android:topLeftRadius="4dp"
        android:topRightRadius="4dp" />

</shape>

如果你需要setFitContents=true，我尝试了通过钩子onStateChanged解决方案，但一旦对话框达到扩展状态，它就会从直角闪烁到圆角。这很烦人。

有一种替代的解决方案，它不会导致闪烁，不需要使用私有api，而且更易于阅读(恕我冒犯)。

查看BottomSheetBehavior的代码，我们发现:

  /** True if Behavior has a non-null value for the @shapeAppearance attribute */
  private boolean shapeThemingEnabled;

事实证明，如果形状主题被禁用，MaterialShapeDrawable将不会被使用。我们在BottomSheetBehavior.onLayout()中找到了这个:

// Only set MaterialShapeDrawable as background if shapeTheming is enabled, otherwise will
// default to android:background declared in styles or layout.
if (shapeThemingEnabled && materialShapeDrawable != null) {
  ViewCompat.setBackground(child, materialShapeDrawable);
}

默认为android:background正是我们所需要的，因为这意味着完全控制如何渲染背景。

我们可以通过创建一个单独的样式来禁用材质主题，并将shapeAppearance和shapeAppearance overlay设置为null:

<style name="Theme.YourApp.NoShapeBottomSheetDialog" parent="Theme.MaterialComponents.BottomSheetDialog">
  <item name="bottomSheetStyle">@style/Theme.YourApp.NoShapeButtonSheet</item>
</style>

<style name="Theme.YourApp.NoShapeButtonSheet" parent="Widget.MaterialComponents.BottomSheet.Modal">
  <item name="shapeAppearance">@null</item>
  <item name="shapeAppearanceOverlay">@null</item>
</style>

扩展BottomSheetDialogFragment和覆盖onCreateDialog:

public Dialog onCreateDialog(@Nullable Bundle savedInstanceState) {
  return new BottomSheetDialog(requireContext(),
                R.style.Theme_Grupin_NoShapeBottomSheetDialog);
}

下面的表格现在是裸露的，没有任何背景。所以我们可以添加任何我们想要的背景，没有动画将被触发。

首先，你应该创建一个可绘制的XML文件，其中包含一个顶部圆角的形状，任何你想要的名称。 我将其命名为bottom rounded_top_shape.xml

<shape
xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle"
>
<solid android:color="@android:color/white" />
<corners
    android:topLeftRadius="16dp"
    android:topRightRadius="16dp"
    />

然后在style.xml中添加这个

<style name="AppBottomSheetDialogTheme" parent="Theme.MaterialComponents.Light.BottomSheetDialog">
    <item name="bottomSheetStyle">@style/AppModalStyle</item>
</style>

<style name="AppModalStyle" parent="Widget.Design.BottomSheet.Modal">
    <item name="android:background">@drawable/rounded_top_shape</item>
</style>

然后在你的应用主题中添加如下所示的行

 <style name="MyAppTheme" parent="Theme.MaterialComponents.Light.Bridge">
    <!-- this line -->
    <item name="bottomSheetDialogTheme">@style/AppBottomSheetDialogTheme</item>
</style>

在STATE_EXPANDED上禁用圆角平化的短解决方案:

@SuppressLint("RestrictedApi")
override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
    val dialog = super.onCreateDialog(savedInstanceState)
    
    //Disable animator that flats the rounded corners
    (dialog as BottomSheetDialog).behavior.disableShapeAnimations()

    return dialog
}

如果您正在使用材质组件主题，您可以通过将背景颜色设置为透明，然后将布局的背景设置为四舍五入来覆盖BottomSheetDialog的默认行为。

    <style name="Theme.MyApp" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <!-- Customize your theme here. -->
        <item name="bottomSheetDialogTheme">@style/BottomSheetDialog</item>
    </style>

    <style name="BottomSheetDialog" parent="ThemeOverlay.MaterialComponents.BottomSheetDialog">
        <item name="bottomSheetStyle">@style/bottomSheetStyleWrapper</item>
    </style>

    <style name="bottomSheetStyleWrapper" parent="Widget.MaterialComponents.BottomSheet.Modal">
        <item name="android:backgroundTint">@android:color/transparent</item>
    </style>

rounded_corner.xml:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <solid android:color="@color/white"/>
    <corners android:topLeftRadius="26dp"
        android:topRightRadius="26dp"/>

</shape>

然后将形状应用到你的底部表格根布局的背景:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/rounded_corner">

    <!-- 
       other layout components
     -->

</LinearLayout>

注意，如果你在底部表单布局中使用Webview这样的组件，你可能无法实现预期的圆角，因为Webview有一个默认的白色背景。在这种情况下，你可以像这样简单地删除背景:

webView.setBackgroundColor(0);

你必须改变底部的主题，以实现顶部圆形布局

创建一个自定义绘制background_bottom_sheet_dialog_fragment.xml:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
   android:shape="rectangle">
    <corners
       android:topLeftRadius="8dp"
        android:topRightRadius="8dp" />
    <padding android:top="0dp" />
    <solid android:color="@color/white" />
</shape>

然后在styles.xml上使用drawable作为背景覆盖bottomSheetDialogTheme:

<!--Bottom sheet-->
<style name="BottomSheet" parent="@style/Widget.Design.BottomSheet.Modal">
    <item 
    name="android:background">@drawable/background_bottom_sheet_dialog_fragment
    </item>
</style>

<style name="BaseBottomSheetDialog" 
    parent="@style/Theme.Design.Light.BottomSheetDialog">
    <item name="android:windowIsFloating">false</item>
    <item name="bottomSheetStyle">@style/BottomSheet</item>
</style>

<style name="BottomSheetDialogTheme" parent="BaseBottomSheetDialog" />

这将改变你的底页的背景布局

BottomSheetDialog

class SheetFragment() : BottomSheetDialogFragment() {

    lateinit var binding: SheetFragmentBinding;

  override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
    val dialog = super.onCreateDialog(savedInstanceState) as BottomSheetDialog;
    val view = View.inflate(context, R.layout.fragment_bottom_sheet, null);

    binding = DataBindingUtil.bind(view)!!;
    binding.viewModel = SheetFragmentVM();

    dialog.setContentView(view);

    var bottomSheetBehavior = BottomSheetBehavior.from(view.parent as View);
    bottomSheetBehavior.setPeekHeight(BottomSheetBehavior.PEEK_HEIGHT_AUTO);

    bottomSheetBehavior.setBottomSheetCallback(object : 
     BottomSheetBehavior.BottomSheetCallback() {
        override fun onStateChanged(bottomSheet: View, newState: Int) {
            if (BottomSheetBehavior.STATE_EXPANDED == newState) {
               // do on STATE_EXPANDED
            }
            if (BottomSheetBehavior.STATE_COLLAPSED == newState) {
                // do on STATE_COLLAPSED
            }

            if (BottomSheetBehavior.STATE_HIDDEN == newState) {
                dismiss()

            }
        }

        override fun onSlide(bottomSheet: View, slideOffset: Float) {
           // do on slide
        }
    })

    return dialog
}