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>

这对我很管用。

创建一个背景绘图(例如命名为shape_rounded_dialog):

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

    <solid android:color="@color/color_white" />
    <corners android:topLeftRadius="16dp"
             android:topRightRadius="16dp" />
</shape>

添加下面的样式:

<style name="AppBottomSheetDialogTheme" 
       parent="Theme.MaterialComponents.Light.BottomSheetDialog">

    <item name="bottomSheetStyle">@style/CustomBottomSheetStyle</item>
</style>

<style name="CustomBottomSheetStyle" 
       parent="Widget.Design.BottomSheet.Modal">

    <item name="android:background">@drawable/shape_rounded_dialog</item>
</style>

在你的DialogFragment中，重写getTheme()方法来返回你的样式。

@Override
public int getTheme() {
    return R.style.AppBottomSheetDialogTheme;
}

使用新的材质组件库，您可以使用样式中的shapeAppearanceOverlay属性自定义组件的形状(注意:它至少需要1.1.0版本)

只需使用BottomSheetDialogFragment覆盖onCreateView方法，然后为BottomSheet对话框定义自定义样式。

在你的应用主题的styles.xml中定义bottomSheetDialogTheme属性:

  <!-- Base application theme. -->
  <style name="AppTheme" parent="Theme.MaterialComponents.Light">
    <!-- Customize your theme here. -->
    <item name="colorPrimary">@color/colorPrimary</item>
    ....
    <item name="bottomSheetDialogTheme">@style/CustomBottomSheetDialog</item>
  </style>

然后用shapeAppearanceOverlay定义你最喜欢的形状

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

  <style name="CustomBottomSheet" parent="Widget.MaterialComponents.BottomSheet">
    <item name="shapeAppearanceOverlay">@style/CustomShapeAppearanceBottomSheetDialog</item>
  </style>

  <style name="CustomShapeAppearanceBottomSheetDialog" parent="">
    <item name="cornerFamily">rounded</item>
    <item name="cornerSizeTopRight">16dp</item>
    <item name="cornerSizeTopLeft">16dp</item>
    <item name="cornerSizeBottomRight">0dp</item>
    <item name="cornerSizeBottomLeft">0dp</item>
  </style>

你可以在你的BottomSheetDialogFragment中获得相同的行为覆盖这个方法(而不是在你的应用主题中添加bottomSheetDialogTheme):

@Override public int getTheme() {
    return R.style.CustomBottomSheetDialog;
  }

在这种情况下，你只在一个BottomSheetDialogFragment中使用这个themeOverlay，而不是在所有的应用程序中。

关于EXPANDED STATE的重要注意事项:

在展开状态下，BottomSheet有平角。你可以在github repo查看官方评论:

我们的设计团队坚持认为圆角表示可滚动内容，而平角表示没有额外的内容。因此，他们不希望我们用fitToContents添加这个更改。

这个行为是由BottomSheetBehavior提供的，不可能重写它。 然而，有一个解决办法->免责声明:它可以停止工作在下一个版本!!

你可以在BottomSheetDialogFragment中添加一个BottomSheetCallback:

  @NonNull @Override public Dialog onCreateDialog(@Nullable Bundle savedInstanceState) {
    Dialog dialog = super.onCreateDialog(savedInstanceState);


    ((BottomSheetDialog)dialog).getBehavior().addBottomSheetCallback(new BottomSheetBehavior.BottomSheetCallback() {

      @Override public void onStateChanged(@NonNull View bottomSheet, int newState) {
        if (newState == BottomSheetBehavior.STATE_EXPANDED) {
          //In the EXPANDED STATE apply a new MaterialShapeDrawable with rounded cornes
          MaterialShapeDrawable newMaterialShapeDrawable = createMaterialShapeDrawable(bottomSheet);
          ViewCompat.setBackground(bottomSheet, newMaterialShapeDrawable);
        }
      }

      @Override public void onSlide(@NonNull View bottomSheet, float slideOffset) {

      }
    });

    return dialog;
  }

  @NotNull private MaterialShapeDrawable createMaterialShapeDrawable(@NonNull View bottomSheet) {
    ShapeAppearanceModel shapeAppearanceModel =

      //Create a ShapeAppearanceModel with the same shapeAppearanceOverlay used in the style
      ShapeAppearanceModel.builder(getContext(), 0, R.style.CustomShapeAppearanceBottomSheetDialog)
        .build();

      //Create a new MaterialShapeDrawable (you can't use the original MaterialShapeDrawable in the BottoSheet)
      MaterialShapeDrawable currentMaterialShapeDrawable = (MaterialShapeDrawable) bottomSheet.getBackground();
      MaterialShapeDrawable newMaterialShapeDrawable = new MaterialShapeDrawable((shapeAppearanceModel));
      //Copy the attributes in the new MaterialShapeDrawable
      newMaterialShapeDrawable.initializeElevationOverlay(getContext());
      newMaterialShapeDrawable.setFillColor(currentMaterialShapeDrawable.getFillColor());
      newMaterialShapeDrawable.setTintList(currentMaterialShapeDrawable.getTintList());
      newMaterialShapeDrawable.setElevation(currentMaterialShapeDrawable.getElevation());
      newMaterialShapeDrawable.setStrokeWidth(currentMaterialShapeDrawable.getStrokeWidth());
      newMaterialShapeDrawable.setStrokeColor(currentMaterialShapeDrawable.getStrokeColor());
      return newMaterialShapeDrawable;
  }

创建一个名为rounded_corners_shape的形状

<shape xmlns:android="http://schemas.android.com/apk/res/android"
       android:shape="rectangle">
    <corners
        android:topLeftRadius="8dp"
        android:topRightRadius="8dp"/>
    <solid android:color="@color/white"/>

</shape>

定义一个样式

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

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

在你的自定义BottomSheetDialogFragment上使用这种风格，它会工作的!

 public class CustomDialogFragment extends BottomSheetDialogFragment {
      @Override
      public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setStyle(STYLE_NORMAL, R.style. AppBottomSheetDialogTheme);
      }

      ...
    }

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
}

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
}