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
}

创建一个自定义绘制的rounded_dialog.xml:

<?xml version="1.0" encoding="utf-8"?>
<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"/>

</shape>

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

<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">       
    <item name="bottomSheetDialogTheme">@style/AppBottomSheetDialogTheme</item>
</style>

<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_dialog</item>
</style>

这将改变你的应用程序的所有底部对话框。

我知道这个问题已经有了一个公认的答案。我想要记录我经历的问题，以及我最终是如何让它工作的，这样它对未来的人是有用的。

Firstly, I was using Theme.AppCompat.Light.DarkActionBar as the parent for our AppTheme. This meant @Gabriele Mariotti solution kept crashing with the error Could not inflate Behavior subclass com.google.android.material.bottomsheet.BottomSheetBehavior. I fixed this by simply changing the parent to Theme.MaterialComponents.Light.DarkActionBar. This did not affect our theme in any way but the RTE was gone. You can also fix this issue by simply including the require items to your style. But I didn't bother figuring out which styles were required by BottomSheetBehavior.

其次，尝试我可能，但我不能得到实际的框架布局(这是BottomSheetDialogFragment)使用圆角。我意识到，将此设置为图像Drawable工作，但不适用于形状或@null。事实证明，这是因为我使用的LinearLayout有一个定义的背景。这覆盖了样式中的任何背景。去掉它最终会产生圆角。

此外，我不需要设置任何背景形状来圆角。@Gabriele Mariotti的解决方案在我做出上述更改后立即起作用。然而，为了设置我想要的背景色，我必须覆盖“backgroundTint”项。

PS:我是Android开发的新手，我正在维护一个旧的应用程序，这是我们学院内部使用的。我不是很熟悉Android的布局系统或材料库。我想这就是为什么我花了3天时间才弄明白。我希望这对将来的人有用。

正如在其他答案中指出的，当state为BottomSheetBehavior时。STATE_EXPANDED角将被压平。

你可以通过设置BottomSheetBehavior的peekHeight属性并使用自定义样式来克服这个问题。

abstract class BaseBottomSheetFragment : BottomSheetDialogFragment(){

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    
        if (state == BottomSheetBehavior.STATE_EXPANDED) {
            val displayMetrics = DisplayMetrics()
            requireActivity().windowManager!!.defaultDisplay!!.getMetrics(displayMetrics)
            (dialog as BottomSheetDialog).behavior.peekHeight = displayMetrics.heightPixels
        } else {
            (dialog as BottomSheetDialog).behavior.state = state
        }
    }

    override fun getTheme(): Int {
        return R.style.CustomBottomSheetDialog
    }
}

CustomBottomSheetDialog风格

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

<style name="CustomMaterialButtonStyle" parent="@style/Widget.MaterialComponents.Button">
    <item name="cornerRadius">@dimen/dialog_bottom_radius</item>
</style>

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

<style name="CustomShapeAppearanceBottomSheetDialog" parent="">
    <item name="android:background">@android:color/transparent</item>
    <item name="backgroundTint">@android:color/transparent</item>
    <item name="cornerFamily">rounded</item>
    <item name="cornerSizeTopRight">@dimen/dialog_bottom_radius</item>
    <item name="cornerSizeTopLeft">@dimen/dialog_bottom_radius</item>
    <item name="cornerSizeBottomRight">0dp</item>
    <item name="cornerSizeBottomLeft">0dp</item>
</style>

在你的BottomsheetDialogFragment类中添加这两个方法。

public void setDialogBorder(Dialog dialog) {
        FrameLayout bottomSheet = (FrameLayout) dialog.getWindow().findViewById(android.support.design.R.id.design_bottom_sheet);
        bottomSheet.setBackground(new ColorDrawable(Color.TRANSPARENT));
        setMargins(bottomSheet, 10, 0, 10, 20);
    }

    private void setMargins(View view, int left, int top, int right, int bottom) {
        if (view.getLayoutParams() instanceof ViewGroup.MarginLayoutParams) {
            ViewGroup.MarginLayoutParams p = (ViewGroup.MarginLayoutParams) view.getLayoutParams();
            p.setMargins(left, top, right, bottom);
            view.requestLayout();
        }
    }

现在调用你的BottomsheetDialogFragment类的setupDialog()方法中的setDialogBorder(dialog)方法。

现在在可绘制文件夹中创建一个形状文件。

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <corners android:radius="20dp" />

    <solid android:color="@color/white" />
    <stroke
        android:width="1dp"
        android:color="@color/transparent" />
</shape>

现在在xml文件中设置父视图组对话框视图的背景。

android:background="@drawable/round_border_white"

完成了! !

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;
  }