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>

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! !

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。

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
}

Koma Yip回答的另一个问题对我有用，你应该试试。

在drawable中创建一个xml，比如dialog_bg.xml <？XML版本="1.0"编码="utf-8"?> <形状xmlns: android = " http://schemas.android.com/apk/res/android " > <固体android:颜色= " @color /白色" / > <corners android:半径="30dp" /> <填充 android:左= " 10 dp” android:顶级= " 10 dp” android:对= " 10 dp” android:底部= " 10 dp " / > < / >形状

把这个放在你的布局XML根节点中:

将其设置为布局XML中的背景 android:背景= " @drawable / dialog_bg”

在onCreateView()中放入:

将对话框的背景设置为透明 dialog.getWindow()。setBackgroundDrawable(新ColorDrawable (Color.TRANSPARENT));

底部对话框设置了默认的白色背景色，这就是为什么角落是不可见的，为了显示它们，你需要通过覆盖底部对话框的风格使对话框的背景透明。

在res/values/styles/styles.xml中定义此样式

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

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

并将此样式设置为您的BottomSheetDialog

View view = getLayoutInflater().inflate(R.layout.chooser_bottom_sheet, null);
BottomSheetDialog dialog = new BottomSheetDialog(this,R.style.BottomSheetDialog); // Style here
dialog.setContentView(view);
dialog.show();