注：解决方式在最后。

一、问题复现

Compose Desktop，AlertDialog代码如下：

        AlertDialog(modifier = Modifier.clip(RoundedCornerShape(10.dp)).fillMaxHeight(0.3f).fillMaxWidth(0.35f).background(Color.White).padding(20.dp),title = null,text = { /* 略 */ },buttons = { /* 略 */ })

这里设置了RoundCornerShape圆角，弹出的对话框如图：

发现是没有圆角的。根据网上查到的信息，设置 AlertDialog 的背景为透明，无效。

当我给这个dialog设置背景色，如：

AlertDialog(modifier = Modifier.clip(RoundedCornerShape(10.dp)).fillMaxHeight(0.3f).fillMaxWidth(0.35f).background(Color.Red)  // 设置背景色为红色.padding(20.dp),title = null,text = { /* 略 */ },buttons = { /* 略 */ })

结果如图：

可以看出，这多半是 AlertDialog 父组件的背景色，而不是 AlertDialog 自身的原因。

二、原因分析

根据 AlertDialog 的源码来看，其界面是通过 dialogProvider 来提供的。

fun AlertDialog(onDismissRequest: () -> Unit,buttons: @Composable () -> Unit,modifier: Modifier = Modifier,title: (@Composable () -> Unit)? = null,text: @Composable (() -> Unit)? = null,shape: Shape = MaterialTheme.shapes.medium,backgroundColor: Color = MaterialTheme.colors.surface,contentColor: Color = contentColorFor(backgroundColor),dialogProvider: AlertDialogProvider = PopupAlertDialogProvider
) {with(dialogProvider) {AlertDialog(onDismissRequest = onDismissRequest) {AlertDialogContent(buttons = buttons,modifier = modifier.width(IntrinsicSize.Min),title = title,text = text,shape = shape,backgroundColor = backgroundColor,contentColor = contentColor)}}
}

而 dialogProvider 的默认实现是 PopupAlertDialogProvider，其中界面部分的代码如下：

Box(modifier = Modifier.fillMaxSize().background(scrimColor).pointerInput(onDismissRequest) {detectTapGestures(onPress = { onDismissRequest() })},contentAlignment = Alignment.Center
) {Surface(Modifier.pointerInput(onDismissRequest) {detectTapGestures(onPress = {// Workaround to disable clicks on Surface background https://github.com/JetBrains/compose-jb/issues/2581})}, elevation = 24.dp) {content()}
}

其中，对话框就是其中的 Surface 部分。而 Surface 的默认背景颜色就是白色。
问题找到了，就好解决了。

三、解决

1. 新建一个 CustomDialogProvider，并复制 PopupAlertDialogProvider 的代码。
   将其中的 Surface 背景改为透明。

import androidx.compose.foundation.background
import androidx.compose.foundation.gestures.detectTapGestures
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.material.AlertDialogProvider
import androidx.compose.material.ExperimentalMaterialApi
import androidx.compose.material.Surface
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.awt.awtEventOrNull
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.input.key.KeyEventType
import androidx.compose.ui.input.key.type
import androidx.compose.ui.input.pointer.pointerInput
import androidx.compose.ui.unit.*
import androidx.compose.ui.window.Popup
import androidx.compose.ui.window.PopupPositionProvider
import java.awt.event.KeyEvent@OptIn(ExperimentalMaterialApi::class)
object CustonDialogProvider : AlertDialogProvider {@Composableoverride fun AlertDialog(onDismissRequest: () -> Unit, content: @Composable () -> Unit) {// Popups on the desktop are by default embedded in the component in which// they are defined and aligned within its bounds. But an [AlertDialog] needs// to be aligned within the window, not the parent component, so we cannot use// [alignment] property of [Popup] and have to use [Box] that fills all the// available space. Also [Box] provides a dismiss request feature when clicked// outside of the [AlertDialog] content.Popup(popupPositionProvider = object : PopupPositionProvider {override fun calculatePosition(anchorBounds: IntRect,windowSize: IntSize,layoutDirection: LayoutDirection,popupContentSize: IntSize): IntOffset = IntOffset.Zero},focusable = true,onDismissRequest = onDismissRequest,onKeyEvent = {if (it.type == KeyEventType.KeyDown && it.awtEventOrNull?.keyCode == KeyEvent.VK_ESCAPE) {onDismissRequest()true} else {false}},) {val scrimColor = Color.Black.copy(alpha = 0.32f) //todo configure scrim color in function argumentsBox(modifier = Modifier.fillMaxSize().background(scrimColor).pointerInput(onDismissRequest) {detectTapGestures(onPress = { onDismissRequest() })},contentAlignment = Alignment.Center) {Surface(Modifier.pointerInput(onDismissRequest) {detectTapGestures(onPress = {// Workaround to disable clicks on Surface background})},elevation = 24.dp,color = Color.Transparent  // 修改背景色为透明) {content()}}}}
}

2. 在 AlertDialog 参数中，加入自定义的 DialogProvider。

        AlertDialog(modifier = Modifier.clip(RoundedCornerShape(10.dp)).fillMaxHeight(0.3f).fillMaxWidth(0.35f).background(Color.White).padding(20.dp),dialogProvider = CustonDialogProvider,  // 添加自定义的 DialogProvidertitle = null,text = { /* 略 */ },buttons = { /* 略 */ })

再次运行代码，发现圆角正常显示了：

当然，我们也可以通过在 Theme 中设置 Surface 的颜色，达到相同的效果：

    MaterialTheme(colors = MaterialTheme.colors.copy(surface = Color.Transparent)) {// 略}

不过这样就会改变所有Surface 的颜色了。