当你使用byproperty delegate时，currentPage的类型不是MutableState<String>，而是String。它被MutableState支持的事实被隐藏了。你有两个选择：
1.删除by关键字并将currentPage用作MutableState：

val currentPage = remember { mutableStateOf("A") }
CrossSlide(targetState = currentPage.value) { screen ->
    when (screen) {
        "A" -> RegistrationScreen(currentPage)
        "B" -> LoginScreen()
    }
}

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1.保留by委托并向RegistrationScreen添加回调-这是首选，因为不建议将State作为函数参数。

@Composable
fun CrossSlideExample() {
    var currentPage by remember { mutableStateOf("A") }

    CrossSlide(targetState = currentPage) { screen ->
        when (screen) {
            "A" -> {
                RegistrationScreen(
                    currentPage = currentPage,
                    onCurrentPageChange = { currentPage = it },
                )
            }
            "B" -> LoginScreen()
        }
    }
}

@Composable
fun RegistrationScreen(
    currentPage: String, // you don't use the value itself here, so not needed
    onCurrentPageChange: (String) -> Unit,
){
    Button(onClick = {
        onCurrentPageChange("B")
    }) {
        Text(text = "GO TO LOGIN")
    }
}

型

问题出在var currentPage by remember { mutableStateOf("A") }语句中。
这里使用的是by委托，它提取值并将currentPage转换为String而不是MutableState<String>。
通过更新代码，

val currentPage = remember { mutableStateOf("A") }

        CrossSlide(targetState = currentPage.value) { screen ->
            when (screen) {
                "A" -> RegistrationScreen(currentPage)
                "B" -> LoginScreen()
            }
        }

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这样，通过避免by委托，您可以直接捕获MutableState对象。
参考：https://kotlinlang.org/docs/delegated-properties.html