composable("page3{name}/? a={age}", null cannot be cast to non-null type kotlin.String

这个错误提示是 Kotlin 的 null 安全机制提醒你，表明你在将一个 null 值转换为非空字符串时出现了问题。

具体来说，这个错误可能是由于传递给 `page3` 函数的 `name` 参数为 null 所导致的。为了解决这个问题，你需要确保 `name` 参数不为 null。你可以使用 Kotlin 的安全调用操作符（`?.`）来避免 null 值引起的错误，例如：

val name: String? = null
val age: Int = 30
val url = composable("page3${name?.let { "/$it" }}/?a=$age")

这里使用了 `let` 函数来检查 `name` 是否为 null，如果不为 null 则在字符串中加上 `"/$it"`，否则返回 null。然后使用安全调用操作符 `?.` 来避免空指针异常。

相关问题

package com.example.handwriting import android.os.Bundle import androidx.activity.ComponentActivity import androidx.activity.compose.setContent import androidx.compose.foundation.layout.fillMaxSize import androidx.compose.material3.MaterialTheme import androidx.compose.material3.Surface import androidx.compose.material3.Text import androidx.compose.runtime.Composable import androidx.compose.ui.Modifier import androidx.compose.ui.tooling.preview.Preview import com.example.handwriting.ui.theme.HandWritingTheme class MainActivity : ComponentActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContent { HandWritingTheme { // A surface container using the 'background' color from the theme Surface( modifier = Modifier.fillMaxSize(), color = MaterialTheme.colorScheme.background ) { Greeting("Android") } } } } } @Composable fun Greeting(name: String, modifier: Modifier = Modifier) { Text( text = "Hello $name!", modifier = modifier ) } @Preview(showBackground = true) @Composable fun GreetingPreview() { HandWritingTheme { Greeting("Android") } }请加 button click 事件 ，启动新activity

好的，我可以帮您在这段代码中添加button click事件，启动新的activity。请参考下面的代码：

package com.example.handwriting

import android.content.Intent
import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.material3.Button
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview
import com.example.handwriting.ui.theme.HandWritingTheme

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            HandWritingTheme {
                // A surface container using the 'background' color from the theme
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    color = MaterialTheme.colorScheme.background
                ) {
                    Greeting("Android")
                    LaunchNewActivityButton()
                }
            }
        }
    }
}

@Composable
fun Greeting(name: String, modifier: Modifier = Modifier) {
    Text(
        text = "Hello $name!",
        modifier = modifier
    )
}

@Composable
fun LaunchNewActivityButton() {
    Button(
        onClick = {
            val intent = Intent(
                LocalContext.current,
                NewActivity::class.java
            )
            startActivity(intent)
        },
        modifier = Modifier.padding(top = 16.dp)
    ) {
        Text("Launch New Activity")
    }
}

@Preview(showBackground = true)
@Composable
fun GreetingPreview() {
    HandWritingTheme {
        Greeting("Android")
    }
}

在这个代码中，我们首先在onCreate()方法中添加了一个新的组件LaunchNewActivityButton()，它是在Material Design组件库中的Button组件的基础上实现的。在这个组件中，我们创建了一个新的Intent对象，指定了当前activity为MainActivity，目标activity为NewActivity，并启动了这个新的activity。您可以根据实际情况，修改NewActivity为您需要启动的目标activity的名称。

安卓开发使用compose 版本 组件 LazyColumn 要实现拖动排序功能 Modifier.draggable() 使用示例kotlin完整代码

非常感谢您的提问！以下是您需要的 Kotlin 代码示例：

import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.itemsIndexed
import androidx.compose.runtime.mutableStateListOf
import androidx.compose.ui.Modifier
import androidx.compose.ui.gesture.scrollorientationlocking.Orientation
import androidx.compose.ui.input.pointer.pointerInput
import androidx.compose.ui.unit.DpOffset

enum class DragState {
    IDLE, DRAGGING
}

@Composable
fun LazyColumnWithDragAndDrop() {
    val items = mutableStateListOf("Item 1", "Item 2", "Item 3")

    var dragState = DragState.IDLE
    var dragIndex by remember { mutableStateOf(-1) }
    var targetIndex by remember { mutableStateOf(-1) }

    LazyColumn {
        itemsIndexed(items) { index, item ->
            val backgroundColor = if (dragIndex == index) {
                Color.Gray
            } else if (targetIndex == index && dragState == DragState.DRAGGING) {
                Color.LightGray
            } else {
                Color.White
            }

            Box(
                Modifier
                    .background(backgroundColor)
                    .fillMaxWidth()
                    .height(50.dp)
                    .pointerInput(Unit) {
                        detectDragGesture { state ->
                            val layoutCoordinates = localLayoutCoordinates
                            val position = layoutCoordinates.offset(state.position)
                            when (state.draggedOrUp) {
                                Dragged -> {
                                    dragIndex = index
                                    dragState = DragState.DRAGGING
                                }
                                Up -> {
                                    if (dragState == DragState.DRAGGING) {
                                        items.move(dragIndex, targetIndex)
                                    }
                                    dragIndex = -1
                                    targetIndex = -1
                                    dragState = DragState.IDLE
                                }
                                Dragging -> {
                                    if (dragState == DragState.DRAGGING) {
                                        val targetY = position.y
                                        val targetIndexTemp = if (targetY < 0) {
                                            0
                                        } else {
                                            val itemCount = items.size
                                            val lastItemPosition = layoutCoordinates.positionOf(Int.MAX_VALUE)
                                            val maxTargetY = lastItemPosition.y
                                            val top = layoutCoordinates.positionOf(0).y
                                            val bottom = maxTargetY + (50.dp.toPx() / 2)
                                            val newTargetY = targetY.coerceIn(top, bottom)
                                            val newTargetIndex = ((newTargetY - (50.dp.toPx() / 2)) / 50.dp.toPx()).toInt()
                                            newTargetIndex.coerceIn(0, itemCount - 1)
                                        }

                                        if (targetIndex != targetIndexTemp) {
                                            targetIndex = targetIndexTemp
                                        }
                                    }
                                }
                                else -> {
                                    dragIndex = -1
                                    targetIndex = -1
                                    dragState = DragState.IDLE
                                }
                            }
                        }
                    }
                    .draggable(
                        orientation = Orientation.Vertical,
                        onDragStarted = {
                            dragIndex = index
                            dragState = DragState.DRAGGING
                        },
                        onDragStopped = {
                            if (dragState == DragState.DRAGGING) {
                                items.move(dragIndex, targetIndex)
                            }
                            dragIndex = -1
                            targetIndex = -1
                            dragState = DragState.IDLE
                        },
                        onDragEnd = {
                            dragIndex = -1
                            targetIndex = -1
                            dragState = DragState.IDLE
                        }
                    )
            ) {
                Text(
                    text = item,
                    modifier = Modifier.align(Alignment.CenterStart)
                )
            }
        }
    }
}

private fun <T> MutableList<T>.move(from: Int, to: Int) {
    val fromItem = this[from]
    removeAt(from)
    add(to, fromItem)
}

这里是一个使用 `Modifier.draggable()` 实现拖动排序的 Android Compose 组件示例。在此代码中，我们使用 `pointerInput()` 拦截滚动事件以捕获拖动手势。拖动时，我们根据每个 item 的位置计算出应该被交换的 item 的索引，然后使用 `MutableList.move()` 实现了交换操作。您可以将此代码与您的项目中使用 Compose 的其他部分集成，以实现拖动排序的功能。

希望这个示例对您有所帮助！如果您有任何疑问，请随时问我。