我正在尝试按照 Google 提供的文档将 DatePicker 添加到我的 Android 应用程序中。我试图做到这一点，以便在按下按钮时出现选择器，允许用户输入日历日期。当我尝试在应用程序中加载此按钮时，它崩溃了，并在 logcat 中给出了 NullPointerException。我相信我需要扩充 XML 视图，但我不确定如何去做。
这是我的AssignmentEntryScreen.kt 文件，我正在其中尝试创建按钮（我已通过注释指示了按钮的相关代码所在位置：
//HEREobject AssignmentEntryDestination : NavigationDestination, ComponentActivity() {
override val route = "assignment_entry"
override val titleRes = R.string.assignment_entry_title
}
//Manages layout of the assignment entry screen
@OptIn(ExperimentalMaterial3Api::class)
@Composable
fun AssignmentEntryScreen(
navigateBack: () -> Unit,
onNavigateUp: () -> Unit,
canNavigateBack: Boolean = true,
viewModel: AssignmentEntryViewModel = viewModel(factory = AppViewModelProvider.Factory)
) {
val coroutineScope = rememberCoroutineScope()
//Title text
Scaffold(
topBar = {
InventoryTopAppBar(
title = stringResource(AssignmentEntryDestination.titleRes),
canNavigateBack = canNavigateBack,
navigateUp = onNavigateUp
)
}
//Body with data entry field and save button
) { innerPadding ->
AssignmentEntryBody(
assignmentUiState = viewModel.assignmentUiState,
onAssignmentValueChange = viewModel::updateUiState,
//Save button
onSaveClick = {
coroutineScope.launch {
viewModel.saveAssignment()
navigateBack()
}
},
modifier = Modifier
.padding(
start = innerPadding.calculateStartPadding(LocalLayoutDirection.current),
top = innerPadding.calculateTopPadding(),
end = innerPadding.calculateEndPadding(LocalLayoutDirection.current),
)
.verticalScroll(rememberScrollState())
.fillMaxWidth()
)
}
}
//Body function to create field entry boxes and save button
@Composable
fun AssignmentEntryBody(
assignmentUiState: AssignmentUiState,
onAssignmentValueChange: (AssignmentDetails) -> Unit,
onSaveClick: () -> Unit,
modifier: Modifier = Modifier
) {
Column(
modifier = modifier.padding(dimensionResource(id = R.dimen.padding_medium)),
verticalArrangement = Arrangement.spacedBy(dimensionResource(id = R.dimen.padding_large))
) {
//Name and due date inputs
AssignmentInputForm(
assignmentDetails = assignmentUiState.assignmentDetails,
onValueChange = onAssignmentValueChange,
modifier = Modifier.fillMaxWidth()
)
//Save button
Button(
onClick = onSaveClick,
enabled = assignmentUiState.isEntryValid,
shape = MaterialTheme.shapes.small,
modifier = Modifier.fillMaxWidth()
) {
Text(text = stringResource(R.string.save_action))
}
}
}
//Name and due date input forums
@Composable
fun AssignmentInputForm(
assignmentDetails: AssignmentDetails,
modifier: Modifier = Modifier,
onValueChange: (AssignmentDetails) -> Unit = {},
enabled: Boolean = true
) {
Column(
modifier = modifier,
verticalArrangement = Arrangement.spacedBy(dimensionResource(id = R.dimen.padding_medium))
) {
//Name forum
Text(text = "Assignment")
OutlinedTextField(
value = assignmentDetails.name,
onValueChange = { onValueChange(assignmentDetails.copy(name = it)) },
label = { Text(stringResource(R.string.assignment_name_req)) },
colors = OutlinedTextFieldDefaults.colors(
focusedContainerColor = MaterialTheme.colorScheme.secondaryContainer,
unfocusedContainerColor = MaterialTheme.colorScheme.secondaryContainer,
disabledContainerColor = MaterialTheme.colorScheme.secondaryContainer,
),
modifier = Modifier.fillMaxWidth(),
enabled = enabled,
singleLine = true
)
//HERE //Date Picker button.
setContentView(R.layout.datepicker)
findViewById<Button>(R.id.pickDate).setOnClickListener {
val newFragment = DatePickerFragment()
newFragment.show(getFragmentManager(),"datePicker")
}
}
}
@Preview(showBackground = true)
@Composable
private fun AssignmentEntryScreenPreview() {
InventoryTheme {
AssignmentEntryBody(assignmentUiState = AssignmentUiState(
AssignmentDetails(
name = "Assignment name", dueDate = Date(5/3/2024)
)
), onAssignmentValueChange = {}, onSaveClick = {})
}
}
datePickerFragment.kt 本身：
package com.example.assignmentNotifier
import android.app.DatePickerDialog
import android.app.Dialog
import android.app.DialogFragment
import android.os.Bundle
import android.widget.DatePicker
import com.example.assignmentNotifier.ui.assignment.AssignmentEntryDestination.setContentView
import java.text.SimpleDateFormat
import java.util.Calendar
import java.util.Locale
@Suppress("DEPRECATION", "OVERRIDE_DEPRECATION")
class DatePickerFragment : DialogFragment(), DatePickerDialog.OnDateSetListener {
private val calendar = Calendar.getInstance()
private val formatter = SimpleDateFormat("MMM. dd, yyyy", Locale.US)
override fun onCreateDialog(savedInstanceState: Bundle): Dialog {
// Use the current date as the default date in the picker.
setContentView(R.layout.datepicker)
val year = calendar.get(Calendar.YEAR)
val month = calendar.get(Calendar.MONTH)
val day = calendar.get(Calendar.DAY_OF_MONTH)
// Create a new instance of DatePickerDialog and return it.
return DatePickerDialog(activity, this, year, month, day)
}
override fun onDateSet(view: DatePicker, year: Int, month: Int, day: Int) {
calendar.set(year, month, day)
returnFormattedDate(calendar.timeInMillis)
}
private fun returnFormattedDate(timestamp: Long): String? {
return formatter.format(timestamp)
}
}
还有 R.layout.datepicker.xml 文件：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<Button
android:id="@+id/pickDate"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Pick date"/>
</LinearLayout>
我的理解是，我需要在调用
findViewById<Button>(R.id.pickDate)任何帮助将不胜感激。谢谢！
您不能像这样混合基于撰写和视图的 UI 层次结构。您可以设置可组合视图或设置根视图，但不能同时执行两者。因此，如果您使用 compose，则永远不会通过 setContentView 设置根视图。
如果您想在撰写层次结构中嵌入视图，则需要按照此处的步骤操作：https://developer.android.com/develop/ui/compose/migrate/interoperability-apis/views-in-compose基本上，您将其包装在合成中的 AndroidView 对象中。如果您需要膨胀 xml，则不会通过 setContentView 来完成。你用
LayoutInflater.from(context).inflate()