MutableStateFlow 在启动应用程序或更改数据库后不发出值

Question

我是 Android 开发和使用房间库创建笔记应用程序的新手。我在 viewModel 中使用 MutableStateFlow 来观察数据库中的数据。但是 MutableStateFlow 在应用程序启动时以及数据库更改后不会发出值。

查看模型代码

package com.example.composenotes.ui.screens

import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.example.composenotes.data.Note
import com.example.composenotes.data.NotesRepository
import dagger.hilt.android.lifecycle.HiltViewModel
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.SharingStarted
import kotlinx.coroutines.flow.StateFlow
import kotlinx.coroutines.flow.stateIn
import javax.inject.Inject

@HiltViewModel
class HomeScreenViewModel @Inject constructor(notesRepository: NotesRepository) :
    ViewModel() {

    private val _notes: MutableStateFlow<List<Note>> = MutableStateFlow(
        notesRepository.getAllNotes().stateIn(
            scope = viewModelScope,
            started = SharingStarted.WhileSubscribed(5_000L),
            initialValue = listOf()
        ).value
    )


    val notes: StateFlow<List<Note>> = _notes
}

编写代码

package com.example.composenotes.ui.screens

import androidx.compose.foundation.clickable
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.items
import androidx.compose.material.icons.Icons
import androidx.compose.material.icons.filled.Add
import androidx.compose.material3.Card
import androidx.compose.material3.FloatingActionButton
import androidx.compose.material3.Icon
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.LaunchedEffect
import androidx.compose.runtime.collectAsState
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.platform.LocalContext
import androidx.compose.ui.text.font.FontWeight
import androidx.compose.ui.unit.dp
import androidx.compose.ui.unit.sp
import androidx.hilt.navigation.compose.hiltViewModel
import com.example.composenotes.data.Note
import com.example.composenotes.ui.NotesAppScreen
import com.example.composenotes.ui.ScaffoldState

@Composable
fun HomeScreen(
    onComposing: (ScaffoldState) -> Unit,
    onNoteClick: (note: Note) -> Unit,
    modifier: Modifier = Modifier,
    onFabClick: () -> Unit,
    viewModel: HomeScreenViewModel = hiltViewModel()
) {
    val context = LocalContext.current

    LaunchedEffect(key1 = Unit) {
        onComposing(
            ScaffoldState(
                topAppBarTitle = context.getString(NotesAppScreen.Start.title),
                fab = {
                    FloatingActionButton(onClick = onFabClick) {
                        Icon(imageVector = Icons.Filled.Add, contentDescription = "Create new note")
                    }
                })
        )
    }

    val notes = viewModel.notes.collectAsState()


    if (notes.value.isEmpty()) {
        NoNotesCreatedMessage()
    } else {
        AllNotes(
            notes = notes.value,
            onNoteClick = onNoteClick,
            modifier = modifier.padding(vertical = 16.dp)
        )
    }

}

@Composable
fun NoNotesCreatedMessage(modifier: Modifier = Modifier) {
    Box(modifier = modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
        Text(text = "No notes created yet!", fontSize = 24.sp, color = Color.LightGray)
    }
}

@Composable
fun AllNotes(notes: List<Note>, onNoteClick: (note: Note) -> Unit, modifier: Modifier = Modifier) {
    LazyColumn(modifier = modifier, content = {
        items(notes) {
            NoteCard(
                note = it, onNoteClick = onNoteClick, modifier = Modifier.padding(
                    horizontal = 16.dp, vertical = 4.dp
                )
            )
        }
    })
}

@Composable
fun NoteCard(note: Note, onNoteClick: (note: Note) -> Unit, modifier: Modifier = Modifier) {
    Card(modifier = modifier
        .clickable {
            onNoteClick(note)
        }
        .fillMaxWidth()) {
        Column(modifier = Modifier.padding(12.dp)) {
            Text(
                text = note.date,
                fontSize = 10.sp,
                modifier = Modifier.padding(bottom = 8.dp),
                color = Color.Gray
            )

            if (!note.title.isNullOrEmpty()) {
                Text(
                    text = note.title,
                    fontWeight = FontWeight.Bold,
                    maxLines = 1,
                    modifier = Modifier.padding(bottom = 4.dp),
                )
            }
            Text(
                text = note.description, maxLines = 1
            )
        }

    }
}

当我在 viewModel 中进行以下更改时，它正在工作。我想知道为什么它不能与 MutableStateFlow 一起使用

package com.example.composenotes.ui.screens

import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.example.composenotes.data.Note
import com.example.composenotes.data.NotesRepository
import dagger.hilt.android.lifecycle.HiltViewModel
import kotlinx.coroutines.flow.SharingStarted
import kotlinx.coroutines.flow.StateFlow
import kotlinx.coroutines.flow.stateIn
import javax.inject.Inject

@HiltViewModel
class HomeScreenViewModel @Inject constructor(notesRepository: NotesRepository) :
    ViewModel() {
    
    val notes: StateFlow<List<Note>> = notesRepository.getAllNotes().stateIn(
        scope = viewModelScope,
        started = SharingStarted.WhileSubscribed(5_000L),
        initialValue = listOf()
    )
}

Answer 1

更换：

private val _notes: MutableStateFlow<List<Note>> = MutableStateFlow(
    notesRepository.getAllNotes().stateIn(
        scope = viewModelScope,
        started = SharingStarted.WhileSubscribed(5_000L),
        initialValue = listOf()
    ).value
)


val notes: StateFlow<List<Note>> = _notes

致：

val notes: StateFlow<List<Note>> = notesRepository.getAllNotes().stateIn(
        scope = viewModelScope,
        started = SharingStarted.WhileSubscribed(5_000L),
        initialValue = listOf()
    )