Jetpack Compose – LazyColumn 不重组

Question

我的 LazyColumn 没有重组，但值正在更新。

如果我向下滚动列表并向上滚动，我会看到 UI

的正确值

主要活动

class MainActivity : AppCompatActivity() {


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MyTheme {
                MyApp()
            }
        }
    }
}

// Start building your app here!
@Composable
fun MyApp(vm: PuppyListViewModel =  viewModel()) {
    val puppers by vm.pups.collectAsState(emptyList())
    Surface(color = MaterialTheme.colors.background) {
        Column {
            Toolbar()
            LazyColumn {
                items(puppers) { pup ->  PuppyUI(pup, vm::seeDetails, vm::togglePuppyAdoption) }
            }
        }
    }
}

视图模型

class PuppyListViewModel : ViewModel() {

    val pups = PuppyRepo.getPuppies().onEach {
        println("FlowEmitted: $it")
    }

    fun togglePuppyAdoption(puppy: Puppy) = viewModelScope.launch {
        PuppyRepo.toggleAdoption(puppy.id)
    }

    fun seeDetails(puppy: Puppy) {
        println("seeDetails $puppy")
    }
}

模特


internal var IDS = 0L

data class Puppy (
    val name: String,
    val tagline: String = "",
    val race: String,
    @DrawableRes val image: Int,
    var adopted: Boolean = false,
    val id: Long = ++IDS,
)

存储库

object PuppyRepo {
    private val changeFlow = MutableStateFlow(0)
    private val pups: List<Puppy>

    private val puppyImages = listOf(
        R.drawable._1,
        R.drawable._2,
        R.drawable._3,
        R.drawable._4,
        R.drawable._5,
        R.drawable._6,
        R.drawable._7,
        R.drawable._8,
        R.drawable._9,
        R.drawable._10,
        R.drawable._11,
        R.drawable._12,
        R.drawable._13,
        R.drawable._14,
        R.drawable._15,
        R.drawable._16,
        R.drawable._17,
        R.drawable._18,
        R.drawable._19,
    )


    private val puppyNames = listOf(
        "Gordie",
        "Alice",
        "Belle",
        "Olivia",
        "Bubba",
        "Pandora",
        "Bailey",
        "Nala",
        "Rosco",
        "Butch",
        "Matilda",
        "Molly",
        "Piper",
        "Kelsey",
        "Rufus",
        "Duke",
        "Ozzy"
    )

    private val puppyTags = listOf(
        "doggo",
        "doge",
        "special dogo",
        "wrinkler",
        "corgo",
        "shoob",
        "puggo",
        "pupper",
        "small dogo",
        "big ol dogo",
        "woofer",
        "floofer",
        "yapper",
        "pupper",
        "good-boye",
        "grizlord",
        "snip-snap dogo"
    )

    private val puppyBreeds = listOf(
        "Labrador Retriever",
        "German Shepard",
        "Golden Retriever",
        "French Bulldog",
        "Bulldog",
        "Beagle",
        "Poodle",
        "Rottweiler",
        "German Shorthaired Pointer",
        "Yorkshire Terrier",
        "Boxer"
    )

    init {
        pups = puppyImages.map { image ->
            val name = puppyNames.random()
            val tagline = puppyTags.random()
            val breed = puppyBreeds.random()
            Puppy(name, tagline, breed, image)
        }
    }

    @OptIn(ExperimentalCoroutinesApi::class)
    fun getPuppies() = changeFlow.flatMapLatest { flowOf(pups) }

    fun getPuppy(puppyId: Long) = flow {
        emit(pups.find { it.id == puppyId })
    }


    suspend fun toggleAdoption(puppyId: Long): Boolean {
        val found = getPuppy(puppyId).first()?.toggleAdoption()?.let { true } ?: false
        if (found) {
            // Trigger a new emission for those that are consuming a Flow from getPuppies
            changeFlow.value = changeFlow.value + 1
        }
        return found
    }


    private fun Puppy.toggleAdoption() {
        adopted = !adopted
    }

}

Flow 小狗正在生成更新值，如您在我的 logcat

中所见

我已经在我的可组合项上放置了打印语句，但在流发出新值后它们没有重新组合。

编辑。

Lookslike Compose 会比较对象的引用，因为那些没有改变，即使流发出新值也不会发生重组（也许是 Compose 上的错误？）

更改了 toggle 功能以重新创建列表元素的实例，如下所示，现在可以使用了。

注意： 我已经将 Puppy.adopted 设为 val 而不是 var


suspend fun toggleAdoption(puppyId: Long): Boolean {
    var found = false
    pups = pups.map {
        val isThePuppy = it.id == puppyId
        found = found || isThePuppy
        if(isThePuppy) it.copy(adopted = !it.adopted) else it.copy()
    }
    if (found) {
        // Trigger a new emission for those that are consuming a Flow from getPuppies
        changeFlow.value = changeFlow.value + 1
    }
    return found
}

Answer 1

The Flow pups is producing updated values as you can see in my logcat

不完全是。

Flow 发出相同 List 的相同 Puppy 对象。我相信 Compose 看到 List 与以前的 List 对象相同，并假定没有变化。

我建议的更改：

使 Puppy 成为不可变的 data class（即没有 var 属性）
摆脱 changeFlow 并拥有 getPuppies() return 一个稳定的 MutableStateFlow<List<Puppy>>（或者让它只是一个 public 属性)
在 toggleAdoption() 中，创建一个新的 Puppy 对象列表并使用它来更新 MutableStateFlow<List<Puppy>>:

    suspend fun toggleAdoption(puppyId: Long) {
        val current = puppies.value // assumes that puppies is a MutableSharedFlow<List<Puppy>>

        val replacement = current.map { if (it.id == puppyId) it.copy(adopted = !it.adopted) else it }

        puppies.value = replacement
    }

Answer 2

这对我有用。

import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.setValue
import androidx.lifecycle.ViewModel

class MyViewModel : ViewModel() {
    var selectables: List<Selectable> by mutableStateOf(List(100) { Selectable(name = "$it") })
        private set

    fun onTapped(tappedItem: Selectable) {
        val index = selectables.indexOf(tappedItem)
        selectables = selectables.toMutableList().also {
            it[index] = tappedItem.copy(selected = !tappedItem.selected)
        }
    }
}

data class Selectable(
    val name: String,
    var selected: Boolean = false,
)

关键部分是：

重新分配列表而不是就地修改它（例如，将 selectables 设为 MutableList 并执行 selectables[index] = tappedItem.copy(selected = !tappedItem.selected) 是行不通的）
重新分配所选项目而不是就地修改它，例如以下行不通

selectables = selectables.toMutableList().also {
    it[index].selected = !tappedItem.selected
}

请注意，您无法使数据 class 不可变，但是，使其不可变将强制您必须制作元素的副本为了更新它。

Jetpack Compose – LazyColumn 不重组

Jetpack Compose – LazyColumn not recomposing

android

kotlin

kotlin-coroutines

android-jetpack-compose

kotlin-flow

编辑。