シーンを使用してカスタム レイアウトを作成する

コレクションでコンテンツを整理 必要に応じて、コンテンツの保存と分類を行います。

@Composable
public fun calculateScene(
    entries: List<NavEntry<T>>,
    onBack: (count: Int) -> Unit,
): Scene<T>?
ScenesSnippets.kt

data class SinglePaneScene<T : Any>(
    override val key: T,
    val entry: NavEntry<T>,
    override val previousEntries: List<NavEntry<T>>,
) : Scene<T> {
    override val entries: List<NavEntry<T>> = listOf(entry)
    override val content: @Composable () -> Unit = { entry.content.invoke(entry.key) }
}

/**
 * A [SceneStrategy] that always creates a 1-entry [Scene] simply displaying the last entry in the
 * list.
 */
public class SinglePaneSceneStrategy<T : Any> : SceneStrategy<T> {
    @Composable
    override fun calculateScene(entries: List<NavEntry<T>>, onBack: (Int) -> Unit): Scene<T> =
        SinglePaneScene(
            key = entries.last().key,
            entry = entries.last(),
            previousEntries = entries.dropLast(1)
        )
}
ScenesSnippets.kt

// --- TwoPaneScene ---
/**
 * A custom [Scene] that displays two [NavEntry]s side-by-side in a 50/50 split.
 */
class TwoPaneScene<T : Any>(
    override val key: Any,
    override val previousEntries: List<NavEntry<T>>,
    val firstEntry: NavEntry<T>,
    val secondEntry: NavEntry<T>
) : Scene<T> {
    override val entries: List<NavEntry<T>> = listOf(firstEntry, secondEntry)
    override val content: @Composable (() -> Unit) = {
        Row(modifier = Modifier.fillMaxSize()) {
            Column(modifier = Modifier.weight(0.5f)) {
                firstEntry.content.invoke(firstEntry.key)
            }
            Column(modifier = Modifier.weight(0.5f)) {
                secondEntry.content.invoke(secondEntry.key)
            }
        }
    }

    companion object {
        internal const val TWO_PANE_KEY = "TwoPane"
        /**
         * Helper function to add metadata to a [NavEntry] indicating it can be displayed
         * in a two-pane layout.
         */
        fun twoPane() = mapOf(TWO_PANE_KEY to true)
    }
}

// --- TwoPaneSceneStrategy ---
/**
 * A [SceneStrategy] that activates a [TwoPaneScene] if the window is wide enough
 * and the top two back stack entries declare support for two-pane display.
 */
class TwoPaneSceneStrategy<T : Any> : SceneStrategy<T> {
    @OptIn(ExperimentalMaterial3AdaptiveApi::class, ExperimentalMaterial3WindowSizeClassApi::class)
    @Composable
    override fun calculateScene(
        entries: List<NavEntry<T>>,
        onBack: (Int) -> Unit
    ): Scene<T>? {

        val windowSizeClass = currentWindowAdaptiveInfo().windowSizeClass

        // Condition 1: Only return a Scene if the window is sufficiently wide to render two panes.
        // We use isWidthAtLeastBreakpoint with WIDTH_DP_MEDIUM_LOWER_BOUND (600dp).
        if (!windowSizeClass.isWidthAtLeastBreakpoint(WIDTH_DP_MEDIUM_LOWER_BOUND)) {
            return null
        }

        val lastTwoEntries = entries.takeLast(2)

        // Condition 2: Only return a Scene if there are two entries, and both have declared
        // they can be displayed in a two pane scene.
        return if (lastTwoEntries.size == 2 &&
            lastTwoEntries.all { it.metadata.containsKey(TwoPaneScene.TWO_PANE_KEY) }
        ) {
            val firstEntry = lastTwoEntries.first()
            val secondEntry = lastTwoEntries.last()

            // The scene key must uniquely represent the state of the scene.
            val sceneKey = Pair(firstEntry.key, secondEntry.key)

            TwoPaneScene(
                key = sceneKey,
                // Where we go back to is a UX decision. In this case, we only remove the top
                // entry from the back stack, despite displaying two entries in this scene.
                // This is because in this app we only ever add one entry to the
                // back stack at a time. It would therefore be confusing to the user to add one
                // when navigating forward, but remove two when navigating back.
                previousEntries = entries.dropLast(1),
                firstEntry = firstEntry,
                secondEntry = secondEntry
            )
        } else {
            null
        }
    }
}
ScenesSnippets.kt

// Define your navigation keys
@Serializable
data object ProductList : NavKey
@Serializable
data class ProductDetail(val id: String) : NavKey

@Composable
fun MyAppContent() {
    val backStack = rememberNavBackStack(ProductList)

    NavDisplay(
        backStack = backStack,
        entryProvider = entryProvider {
            entry<ProductList>(
                // Mark this entry as eligible for two-pane display
                metadata = TwoPaneScene.twoPane()
            ) { key ->
                Column {
                    Text("Product List")
                    Button(onClick = { backStack.add(ProductDetail("ABC")) }) {
                        Text("View Details for ABC (Two-Pane Eligible)")
                    }
                }
            }

            entry<ProductDetail>(
                // Mark this entry as eligible for two-pane display
                metadata = TwoPaneScene.twoPane()
            ) { key ->
                Text("Product Detail: ${key.id} (Two-Pane Eligible)")
            }
            // ... other entries ...
        },
        // Simply provide your custom strategy. NavDisplay will fall back to SinglePaneSceneStrategy automatically.
        sceneStrategy = TwoPaneSceneStrategy<Any>(),
        onBack = { count ->
            repeat(count) {
                if (backStack.isNotEmpty()) {
                    backStack.removeLastOrNull()
                }
            }
        }
    )
}
ScenesSnippets.kt

@Serializable
object ProductList : NavKey

@Serializable
data class ProductDetail(val id: String) : NavKey

@Serializable
data object Profile : NavKey

class MaterialListDetailActivity : ComponentActivity() {

    @OptIn(ExperimentalMaterial3AdaptiveApi::class)
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setContent {
            Scaffold { paddingValues ->
                val backStack = rememberNavBackStack(ProductList)
                val listDetailStrategy = rememberListDetailSceneStrategy<Any>()

                NavDisplay(
                    backStack = backStack,
                    modifier = Modifier.padding(paddingValues),
                    onBack = { keysToRemove -> repeat(keysToRemove) { backStack.removeLastOrNull() } },
                    sceneStrategy = listDetailStrategy,
                    entryProvider = entryProvider {
                        entry<ProductList>(
                            metadata = ListDetailSceneStrategy.listPane(
                                detailPlaceholder = {
                                    ContentYellow("Choose a product from the list")
                                }
                            )
                        ) {
                            ContentRed("Welcome to Nav3") {
                                Button(onClick = {
                                    backStack.add(ProductDetail("ABC"))
                                }) {
                                    Text("View product")
                                }
                            }
                        }
                        entry<ProductDetail>(
                            metadata = ListDetailSceneStrategy.detailPane()
                        ) { product ->
                            ContentBlue("Product ${product.id} ", Modifier.background(PastelBlue)) {
                                Column(horizontalAlignment = Alignment.CenterHorizontally) {
                                    Button(onClick = {
                                        backStack.add(Profile)
                                    }) {
                                        Text("View profile")
                                    }
                                }
                            }
                        }
                        entry<Profile>(
                            metadata = ListDetailSceneStrategy.extraPane()
                        ) {
                            ContentGreen("Profile")
                        }
                    }
                )
            }
        }
    }
}
MaterialScenesSnippets.kt