Compose状态保存rememberSaveable原理解析

//保存列表状态
val list = rememberSaveable {
    mutableListOf<String>()
}
//保存普通状态
var value by rememberSaveable {
    mutableStateOf("")
}

data class User(
    val name: String = ""
)
val user = rememberSaveable {
    User()
}

@Composable
fun <T : Any> rememberSaveable(
    vararg inputs: Any?,
    saver: Saver<T, out Any> = autoSaver(),
    key: String? = null,
    init: () -> T
): T {
    //...
    // 通过 CompositionLocal 获取 SaveableStateRegistry
    val registry = LocalSaveableStateRegistry.current
    // 通过 init 获取需要保存的数据
    val value = remember(*inputs) {
        // registry 根据 key 恢复数据，恢复的数据是一个 Saveable
        val restored = registry?.consumeRestored(finalKey)?.let {
            // 使用 Saver 将 Saveable 转换为业务类型
            saver.restore(it)
        }
        restored ?: init()
    }
    // 用一个 MutableState 保存 Saver，主要是借助 State 的事务功能避免一致性问题发生
    val saverHolder = remember { mutableStateOf(saver) }
    saverHolder.value = saver
    if (registry != null) {
        DisposableEffect(registry, finalKey, value) {
            //ValueProvider：通过 Saver#save 存储数据
            val valueProvider = {
                with(saverHolder.value) { SaverScope { registry.canBeSaved(it) }.save(value) }
            }
            //试探数值是否可被保存
            registry.requireCanBeSaved(valueProvider())
            //将ValueProvider 注册到 registry ，等到合适的时机被调用
            val entry = registry.registerProvider(finalKey, valueProvider)
            onDispose {
                entry.unregister()
            }
        }
    }
    return value
}

private val AutoSaver = Saver<Any?, Any>(
    save = { it },
    restore = { it }
)

override fun setContent(content: @Composable () -> Unit) {
    //...
    ProvideAndroidCompositionLocals(owner, content)
    //...
}
@Composable
@OptIn(ExperimentalComposeUiApi::class)
internal fun ProvideAndroidCompositionLocals(
    owner: AndroidComposeView,
    content: @Composable () -> Unit
) {
    val view = owner
    val context = view.context
    //...
    val viewTreeOwners = owner.viewTreeOwners ?: throw IllegalStateException(
        "Called when the ViewTreeOwnersAvailability is not yet in Available state"
    )
    val saveableStateRegistry = remember {
        DisposableSaveableStateRegistry(view, viewTreeOwners.savedStateRegistryOwner)
    }
    //...
    CompositionLocalProvider(
        //...
        LocalSaveableStateRegistry provides saveableStateRegistry,
        //...
    ) {
        ProvideCommonCompositionLocals(
            owner = owner,
            //...
            content = content
        )
    }
}

internal fun DisposableSaveableStateRegistry(
    id: String,
    savedStateRegistryOwner: SavedStateRegistryOwner
): DisposableSaveableStateRegistry {
    //基于 id 创建 key
    val key = "${SaveableStateRegistry::class.java.simpleName}:$id"
    // 基于 key 获取 bundle 数据
    val androidxRegistry = savedStateRegistryOwner.savedStateRegistry
    val bundle = androidxRegistry.consumeRestoredStateForKey(key)
    val restored: Map<String, List<Any?>>? = bundle?.toMap()
    // 创建 saveableStateRegistry，传入 restored 以及 canBeSaved
    val saveableStateRegistry = SaveableStateRegistry(restored) {
        canBeSavedToBundle(it)
    }
    val registered = try {
        androidxRegistry.registerSavedStateProvider(key) {
            //调用 register#performSave 并且转为 Bundle
            saveableStateRegistry.performSave().toBundle()
        }
        true
    } catch (ignore: IllegalArgumentException) {
        false
    }
    return DisposableSaveableStateRegistry(saveableStateRegistry) {
        if (registered) {
            androidxRegistry.unregisterSavedStateProvider(key)
        }
    }
}

internal class DisposableSaveableStateRegistry(
    saveableStateRegistry: SaveableStateRegistry,
    private val onDispose: () -> Unit
) : SaveableStateRegistry by saveableStateRegistry {
    fun dispose() {
        onDispose()
    }
}

val saveableStateRegistry = SaveableStateRegistry(restored) {
    canBeSavedToBundle(it)
}
fun SaveableStateRegistry(
    restoredValues: Map<String, List<Any?>>?,
    canBeSaved: (Any) -> Boolean
): SaveableStateRegistry = SaveableStateRegistryImpl(restoredValues, canBeSaved)

private fun canBeSavedToBundle(value: Any): Boolean {
    // SnapshotMutableStateImpl is Parcelable, but we do extra checks
    if (value is SnapshotMutableState<*>) {
        if (value.policy === neverEqualPolicy<Any?>() ||
            value.policy === structuralEqualityPolicy<Any?>() ||
            value.policy === referentialEqualityPolicy<Any?>()
        ) {
            val stateValue = value.value
            return if (stateValue == null) true else canBeSavedToBundle(stateValue)
        } else {
            return false
        }
    }
    for (cl in AcceptableClasses) {
        if (cl.isInstance(value)) {
            return true
        }
    }
    return false
}
private val AcceptableClasses = arrayOf(
    Serializable::class.java,
    Parcelable::class.java,
    String::class.java,
    SparseArray::class.java,
    Binder::class.java,
    Size::class.java,
    SizeF::class.java
)

    override fun consumeRestored(key: String): Any? {
        val list = restored.remove(key)
        return if (list != null && list.isNotEmpty()) {
            if (list.size > 1) {
                restored[key] = list.subList(1, list.size)
            }
            list[0]
        } else {
            null
        }
    }

    override fun registerProvider(key: String, valueProvider: () -> Any?): Entry {
        require(key.isNotBlank()) { "Registered key is empty or blank" }
        @Suppress("UNCHECKED_CAST")
        valueProviders.getOrPut(key) { mutableListOf() }.add(valueProvider)
        return object : Entry {
            override fun unregister() {
                val list = valueProviders.remove(key)
                list?.remove(valueProvider)
                if (list != null && list.isNotEmpty()) {
                    // if there are other providers for this key return list back to the map
                    valueProviders[key] = list
                }
            }
        }
    }

    override fun performSave(): Map<String, List<Any?>> {
        val map = restored.toMutableMap()
        valueProviders.forEach { (key, list) ->
            if (list.size == 1) {
                val value = list[0].invoke()
                if (value != null) {
                    check(canBeSaved(value))
                    map[key] = arrayListOf<Any?>(value)
                }
            } else {
                map[key] = List(list.size) { index ->
                    val value = list[index].invoke()
                    if (value != null) {
                        check(canBeSaved(value))
                    }
                    value
                }
            }
        }
        return map
    }

 androidxRegistry.registerSavedStateProvider(key) {
            //调用 register#performSave 并且转为 Bundle
            saveableStateRegistry.performSave().toBundle()
        }

internal fun DisposableSaveableStateRegistry(
    id: String,
    savedStateRegistryOwner: SavedStateRegistryOwner
): DisposableSaveableStateRegistry {
    val key = "${SaveableStateRegistry::class.java.simpleName}:$id"
    val androidxRegistry = savedStateRegistryOwner.savedStateRegistry
    //...
 }

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        ContextAware,
        LifecycleOwner,
        ViewModelStoreOwner,
        HasDefaultViewModelProviderFactory,
        SavedStateRegistryOwner, // ComponentActivity 是一个 SavedStateRegistryOwner
        OnBackPressedDispatcherOwner,
        ActivityResultRegistryOwner,
        ActivityResultCaller {
    //...
    public final SavedStateRegistry getSavedStateRegistry() {
        return mSavedStateRegistryController.getSavedStateRegistry();
    }
    //...
}

protected void onCreate(@Nullable Bundle savedInstanceState) {
    mSavedStateRegistryController.performRestore(savedInstanceState);
    //...
}
protected void onSaveInstanceState(@NonNull Bundle outState) {
    //...
    mSavedStateRegistryController.performSave(outState);
}

fun performSave(outBundle: Bundle) {
    //...
    val it: Iterator<Map.Entry<String, SavedStateProvider>> =
        this.components.iteratorWithAdditions()
    while (it.hasNext()) {
        val (key, value) = it.next()
        components.putBundle(key, value.saveState())
    }
    if (!components.isEmpty) {
        outBundle.putBundle(SAVED_COMPONENTS_KEY, components)
    }
}