NavBackStackEntryProvider.kt

/*
 * Copyright 2021 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package androidx.navigation.compose

import androidx.compose.runtime.Composable
import androidx.compose.runtime.CompositionLocalProvider
import androidx.compose.runtime.saveable.SaveableStateHolder
import androidx.compose.ui.platform.LocalLifecycleOwner
import androidx.compose.ui.platform.LocalSavedStateRegistryOwner
import androidx.lifecycle.SavedStateHandle
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewmodel.compose.LocalViewModelStoreOwner
import androidx.lifecycle.viewmodel.compose.viewModel
import androidx.navigation.NavBackStackEntry
import java.lang.ref.WeakReference
import java.util.UUID

/**
 * Provides [this] [NavBackStackEntry] as [LocalViewModelStoreOwner], [LocalLifecycleOwner] and
 * [LocalSavedStateRegistryOwner] to the [content] and saves the [content]'s saveable states with
 * the given [saveableStateHolder].
 *
 * @param saveableStateHolder The [SaveableStateHolder] that holds the saved states. The same
 * holder should be used for all [NavBackStackEntry]s in the encapsulating [Composable] and the
 * holder should be hoisted.
 * @param content The content [Composable]
 */
@Composable
public fun NavBackStackEntry.LocalOwnersProvider(
    saveableStateHolder: SaveableStateHolder,
    content: @Composable () -> Unit
) {
    CompositionLocalProvider(
        LocalViewModelStoreOwner provides this,
        LocalLifecycleOwner provides this,
        LocalSavedStateRegistryOwner provides this
    ) {
        saveableStateHolder.SaveableStateProvider(content)
    }
}

@Composable
private fun SaveableStateHolder.SaveableStateProvider(content: @Composable () -> Unit) {
    val viewModel = viewModel<BackStackEntryIdViewModel>()
    // Stash a reference to the SaveableStateHolder in the ViewModel so that
    // it is available when the ViewModel is cleared, marking the permanent removal of this
    // NavBackStackEntry from the back stack. Which, because of animations,
    // only happens after this leaves composition. Which means we can't rely on
    // DisposableEffect to clean up this reference (as it'll be cleaned up too early)
    viewModel.saveableStateHolderRef = WeakReference(this)
    SaveableStateProvider(viewModel.id, content)
}

internal class BackStackEntryIdViewModel(handle: SavedStateHandle) : ViewModel() {

    private val IdKey = "SaveableStateHolder_BackStackEntryKey"

    // we create our own id for each back stack entry to support multiple entries of the same
    // destination. this id will be restored by SavedStateHandle
    val id: UUID = handle.get<UUID>(IdKey) ?: UUID.randomUUID().also { handle.set(IdKey, it) }

    lateinit var saveableStateHolderRef: WeakReference<SaveableStateHolder>

    // onCleared will be called on the entries removed from the back stack. here we notify
    // SaveableStateProvider that we should remove any state is had associated with this
    // destination as it is no longer needed.
    override fun onCleared() {
        super.onCleared()
        saveableStateHolderRef.get()?.removeState(id)
        saveableStateHolderRef.clear()
    }
}