LiveDataAdapter.kt

/*
 * Copyright 2020 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.compose.runtime.livedata

import androidx.compose.runtime.Composable
import androidx.compose.runtime.DisposableEffect
import androidx.compose.runtime.State
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.ui.platform.LocalLifecycleOwner
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleOwner
import androidx.lifecycle.LiveData
import androidx.lifecycle.Observer

/**
 * Starts observing this [LiveData] and represents its values via [State]. Every time there would
 * be new value posted into the [LiveData] the returned [State] will be updated causing
 * recomposition of every [State.value] usage.
 *
 * The inner observer will automatically be removed when this composable disposes or the current
 * [LifecycleOwner] moves to the [Lifecycle.State.DESTROYED] state.
 *
 * @sample androidx.compose.runtime.livedata.samples.LiveDataSample
 */
@Composable
fun <T> LiveData<T>.observeAsState(): State<T?> = observeAsState(value)

/**
 * Starts observing this [LiveData] and represents its values via [State]. Every time there would
 * be new value posted into the [LiveData] the returned [State] will be updated causing
 * recomposition of every [State.value] usage.
 *
 * The [initial] value will be used only if this LiveData is not already
 * [initialized][isInitialized]. Note that if [T] is a non-null type, it is your
 * responsibility to ensure that any value you set on this LiveData is also non-null.
 *
 * The inner observer will automatically be removed when this composable disposes or the current
 * [LifecycleOwner] moves to the [Lifecycle.State.DESTROYED] state.
 *
 * @sample androidx.compose.runtime.livedata.samples.LiveDataWithInitialSample
 */
@Composable
fun <R, T : R> LiveData<T>.observeAsState(initial: R): State<R> {
    val lifecycleOwner = LocalLifecycleOwner.current
    val state = remember {
        @Suppress("UNCHECKED_CAST") /* Initialized values of a LiveData<T> must be a T */
        mutableStateOf(if (isInitialized) value as T else initial)
    }
    DisposableEffect(this, lifecycleOwner) {
        val observer = Observer<T> { state.value = it }
        observe(lifecycleOwner, observer)
        onDispose { removeObserver(observer) }
    }
    return state
}