Android Weekly Issue #414

ViewModel and SavedStateHandle: always retain state

依赖:

implementation "androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version"

构造:

class MyViewModel(private val state: SavedStateHandle) : ViewModel() {
[...]
}

实例化:

override val model by viewModels<MyViewModel>()

如果有其他参数:

class MyViewModelFactory(owner: SavedStateRegistryOwner,
                                     private val myId: Int,
                                     defaultArgs: Bundle? = null
) : AbstractSavedStateViewModelFactory(owner, defaultArgs) {

    override fun <T : ViewModel?> create(
        key: String,
        modelClass: Class<T>,
        handle: SavedStateHandle
    ): T = MyViewModel(handle, myId) as T
}

实例化:

override val model by viewModels<MyViewModel> {
    MyViewModelFactory(this, args.myId)
}

拿值:

private val itemsLiveData = savedStateHandle.getLiveData<Item>("itemsKey")

杀死进程的命令:

adb shell am kill your.package.name

Kotlin withContext vs Async-await

假设有这么两个方法:

private suspend fun doLongRunningTaskOne(): String {
    delay(5000)
    return "One"
}

private suspend fun doLongRunningTaskTwo(): String {
    delay(5000)
    return "Two"
}

用async, 把两个返回值加起来:

fun startLongRunningTaskInParallel() {
    viewModelScope.launch {
        val resultOneDeferred = async { doLongRunningTaskOne() }
        val resultTwoDeferred = async { doLongRunningTaskTwo() }
        val combinedResult = resultOneDeferred.await() + resultTwoDeferred.await()
    }
}

如果用withContext:

fun startLongRunningTaskInParallel() {
    viewModelScope.launch {
        val resultOne = withContext(Dispatchers.IO) { doLongRunningTaskOne() }
        val resultTwo = withContext(Dispatchers.IO) { doLongRunningTaskTwo() }
        val combinedResult = resultOne + resultTwo
    }
}

这样是串行的.