Kotlin-在运行函数之前，等待观察到多个LiveData

Question

我正在使用viewModel从房间数据库中提取实时数据。我有2个LiveData，我从我的viewModel中提取，然后我将运行一个函数从我的服务器中提取数据。我需要在运行我的函数之前设置这两个值，该函数将从我的服务器获取信息，因为这些值是帖子正文的一部分。

这是我活动的一部分

var locationSeleted:Long=0L
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = DataBindingUtil.setContentView<ActivityBinding>(this,R.layout.activity)
//This is to get the viewmodel
val application = requireNotNull(this).application
val dataSource = Database.getInstance(application).DatabaseDao
val viewModelFactory = ViewModelFactory(dataSource, application)
val ViewModel = ViewModelProviders.of(this,viewModelFactory).get(ViewModel::class.java)
binding.ViewModel = ViewModel
binding.setLifecycleOwner (this)
//Get location from livedata
ViewModel.Location.observe(this, Observer {location ->
if (location == null){
//do something
}
else{
locationSeleted = location.locationId
}
})
//listens to the live data of the products value
ViewModel.Products.observe(this, Observer{products ->

if (products == null){
//do something
}
else{
myfunction(products)
}
})

这是我的视图模型的一部分

class ViewModel(val database: DatabaseDao, application: Application) : AndroidViewModel(application) {

//This gets the location selected
var Location: LiveData<LocationSelect> = database.get_location()
//This line gets the products
var Products: LiveData<Array<Products>> = database.get_products()

我目前的代码可以工作，但我可以想象，如果Location LiveData被延迟，那么myfunction将在没有位置的情况下运行，Post调用将为false。我的问题是，当位置和产品都从viewModel设置时，我如何调用myfunction？或者有更好的方法。

谢谢

Answer 1

编辑：这可以使用中间流简洁地完成，因为流有像combine这样方便的运算符。

val locationAndProductsLiveData: LiveData<Pair<LocationSelect, Array<Products>>> = 
location.asFlow().combine(products.asFlow()) { location, products ->
location to products
}.asLiveData()

---

您可以使用MediatorLiveData，因此您只需观察一件事，并且它仅在收到两项时以及之后的每次更改时才会触发。

在ViewModel中：

val locationAndProductsLiveData: LiveData<Pair<LocationSelect, Array<Products>>> = 
object: MediatorLiveData<Pair<LocationSelect, Array<Products>>>() {
var location: LocationSelect? = null
var products: Array<Products>? = null
init {
addSource(location) { location ->
this.location = location 
products?.let { value = location to it }
}
addSource(products) { products ->
this.products = products 
location?.let { value = it to products }
}
}
}

在您的活动中：

viewModel.locationAndProductsLiveData.observe(this) { (location, products) ->
locationSelected = location.Id
myFunction(products)
}

Answer 2

您可以使用实时数据进行观察。在您的视图模型中：

val _weHaveAllTheData = MutableLiveData(pairToShowSpinner)
val weHaveAllTheData: LiveData<Pair<Boolean, Boolean>> = _weHaveAllTheData
fun updateTheData(int firstOrSecond: Int) { 
val pair = _weHaveAllTheData.value
if (firstOrSecond = 1) {

val secondPair = pair?.second as Boolean
val pairInObservable = Pair(true, secondPair)
_weHaveAllTheData.value = pairInObservable
} else {
val firstPair = pair?.first as Boolean
val pairInObservable = Pair(firstPair, true)
_weHaveAllTheData.value = pairInObservable 
}
}

在您的活动中：

//Get location from livedata
ViewModel.Location.observe(this, Observer {location ->
if (location == null){
//do something
}
else{
locationSeleted = location.locationId
}
ViewModel.updateTheData(1)

})
//listens to the live data of the products value
ViewModel.Products.observe(this, Observer{products ->

if (products == null){
//do something
}
else{
ViewModel.updateTheData(2)

}
})
ViewModel.weHaveAllTheData.observe(lifecycleOwner, Observer {
val positivePair = Pair(first = true, second = true)
if (it == positivePair)
myfunction(ViewModel.Products)
})