Kotlin-在运行功能之前等待观察多个LiveData

如何解决Kotlin-在运行功能之前等待观察多个LiveData

我正在使用viewModel从房间数据库中提取实时数据。我从ViewModel中提取了2个LiveData，然后将运行一个从服务器中提取数据的函数。在运行从服务器获取信息的函数之前，我需要同时设置两个值，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()

我当前的代码有效，但是我可以想象如果Location LiveData被延迟，则myfunction将在没有位置的情况下运行，而Post调用将为false。我的问题是，当从viewModel设置位置和产品时如何调用myfunction？还是有更好的方法做到这一点。

谢谢

解决方法

您可以使用MediatorLiveData，因此您只需要观察一件事，并且仅在接收到两个项目以及此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)
}

,

您可以观察实时数据。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)
    })