Kotlin

Question

我正在使用 viewModel 从房间数据库中提取实时数据。我有 2 个从我的 viewModel 中提取的 LiveData，然后我将运行一个函数从我的服务器中提取数据。我需要在运行设置将从我的服务器获取信息的函数之前设置这两个值，因为这些值是 post.

正文的一部分

这是我的一部分 activity

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，但我可以想象，如果 Location LiveData 被延迟，那么我的函数将运行没有位置，并且 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。

谢谢你

Answer 1

您可以通过实时数据进行观察。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 
    }

}

在你的activity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)
    })

Answer 2

您可以使用 MediatorLiveData，这样您只需观察一件事，它只会在收到两个项目时触发，并在之后的每次更改时触发。

在视图模型中：

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 }
            }
        }
    }

在你的Activity中：

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

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