LiveData Transformations.map() with multiple arguments

问题

I have a value in the UI that it's value depends on two LiveData objects. Imagine a shop where you need a subtotal = sum of all items price and a total = subtotal + shipment price. Using Transformations we can do the following for the subtotal LiveData object (as it only depends on itemsLiveData):

val itemsLiveData: LiveData<List<Items>> = ...
val subtotalLiveData = Transformations.map(itemsLiveData) { 
   items ->
       getSubtotalPrice(items)
}

In the case of the total it would be great to be able to do something like this:

val shipPriceLiveData: LiveData<Int> = ...
val totalLiveData = Transformations.map(itemsLiveData, shipPriceLiveData) { 
   items, price ->
       getSubtotalPrice(items) + price
}

But, unfortunately, that's not possible because we cannot put more than one argument in the map function. Anyone knows a good way of achieving this?

回答1:

At the end I used MediatorLiveData to achieve the same objective.

fun mapBasketTotal(source1: LiveData<List<Item>>, source2: LiveData<ShipPrice>): LiveData<String> {
    val result = MediatorLiveData<String>()
    uiThread {
        var subtotal: Int = 0
        var shipPrice: Int = 0
        fun sumAndFormat(){ result.value = format(subtotal + shipPrice)}
        result.addSource(source1, { items ->
            if (items != null) {
                subtotal = getSubtotalPrice(items)
                sumAndFormat()
            }
        })
        result.addSource(source2, { price ->
            if (price != null) {
                shipPrice = price
                sumAndFormat()
            }
        })
    }
    return result
}

回答2:

I come up with another solution.

class PairLiveData<A, B>(first: LiveData<A>, second: LiveData<B>) : MediatorLiveData<Pair<A?, B?>>() {
    init {
        addSource(first) { value = it to second.value }
        addSource(second) { value = first.value to it }
    }
}

class TripleLiveData<A, B, C>(first: LiveData<A>, second: LiveData<B>, third: LiveData<C>) : MediatorLiveData<Triple<A?, B?, C?>>() {
    init {
        addSource(first) { value = Triple(it, second.value, third.value) }
        addSource(second) { value = Triple(first.value, it, third.value) }
        addSource(third) { value = Triple(first.value, second.value, it) }
    }
}

fun <A, B> LiveData<A>.combine(other: LiveData<B>): PairLiveData<A, B> {
    return PairLiveData(this, other)
}

fun <A, B, C> LiveData<A>.combine(second: LiveData<B>, third: LiveData<C>): TripleLiveData<A, B, C> {
    return TripleLiveData(this, second, third)
}

Then, you can combine multiple source.

val totalLiveData = Transformations.map(itemsLiveData.combine(shipPriceLiveData)) {
    // Do your stuff
}

If you want to have 4 or more sources, you need to create you own data class because Kotlin only has Pair and Triple.

In my opinion, there is no reason to run with uiThread in Damia's solution.

回答3:

You can use switchMap() for such case, because it returns LiveData object which can be Transformations.map()

In below code I am getting sum of final amount of two objects onwardSelectQuote and returnSelectQuote

finalAmount = Transformations.switchMap(onwardSelectQuote) { data1 ->
            Transformations.map(returnSelectQuote) { data2 -> ViewUtils.formatRupee((data1.finalAmount!!.toFloat() + data2.finalAmount!!.toFloat()).toString())
            }
        }

来源：https://stackoverflow.com/questions/47572913/livedata-transformations-map-with-multiple-arguments