为什么即使我更改了值，我的地图 livedata 也没有被调用？

Question

我是 MVVM 和 Android 架构组件的新手。所以我有这样的存储库

object RestaurantRepository {

    val restaurants : LiveData<ArrayList<Restaurant>> = RestaurantClient.restaurants

    private var count : LiveData<Int> = Transformations.map(restaurants) {

        Log.d("debugLog","map")

        it.size
    }

    fun searchRestaurants(query: String, latitude:Double, longitude: Double) {

        mQuery = query

        RestaurantClient.searchRestaurants(
            query = query,
            latitude = latitude,
            longitude = longitude,
            start = 0
        )
    }

}

我有客户使用 Retrofit 获取数据

object RestaurantClient {

    val restaurants = MutableLiveData<ArrayList<Restaurant>>()


    private val restaurantService = RetrofitServiceGenerator.getInstance(RestaurantAPI::class.java)

    fun searchRestaurants(query: String, latitude:Double, longitude: Double, start: Int) {

        val call = restaurantService.searchRestaurants(
            radius = 2000,
            query = query,
            latitude = latitude,
            longitude = longitude,
            start = start,
            count = NUMBER_OF_DOCUMENT_PER_PAGE
        )



        call.enqueue(object: Callback<RestaurantListBaseResponse> {

            override fun onFailure(call: Call<RestaurantListBaseResponse>, t: Throwable) {

                // if there is an error while sending data to server or while parsing the data

            }

            override fun onResponse(call: Call<RestaurantListBaseResponse>, response: Response<RestaurantListBaseResponse>) {

                if (response.isSuccessful) {

                    val listOfRestaurants = response.body()!!.restaurants
                    val restos = ArrayList<Restaurant>()

                    for (i in listOfRestaurants ) {
                        restos.add(i.restaurant)
                    }

                    restaurants.postValue(restos)


            }

        })


    }

我相信，我从服务器获取数据，并通过 RestaurantClient 中的 restaurants MutableLiveData 获取数据 post。所以我假设因为 Livedata 发生了变化，它会触发 Transformations.map ，但是 count 的登录从未出现在我的 Logcat.

中

此计数仅在 RestaurantRepository 中是私有的，这只是为了测试，因为我仍在学习 livedata

Answer 1

根据docs

The transformations aren't calculated unless an observer is observing the returned LiveData object. 因此，您需要订阅 count LiveData 才能使转换工作。

为什么即使我更改了值，我的地图 livedata 也没有被调用？

why my map livedata is not called even though I have change the value?

android

android-livedata

android-architecture-components

android-jetpack

mutablelivedata