平面图操作后获取列表<单一<天气>>

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Android Studio 3.6.3

我正在使用RxJava将一些记录插入到天气和天气预报表中。这些表之间存在外键关系。预报表有天气的描述。天气ID是“预测”表中的外键。

我已经写了评论来解释我在做什么

private fun createWeatherForecast() {
        val disposable = databaseService
            .weatherDao()
            // Insert the weather description 
            .insert(WeatherTable(icon= "icon", code = (10..1000).shuffled().first(), description = "cloudy with a chance of meatballs"))
            // Insert the Forecast. A weather ID is returned and used as a foregin key in the forecast table.
            .flatMap { weatherId ->
                databaseService.forecastDao().insert(
                    ForecastTable(
                        temp = 45.2F,
                        highTemp = 50.5F,
                        lowTemp = 34.8F,
                        feelLikeMinTemp = 30.5F ,
                        feelLikeMaxTemp = 49.9F,
                        validDate = "today",
                        weatherId = weatherId)
                )
            }
            // Get the forecast weather from the Forecast Table
            .flatMap {
                databaseService.forecastDao().getAllForecast()
            }
            // Get the weather by WeatherID that was inserted in the Forecast table and loop
            .flatMap { forecastList ->
                Single.just(forecastList.map {
                    databaseService.weatherDao().getWeatherById(it.weatherId)
                })
            }
            .subscribeOn(schedulerProvider.backgroundScheduler())
            .observeOn(schedulerProvider.uiScheduler())
            .subscribeBy(
            // This is the confusion as I expect a List<WeatherTable> but instead its List<Single<WeatherTable>>
                onSuccess = {
                    println("Weather forecast $it")
                },
                onError = { println(it.message) }
            )
    }

只是想知道为什么我在onSuccess中获得了List>。我认为应该是清单

只是一些问题： 1）为什么它是列表>而不是列表？ 2）我可以使用List>做什么？

非常感谢您的任何建议，