从Observable获取最后一个值

怎样才能 combineLatest 用于返回 BehaviorSubject ?我正试图从观测数据中得到最后一个值。

var things: Observable<Thing>

// driven by UI
var selectedThingIndex: PublishSubject<Int>

// this is a BehaviorSubject, because I need to get the latest value outside a subscriber
var currentThing: BehaviorSubject<Thing> = BehaviorSubject.combineLatest(things, selectedThingIndex) { things, index in
    things[index]
}

// Get the last value
let thing = currentThing.value()

我不能编译这个,因为 组合最 返回一个 Observable ,这似乎不适用于 行为主体 . 我尝试了一个明确的演员阵容,例如 as? BehaviorSubject<Thing?> 但结果是零。