（*useful）标记：目前觉得有用的函数
//FIXME 标记：待补充

Transform：（转换Next event）

转换由可观察序列发出的下一个事件元素的操作。

map：（*useful）将elements emitted 的事件转换后发出一个新的Observable sequence

转换elemet

    let disposeBag = DisposeBag()
    Observable.of(1, 2, 3)
        .map { $0 * $0 }
        .subscribe(onNext: { print($0) })
        .disposed(by: disposeBag)

打印：
--- map example ---
1
4
9

flatMap and flatMapLatest：

flatMapLatest实际上是Map和switchLatest的组合。

   let disposeBag = DisposeBag()
    
    struct Player {
        var score: Variable<Int>
    }
    
    let 👦🏻 = Player(score: Variable(80))
    let 👧🏼 = Player(score: Variable(90))
    
    let player = Variable(👦🏻)
    
    player.asObservable()
        .flatMap { $0.score.asObservable() } // Change flatMap to flatMapLatest and observe change in printed output
        .subscribe(onNext: { print($0) })
        .disposed(by: disposeBag)
    
    👦🏻.score.value = 85
    
    player.value = 👧🏼
    
    👦🏻.score.value = 95 // Will be printed when using flatMap, but will not be printed when using flatMapLatest
    
    👧🏼.score.value = 100

打印： //FIXME :flatMap理解不到位
80
85
90
95 //Will be printed when using flatMap, but will not be printed when using flatMapLatest
100

scan：累加器

    let disposeBag = DisposeBag()
    
    Observable.of(10, 100, 1000)
        .scan(2) { aggregateValue, newValue in
            aggregateValue + newValue
        }
        .subscribe(onNext: { print($0) })
        .disposed(by: disposeBag)

打印：
--- scan example ---
12
112
1112