combine
ReplaceNil
在emit element的时候，用来处理nil
var subscriptions = Set<AnyCancellable>()
example(of: "replaceNil") {
["A", nil, "C"].publisher.eraseToAnyPublisher().replaceNil(with: "-").sink(receiveValue: { print($0) }).store(in: &subscriptions)
}
/// 输出 A - C
ReplaceRmpty
let empty = Empty<Int, Never>()
empty.replaceEmpty(with: 1).sink(receiveCompletion: { print($0) }, receiveValue: { print($0) }).store(in: &subscriptions)
///输出 1 finished
Incrementally transforming output
scan(::)
scan 有点像reduce
var dailyGainLoss: Int { .random(in: -10...10) }
let august = (0..<22).map({ _ in dailyGainLoss }).publisher
august.scan(50) { latest, current in
max(0, latest + current)
}.sink(receiveValue: { _ in }).store(in: &subscriptions)
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