RxSwiftで引数付きの関数を追加 (extension) する場合は気をつけるという話 ref: https://qiita.com/YutoMizutani/items/de082fde1c7c3b111195

file0.swift

extension Observable where E == Int {
    func add(_ value: Int) -> Observable<Int> {
        return map { $0 + value }
    }
}

file1.swift

let subject = PublishSubject<Int>()
var value = 10

_ = subject
    .add(value)
    .subscribe(onNext: { print($0) })

value = 5
subject.onNext(0) // 10

file2.swift

extension Observable where E == Int {
    func add(_ value: @escaping () -> Int) -> Observable<Int> {
        return map { $0 + value() }
    }
}

file3.swift

_ = subject
    .add({ value })
    .subscribe(onNext: { print($0) })

file4.swift

extension Observable where E == Int {
    func add(_ value: @escaping @autoclosure () -> Int) -> Observable<Int> {
        return map { $0 + value() }
    }
}

file5.swift

let subject = PublishSubject<Int>()
var value = 10

_ = subject
    .add(value)
    .subscribe(onNext: { print($0) })

value = 5
subject.onNext(0) // 5
value = 20
subject.onNext(0) // 20