[Swift] DispatchGroup の実行途中で enter 数と leave 数が同じになっても残りの enter が実行されるか調べた時のメモ

結果：実行される

---

実行したコード：

import Foundation

let dispatchGroup = DispatchGroup()

for i in 0..<3 {

    print(i, "enter start")
    dispatchGroup.enter()
    print(i, "enter did")

    DispatchQueue.main.async {

        DispatchQueue.main.async {

            print(i, "- inner enter start")
            dispatchGroup.enter()
            print(i, "- inner enter did")

            sleep(1)

            print(i, "- inner leave start")
            dispatchGroup.leave()
            print(i, "- inner leave did")

        }

        print(i, "leave start")
        dispatchGroup.leave()
        print(i, "leave  did")
    }

    sleep(1)

}

dispatchGroup.notify(queue: .main) {
    print("notify")
}

結果：

0 enter start
0 enter did
1 enter start
1 enter did
2 enter start
2 enter did
0 leave start
0 leave  did
1 leave start
1 leave  did
2 leave start
2 leave  did
0 - inner enter start
0 - inner enter did
0 - inner leave start
0 - inner leave did
1 - inner enter start
1 - inner enter did
1 - inner leave start
1 - inner leave did
2 - inner enter start
2 - inner enter did
2 - inner leave start
2 - inner leave did
notify

まとめると、

1. 外側が全部 enter
2. 外側が全部 leave
3. 内側がそれぞれ　enter → leave
4. notify

途中で enter 数 == leave 数　になっても、
まだ enter していない部分があれば
notify されないみたい。