DispatchGroup操作

最近一個案子有個需求，需要call 2隻API，並且得到2隻API回傳的結果後，才做資料處理及UI顯示。

做法1: 2隻API沒有前後順序

Code:

print("Group created")

//產生兩個task

let task1 = DispatchQueue(label: "task1")

let task2 = DispatchQueue(label: "task2")

//產生DispatchGroup

let group = DispatchGroup()

group.enter()

task1.async(group: group, qos: .default) {

    print("task1 start")

    sleep(10)

    print("task1 end")

    group.leave()

}

group.enter()

task2.async(group: group, qos: .default) {

    print("task2 start")

    sleep(5)

    print("task2 end")

    group.leave()

}

group.notify(queue: .main, execute:{

    print("All task is done")

    //self.reloadUI()

})

Result:

Group created

task1 start

task2 start

task2 end

task1 end

All task is done

可以看到task1執行10秒，task2執行5秒，所以task2先結束了，並沒有按照順序執行

做法2: API有先後順序，加入group.wait()

print("Group created")

//產生兩個task

let task1 = DispatchQueue(label: "task1")

let task2 = DispatchQueue(label: "task2")

//產生DispatchGroup

let group = DispatchGroup()

group.enter()

task1.async(group: group, qos: .default) {

    print("task1 start")

    sleep(10)

    print("task1 end")

    group.leave()

}

group.wait()

group.enter()

task2.async(group: group, qos: .default) {

    print("task2 start")

    sleep(5)

    print("task2 end")

    group.leave()

}

group.notify(queue: .main, execute:{

    print("All task is done")

    //self.reloadUI()

})

Result:

Group created

task1 start

task1 end

task2 start

task2 end

All task is done

可以看到兩個task按照順序執行了

參考資料:
https://medium.com/@shawnloveapp/dispatchgroup操作-6365cb4f0fdc