Swift调度组在任务完成前通知

更新时间: 上述问题实际上按原样正确运行(正如rmaddy指出的!)

我将这个错误答案保存在下面,以防其他人对DispatchQueue的异步(组:)方法行为感到困惑,因为 Apple's swift doc on it 目前很糟糕。

在每次调用async()之前,需要调用组的enter(),然后在每个async()块的末尾调用组的leave(),但是 在内部 街区。它基本上就像一个refcount,当它达到零(没有输入剩余值)时,就会调用notify块。

let group = DispatchGroup()
let queueImage = DispatchQueue(label: "com.image")
let queueVideo = DispatchQueue(label: "com.video")

group.enter()
queueImage.async(group: group) {
    sleep(2)
    print("image")
    group.leave()
}

group.enter()
queueVideo.async(group: group) {
    sleep(3)
    print("video")
    group.leave()
}

group.notify(queue: .main) {
    print("all finished.")
}

一般回答:(Swift 5)

let yourDispatchGroup = DispatchGroup()

yourDispatchGroup.enter()
task1FunctionCall {
  yourDispatchGroup.leave() //task 1 complete
}

yourDispatchGroup.enter()
task2FunctionCall {
  yourDispatchGroup.leave() //task 2 complete
}

.. ..
yourDispatchGroup.enter()
tasknFunctionCall {
  yourDispatchGroup.leave() //task n complete
}

dispatchGroup.notify(queue: .main) {
  //This is invoked when all the tasks in the group is completed.
}

如果您的 DispatchGroup 是一个 lazy var ,尽量不要调用 notify 方法。

lazy var dispatchGroup: DispatchGroup = {
    let dispatchGroup = DispatchGroup()
    
    // not call here dispatchGroup.notify(...

    return dispatchGroup
}()

你需要打电话给所有 enter 方法之前的 通知 方法:

dispatchGroup.enter()

dispatchQueue.async(group: dispatchGroup) {
    // ...
    self.dispatchGroup.leave()
}

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