ios - 使用dispatch_group_async从嵌套异步调用获取通知

Question

所以，当我想要做一些异步的事情时，我会遇到这样的情况:在 Facebook 上发布状态并在发布完成后收到通知。我本来希望dispatch_group_async能够完成这项工作，但现在我碰壁了。

这就是此时逻辑的样子。

功能

func saveAndPost() {
    //1. save
    dispatch_group_async(group, queue) { () -> Void in
        print("saving to DB")
        //some func
        print("saved to DB")
    }
    //2. post
    dispatch_group_async(group, queue, { () -> Void in
        print("publishing on FB")
        //some func
        //async request to ACAccount {
            print("access")
            //async performRequestWithHandler{
                print("posted")
                //anwser from request
                }
            }
        }
        print("published on FB")
    })

    dispatch_group_notify(group, queue, { () -> Void in
        print("done")
        dispatch_async(dispatch_get_main_queue(), { () -> Void in
            //go to next view
        })
    })
}

输出

saving to DB
saved to DB
publishing on FB
published on FB
done
post
posted

我的目标是在整个发布过程完成后收到通知。也许这对于dispatch_group_async是不可能的，我必须使用KVO甚至PromiseKit？

解决方案

David 让我思考，dispatch_group_enter 和“dispatch_group_leave”正是我所需要的。所以此时我的逻辑是这样的。

func saveAndPost() {
    //1. save
    dispatch_group_async(group, queue) { () -> Void in
        print("saving to DB")
        //some func
        print("saved to DB")
    }
    //2. post
    dispatch_group_enter(group)
        print("publishing on FB")
        //some func
        //async request to ACAccount {
            print("access")
            //async performRequestWithHandler{
                print("posted")
                //anwser from request
                dispatch_group_leave(group)
                }
            }
        }
        print("published on FB")

    dispatch_group_notify(group, queue, { () -> Void in
        print("done")
        dispatch_async(dispatch_get_main_queue(), { () -> Void in
            //go to next view
        })
    })
}

如您所见，2. 部分现已修改。

Answer 1

我在处理异步调用时解决这个问题的首选方法:

var asyncGroup = dispatch_group_create()

dispatch_group_enter(asyncGroup)
asyncCall1.start() {


  //end of callback
  dispatch_group_leave(asyncGroup)
}

dispatch_group_enter(asyncGroup)
asyncCall2.start() {

    asyncCall3.start() {

        asyncCall4.start() {

            dispatch_group_leave(asyncGroup)
        }

    }
}

dispatch_group_notify(asyncGroup, dispatch_get_main_queue(), {
    println("done")
})

asyncCall1.start() { ... } 和 asyncCall2.start() { ... } 只是一些伪代码，向您展示它是如何工作的。

每个 dispatch_group_enter 调用都需要通过 dispatch_group_leave 调用来平衡，否则您将永远不会进入 dispatch_group_notify block 。