ios - 多个完成 block 完成后执行的函数

Question

我有一个函数，我只想在两个完成 block 完成时才执行(而且无法判断先完成哪个)。以下是我的尝试。然而，它非常困惑，如果我要等待三个或更多的完成 block ，我会到处都有标志。我想知道是否有更漂亮的方法。

class TestClass: UIViewController {
    var blockOneComplete = false
    var blockTwoComplete = false

    func blockOneDownloadImageDescription(completion:()->Void) {
        downloadAsyncWithCompletion {
            blockOneComplete = true 
            if self.blockTwoComplete == true {
                self.allDataDownloadCompleted()
            } else {
                // Do nothing and wait for block Two to complete
            }
        }
    }

    func blockTwoDownloadImageData(completion:()->Void) {
        downloadAsyncWithCompletion {
            blockTwoComplete = true 
            if self.blockOneComplete == true {
                self.allDataDownloadCompleted()
            } else {
                // Do nothing and wait for block One to complete
            }
        }
    }

    func allDataDownloadComplete() {
        // Execute this funciton after all Async Download has complete
    }
}

-- 更新最终结果 --事实证明，该网站中概述的内容正是我所需要的 Using dispatch groups to wait for multiple web services

我相信这不是评论中提到的 SO 问题的重复，因为最终解决方案包括 dispatch_group_enter 和 dispatch_group_leave

Answer 1

最好的选择是使用 dispatch_group

class TestClass: UIViewController {

    var group : dispatch_group_t = dispatch_group_create()

    override func viewDidLoad() {
        super.viewDidLoad()
        dispatch_group_notify(group, dispatch_get_main_queue()) { 
            allDataDownloadComplete()
        }
    }

    func blockOneDownloadImageDescription(completion:()->Void) {
        dispatch_group_enter(group)
        downloadAsyncWithCompletion {
            dispatch_group_leave(group)
        }
    }

    func blockTwoDownloadImageData(completion:()->Void) {
        dispatch_group_enter(group)
        downloadAsyncWithCompletion {
            dispatch_group_leave(group)
        }
    }

    func allDataDownloadComplete() {
        // Execute this funciton after all Async Download has complete
    }
}