同步嵌套的异步任务

Question

我有嵌套的异步任务.此流程后面的函数:loadEpisodes(加载剧集列表) - >使用完成中的数组循环每个第一集并为特定剧集加载评论(另外一个异步任务).

问题是:在完成注释加载任务之前执行了comletion(fullyEpisodes).我试图使用Dispatch Group(第二个代码块),但它不起作用.

func loadComments(comletion: @escaping ([Episode]) -> Void){
loadEpisodes(completion: {
    episodes in
    var fullyEpisodes = [Episode]()
    for episode in episodes {
        WebService().load(resource: episode.comment, comletion: {
            comments in
            if let comments = comments {
                let _episode = Episode(id: episode.id, title: episode.title, comments: comments)
                fullyEpisodes.append(_episode)
                print("done")
            }
        })
    }
    comletion(fullyEpisodes)

})
}

实施调度组:

func loadComments(comletion: @escaping ([Episode]) -> Void){
    loadEpisodes(completion: {
        episodes in
        var fullyEpisodes = [Episode]()
        let group = DispatchGroup()
        for episode in episodes {
            group.enter()
            WebService().load(resource: episode.comment, comletion: {
                comments in
                if let comments = comments {
                    let _episode = Episode(id: episode.id, title: episode.title, comments: comments)
                    fullyEpisodes.append(_episode)
                    print("done")
                }
            })
            group.leave()
        }
        group.wait()
        group.notify(queue: .main, execute: {
            comletion(fullyEpisodes)
        })

    })
}

当我尝试通过"print("something")"(不是新的异步任务)替换注释加载请求时,Dispatch组正在运行.

Answer 1

在第二个例子中,(a)group.leave()进入load()完成处理程序关闭; (b)group.wait()完全删除.

func loadComments(comletion: @escaping ([Episode]) -> Void){
    loadEpisodes(completion: {
        episodes in
        var fullyEpisodes = [Episode]()
        let group = DispatchGroup()
        for episode in episodes {
            group.enter()
            WebService().load(resource: episode.comment, comletion: {
                comments in
                if let comments = comments {
                    let _episode = Episode(id: episode.id, title: episode.title, comments: comments)
                    fullyEpisodes.append(_episode)
                    print("done")
                }
                group.leave()
            })
            // group.leave()
        }
        //group.wait()
        group.notify(queue: .main, execute: {
            comletion(fullyEpisodes)
        })

    })
}

或者,使用尾随闭包语法清除一点并修复completion拼写:

func loadComments(completion: @escaping ([Episode]) -> Void) {
    loadEpisodes { episodes in
        var fullyEpisodes = [Episode]()
        let group = DispatchGroup()
        for episode in episodes {
            group.enter()
            WebService().load(resource: episode.comment) { comments in
                if let comments = comments {
                    let _episode = Episode(id: episode.id, title: episode.title, comments: comments)
                    fullyEpisodes.append(_episode)
                }
                group.leave()
            }
        }
        group.notify(queue: .main) {
            completion(fullyEpisodes)
        }
    }
}