使用DispatchGroup在for-in循环中处理异步回调仅在所有循环成功时有效

下面的代码是该任务的概要。查询数据库时,它返回一个结果集合,该集合在搜索特定属性时循环,如果找到该属性,则立即查询文件存储,其异步完成处理程序在循环中返回文件。因为我在for-in循环中处理异步回调,所以我使用 DispatchGroup someIdentifier 财产。如果集合中的一个文档没有该属性,则分派组从不调用 notify()

someDatabaseQuery.retrieveSomeData { (data, error) in

    guard let data = data, error == nil else {
        return
    }

    // database has retrieved data, create dispatch group
    let dispatchGroup = DispatchGroup()

    for document in data { // loop through collection

        // check if document has some identifier
        guard let someIdentifier = document["someIdentifier"] as? String else {
            return
        }

        dispatchGroup.enter() // identifier found, enter dispatch

        // perform async operation inside loop
        Filestorage.getSomeFile(forURL: someIdentifier) { (data, error) in

            guard let file = data, error == nil else {
                return
            }

            // download the file
            dispatchGroup.leave() // leave dispatch

        }

    }

    dispatchGroup.notify(queue: .main) {

        // all data grabbed, load table

    }

}