Firebase 未按正确顺序下载 GIF - iOS

Question

所以我使用 Firebase 存储存储了 4 个 gif，我正在使用实时数据库来获取它们的下载 URL 并将它们附加到一个数组中。问题是，当我想通过遍历数组来下载它们时，它们并不总是以正确的顺序下载。其实每次都是随机下载的。

到目前为止我的代码：

    if self.downloadURLS.isEmpty == true {
    } else {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)) {
            var newItems: [UIImage] = [UIImage]()

            for (index, element) in self.downloadURLS.enumerate() {
                 print("Item \(index): \(element)")
                self.storageRef = self.storage.referenceForURL(element)
                self.storageRef.dataWithMaxSize(5 * 1024 * 1024) { (data, error) -> Void in
                    if (error != nil) {
                        // Uh-oh, an error occurred!
                    } else {
                        self.image = UIImage(gifData: data!, levelOfIntegrity: 1)
                        print("Downloaded \(index)")
                        newItems.append(self.image)
                    }
                }


                self.items = newItems
            }
        }

        print(downloadURLS.joinWithSeparator("\n"))
    }

有什么方法可以确保他们下载的顺序正确，或者我可以在最后使用某种排序方法吗？上次完成下载的顺序是：2,3,0,1...任何帮助都会很棒！

Answer 1

我猜你的处理程序是异步调用的。我建议你事先声明你的newItems 数组并添加一个dispatch_group_t 来跟踪每张图片的下载时间：

var newItems = [UIImage?](count: downloadURLS.count, repeatedValue: nil)
let group = dispatch_group_create()

for (index, element) in self.downloadURLS.enumerate() {
    dispatch_group_enter(group)
    self.storageRef = self.storage.referenceForURL(element)
    self.storageRef.dataWithMaxSize(5 * 1024 * 1024) { (data, error) -> Void in
        if (error != nil) {
            // element at this index stays nil
        } else {
            // Set the element at this index
            newItems[index] = UIImage(gifData: data!, levelOfIntegrity: 1)
        }
        dispatch_group_leave(group)
    }
}

// The following gets called asynchronously when every image has been downloaded (or failed to)
dispatch_group_notify(group, dispatch_get_main_queue()) {
    // This will throw an error because newItems is now an array of Optional UIImages
    // It contains nil when there was an error while getting a specific item
    self.items = newItems
}

另外：在for 循环中分配self.storageRef 是否正确？我相信您应该改用局部变量：

let ref = self.storage.referenceForURL(element)
ref.dataWithMaxSize ...