Question

我正在使用HanekeSwift检索缓存数据，然后在每次出现视图时将其设置为swipeView中的标签。我的代码检索数据没问题，但由于cache.fetch()是异步的，当我调用我的方法更新视图时，我的标签设置为nil。无论如何要告诉swift在加载视图之前等待我的缓存数据？

见下面的代码：

override func viewWillAppear(animated: Bool) {
    updateEntries() // updates entries from cache when view appears
}

func updateEntries() {
    guard let accessToken = NSUserDefaults.standardUserDefaults().valueForKey("accessToken") as? String else { return }
    guard let cachedEntryKey = String(accessToken) + "food_entries.get" as? String else { return }
    cache.fetch(key: cachedEntryKey).onSuccess { data in
        ...
        // if successful, set labels in swipeView to data retrieved from cache
        ...
        dispatch_group_leave(dispatchGroup)
    } .onFailure { error in
        print(error)
        ...
        // if unsuccessful, call servers to retrieve data, set labels in swipeView to that data
        ...
        dispatch_group_leave(dispatchGroup)
    }
}

当我单步执行上面的代码时，它始终显示视图，然后进入缓存块。如何使viewWillAppear（）允许updateEntries（）完成，并且在执行缓存块之前不返回它？非常感谢！[/ p>

更新1：

下面的解决方案工作得很好，我的调用是按照正确的顺序进行的（我在通知块中的print语句在缓存检索后执行），但我的视图只在服务器是的时使用非零值更新标签调用。也许我在通知组中混淆了错误的代码？

override func viewWillAppear(animated: Bool) {
    self.addProgressHUD()
    updateEntries() // updates entries from cache when view appears
}

func updateEntries() {
    guard let accessToken = NSUserDefaults.standardUserDefaults().valueForKey("accessToken") as? String else { return }
    guard let cachedEntryKey = String(accessToken) + "food_entries.get" as? String else { return }

    let dispatchGroup = dispatch_group_create()
    dispatch_group_enter(dispatchGroup)

    dispatch_group_async(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
        cache.fetch(key: cachedEntryKey).onSuccess { data in
            ...
            // if successful, set labels in swipeView to data retrieved from cache
            ...
        } .onFailure { error in
            print(error)
            ...
            // if unsuccessful, call servers to retrieve data, set labels in swipeView to that data
            ...
        }
    }

    dispatch_group_notify(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
        print("Retrieved Data")
        self.removeProgressHUD()
    }

}

更新2：

另外，当我切换视图时，我在控制台中收到此警告。我想我用上面的代码锁定主线程

“此应用程序正在从后台线程修改自动布局引擎，这可能导致引擎损坏和奇怪的崩溃。这将在将来的版本中导致异常。”

Answer 1

好的建议大家对此有所帮助。以为我明白了。我需要确保我的缓存块不阻塞主队列。见下面的代码

修改

感谢@Rob帮助我做出适当的调整以使这项工作

let dispatchGroup = dispatch_group_create() dispatch_group_enter(dispatchGroup) cache.fetch(key: cachedEntryKey).onSuccess { data in ... // if successful, set labels in swipeView to data retrieved from cache ... dispatch_group_leave(dispatchGroup) } .onFailure { error in print(error) ... // if unsuccessful, call servers to retrieve data, set labels in swipeView to that data ... dispatch_group_leave(dispatchGroup) } dispatch_group_notify(dispatchGroup, dispatch_get_main_queue()) { print("Retrieved Data") self.removeProgressHUD() }

Answer 2

注意：

在调用异步方法之前输入group
离开组是各自的完成/失败处理程序
将通知块中的UI更新分派到主队列

因此：

func updateEntries() {
    guard let accessToken = NSUserDefaults.standardUserDefaults().valueForKey("accessToken") as? String else { return }
    guard let cachedEntryKey = String(accessToken) + "food_entries.get" as? String else { return }

    let group = dispatch_group_create()
    dispatch_group_enter(group)

    cache.fetch(key: cachedEntryKey).onSuccess { data in
        ...
        // if successful, set labels in swipeView to data retrieved from cache
        ...
        dispatch_group_leave(group)
    } .onFailure { error in
        print(error)
        ...
        // if unsuccessful, call servers to retrieve data, set labels in swipeView to that data
        ...
        dispatch_group_leave(group)
    }

    dispatch_group_notify(group, dispatch_get_main_queue()) {
        print("Retrieved Data")
        self.removeProgressHUD()
    }

}

Answer 3

以下是您可以暂存加载屏幕的简单示例。我只是创建一个提醒视图，您也可以创建自定义加载指标视图。

let alert = UIAlertController(title: "", message: "please wait ...", preferredStyle: .alert)

override func viewWillAppear(animated: Bool) {
    self.present(alert, animated: true, completion: nil)
    updateEntries() // updates entries from cache when view appears
}

func updateEntries() {
    guard let accessToken = UserDefaults.standard.value(forKey: "accessToken") as? String,
          let cachedEntryKey = (accessToken + "food_entries.get") as? String else { 
      return 
    }
    cache.fetch(key: cachedEntryKey).onSuccess { data in
        ...
             // update value in your UI
             alert.dismiss(animated: true, completion: nil)
        ...
        } .onFailure { error in
            print(error)
            ...
                // if unsuccessful, call servers to retrieve data, set labels in swipeView to that data
            ...
    }
}

Answer 4

虽然我完全赞同@ozgur关于从UX角度展示某种加载指示器，但我认为学习如何使用Grand Central Dispatch（Apple的异步等待的原生解决方案）的好处可能会有所帮助你是长期的。

在运行某种类型的完成处理程序之前，您可以使用dispatch_groups等待代码块完全完成运行。

来自Apple's documentation：

调度组是一种监视一组块的机制。您的应用程序可以根据您的需要同步或异步监视组中的块。通过扩展，一个组可以用于同步代码，这取决于其他任务的完成。

[...]

调度组会跟踪未完成的块数，并且GCD会保留该组，直到其所有关联的块完成执行。

以下是dispatch_groups的实例示例：

let dispatchGroup = dispatch_group_create()

dispatch_group_async(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
    // Run whatever code you need to in here. It will only move to the final
    // dispatch_group_notify block once it reaches the end of the block.
}

dispatch_group_notify(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
    // Code in here only runs once all dispatch_group_async blocks associated
    // with the dispatchGroup have finished completely.
}

关于dispatch_groups的重要部分是，它们允许您同时运行多个异步块，并等待所有异步块在运行最终完成处理程序之前完成。换句话说，您可以根据需要将dispatch_group_async块与dispatchGroup关联起来。

如果你想使用加载指示器方法（你应该），你可以运行代码来显示加载指示符，然后进入带有完成处理程序的dispatch_group以删除加载指示符并加载数据dispatch_group完成后进入视图。

Swift：在视图出现之前从异步调用中检索值

4 个答案: