Swift: 在视图出现之前从异步调用中检索值

Question

我正在使用 HanekeSwift 检索缓存数据，然后在每次视图出现时将其设置为 swipeView 中的标签。我的代码检索数据没问题，但是因为 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 等到我的缓存数据被检索后再加载视图？

查看下面的代码：

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，它总是显示视图，然后单步进入缓存块。如何让 viewWillAppear() 允许 updateEntries() 完成，而不是 return 在执行缓存块之前退出它？提前致谢！

更新 1：

下面的解决方案运行良好，我的调用顺序正确（我在通知块中的打印语句在缓存检索后执行），但我的视图仅在服务器被调用。也许我在通知组中输入了错误的代码？

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()
    }

}

更新二：

此外，我在切换视图时在控制台中收到此警告。我想我用上面的代码锁定了主线程

"This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes. This will cause an exception in a future release."

Answer 1

这是一个简单的示例，您可以展示一个加载屏幕。我只是创建了一个警报视图，您也可以创建自定义 加载指示器视图 。

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 2

虽然我完全同意@ozgur 关于从用户体验的角度显示某种加载指示器的观点，但我认为学习如何使用 Grand Central Dispatch（Apple 的异步等待本地解决方案）的好处可能会帮助您长期。

您可以使用 dispatch_groups 等待代码块完全完成运行ning，然后运行ning 某种完成处理程序。

来自Apple's documentation：

A dispatch group is a mechanism for monitoring a set of blocks. Your application can monitor the blocks in the group synchronously or asynchronously depending on your needs. By extension, a group can be useful for synchronizing for code that depends on the completion of other tasks.

[...]

The dispatch group keeps track of how many blocks are outstanding, and GCD retains the group until all its associated blocks complete execution.

下面是 dispatch_groups 的一个例子：

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之前完成处理程序。换句话说，您可以根据需要将任意多的 dispatch_group_async 块与 dispatchGroup 相关联。

如果您想使用加载指示器方法（您应该这样做），您可以运行代码来显示加载指示器，然后移动到 dispatch_group 带有完成处理程序以删除dispatch_group 完成后加载指示器和加载数据进入视图。

Answer 3

每个人的好的建议在这方面帮助很大。想我明白了。我需要确保我的缓存块没有阻塞主队列。请参阅下面的代码

编辑

感谢@Rob 帮助我进行适当的调整以完成这项工作

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 4

注意：

调用异步方法前进群
离开组是各自的 completion/failure 个处理程序
调度UI通知块中的更新到main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()
    }

}

Swift: 在视图出现之前从异步调用中检索值

Swift: Retrieve value from asynchronous call before view appears

closures

asynchronous

grand-central-dispatch

swift

haneke