Swift:在 View 出现之前从异步调用中检索值

Question

我使用 HanekeSwift 检索缓存的数据，然后在每次 View 出现时将其设置为 swipeView 中的标签。我的代码检索数据没有问题，但由于 cache.fetch() 是异步的，当我调用我的方法来更新 View 时，我的标签被设置为 nil。无论如何，有没有办法告诉 swift 在加载 View 之前等待我的缓存数据被检索？

参见下面的代码:

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，它总是显示 View ，然后单步进入缓存 block 。如何使 viewWillAppear() 允许 updateEntries() 完成并且在执行缓存 block 之前不返回？提前非常感谢!

更新1:

下面的解决方案工作得很好，我的调用是按正确的顺序进行的(通知 block 中的打印语句在缓存检索后执行)，但我的 View 仅在服务器启动时用非零值更新其标签叫。也许我在通知组中添加了错误的代码？

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:

此外，当我切换 View 时，我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

“此应用程序正在从后台线程修改自动布局引擎，这可能会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

注意:

• 调用异步方法之前进入组
• 离开组是每个相应的完成/失败处理程序
• 将通知 block 中的 UI 更新调度到主队列

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()
    }

}