Swift viewDidLoad 和 viewWillAppear 之间的长时间延迟

Question

我遇到了一个问题，我在尝试展示 ViewController 时遇到了很长的延迟。当用户单击需要高级访问权限的 UITableViewCell 时，我试图显示升级警报。在显示的 ViewController 中，我放置了调试代码：

override func viewDidLoad() {
    super.viewDidLoad()

    println("\(NSDate()) viewDidLoad")
    // Set Navigation Title font and color
    self.navigationController?.navigationBar.titleTextAttributes = [NSFontAttributeName: UIFont(name: "UbuntuCondensed-Regular", size: 22)!,
        NSForegroundColorAttributeName: UIColor.whiteColor()]
    println("\(NSDate()) end of viewDidLoad")
}

override func viewWillAppear(animated: Bool) {
    println("\(NSDate()) before super.viewWillAppear(animated)")
    super.viewWillAppear(animated)
    println("\(NSDate()) after super.viewWillAppear(animated)")
}

override func viewDidAppear(animated: Bool) {
    println("\(NSDate()) before super.viewDidAppear(animated)")
    super.viewDidAppear(animated)

    println("\(NSDate()) after super.viewDidAppear(animated)")
}

println 语句导致：

2015-06-23 16:36:54 +0000 viewDidLoad
2015-06-23 16:36:54 +0000 end of viewDidLoad
2015-06-23 16:36:57 +0000 before super.viewWillAppear(animated)
2015-06-23 16:36:57 +0000 after super.viewWillAppear(animated)
2015-06-23 16:36:58 +0000 before super.viewDidAppear(animated)
2015-06-23 16:36:58 +0000 after super.viewDidAppear(animated)

如您所见，viewDidLoad 结束与 viewWillAppear 开始之间有 3 秒的延迟。我不明白为什么会这样。我在 ViewController 中以编程方式创建视图，因此此处未使用故事板。

这是我必须展示我的代码 ViewController:

// Create the upgrade view contorller
let upgradeVC = UpgradeViewController()
// Set the presentation context
self.providesPresentationContextTransitionStyle = true
self.definesPresentationContext = true
// Set the upgrade controller to be modal over current context
upgradeVC.modalPresentationStyle = UIModalPresentationStyle.OverCurrentContext
// Show the view controller
self.navigationController?.presentViewController(upgradeVC, animated: true, completion: nil)

Answer 1

创建和呈现视图控制器应该在主线程上完成。

（当视图更新延迟时，几乎总是意味着您错误地处理了后台线程。）

Answer 2

我把它放在那里是为了让人们理解 viewDidLoad 和 viewDidAppear 时间延迟，如果你在 UITableViewController didSelectRowAt 中，你没有做错什么。

Swifts UITableViewController didSelectRowAt 回调中显然存在隐藏的 async await 后台线程错误：

如果您的代码看起来像这样，它可能会在 viewDidLoad 和 viewDidAppear

之间滞后 2 或 3 秒

class ViewControllerList: UITableViewController{
   override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath)       
   {
        let storyBoard = UIStoryboard(name: "Display", bundle:nil)
        let displayView = storyBoard.instantiateViewController(withIdentifier: "ViewControllerDisplay") as! ViewControllerDisplay
        self.present(displayView, animated: true, completion: nil)
   }
}

（尽管你没有做任何值得的事）

这就是您在 Swift 4

中加速的方式

class ViewControllerList: UITableViewController{
   override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath)       
   {
        DispatchQueue.global(qos: .background).async {
            // Background Thread
            DispatchQueue.main.async {
                // Run UI Updates or call completion block
                let storyBoard = UIStoryboard(name: "Display", bundle:nil)
                let displayView = storyBoard.instantiateViewController(withIdentifier: "ViewControllerDisplay") as! ViewControllerDisplay
                self.present(displayView, animated: true, completion: nil)
            }
        }
   }
}

Swift viewDidLoad 和 viewWillAppear 之间的长时间延迟

Swift Long Delay Between viewDidLoad and viewWillAppear

uitableview

uiviewcontroller

ios

uimodalpresentationstyle

swift