问题
I'm encountering an problem where I am having a long delay when trying to present a ViewController
. I am trying to display an upgrade alert when a user clicks on a UITableViewCell
which requires premium access. In the ViewController
being presented, I put debug code:
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)")
}
The println
statement resulted in:
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)
As you can see there is a 3 second delay between the end of viewDidLoad
and the start of viewWillAppear
. I can't figure out why this is occurring. I am creating the views programmatically within the ViewController
, so the storyboard is not being used here.
This is the code I have to present my 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)
回答1:
Creating and presenting the view controller should be done on the main thread.
(When view updates are delayed, it nearly always means you're erroneously working on a background thread.)
回答2:
I am putting this out there so people understand the viewDidLoad
and viewDidAppear
time delay is nothing you did wrong if you are in UITableViewController
didSelectRowAt
.
There is clearly a hidden async
await
background thread bug inside Swifts UITableViewController
didSelectRowAt
callback:
If your code looks something like this it may well lag 2 or 3 seconds in between viewDidLoad
and viewDidAppear
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)
}
}
(despite you doing nothing to deserve it)
this is how you speed it up in Swift 4
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)
}
}
}
}
来源:https://stackoverflow.com/questions/31008850/swift-long-delay-between-viewdidload-and-viewwillappear