Showing hidden view really slow

Question

I have a setup as such:

self.webView = WKWebView(frame: frame, configuration: WKWebViewConfiguration())
self.externView = UIView(frame: frame)
self.externWebView = WKWebView(frame: subFrame, configuration: WKWebViewConfiguration())
let StackView = UIStackView(frame: frame)


self.externView?.addSubview(StackView)
self.externView?.backgroundColor = UIColor.blackColor()

view.addSubview(self.webView!)
self.externView?.hidden = true
StackView.addSubview(self.externWebView!)
view.addSubview(self.externView!)

This creates the initial setup I want, this allows for 1 web view to act as the main one, which can launch and external one to render something else, that is initially hidden.

The problem I'm running into, is that from a worker thread I'm calling

WKExteralLoader.externView!.hidden = false;
WKExteralLoader.webView!.hidden = true;

which can take up to 30 seconds to take effect; however, its inverse is nearly instantaneous.

WKExteralLoader.webView!.hidden = false;
WKExteralLoader.externView!.hidden = true;

Any ideas what could cause this problem?

Thanks!

score 4 · Answer 1 · answered Jun 27 '16 at 15:01

4

Run the hide/show code on the UI thread

 dispatch_async(dispatch_get_main_queue()) {
        WKExteralLoader.webView!.hidden = false;
        WKExteralLoader.externView!.hidden = true;
    }

answered Jun 27 '16 at 15:01

MobileMon

8,341
5
56
75

1

**Swift 4:** `dispatch_get_main_queue()` has been replaced with `DispatchQueue.main.async()`. – Faizan Mubasher Feb 14 '18 at 07:30

Duncan C · Accepted Answer · 2018-02-14T13:23:05.947

@MobileMon told you how to fix your problem, but not why.

The UIKit is not thread-safe, meaning that must make all UIKit calls from the main thread. Setting a view's hidden flag is a UIKit call, so it needs to be done from the main thread.

If you manipulate UIView objects from a background thread results are undefined. Most of the time the result is that it takes a long time for the change to take effect (what you are seeing), but sometimes the change doesn't happen at all, sometimes the result is bizarre visual effects, and sometimes your app will crash.

Whenever you manipulate a UIView object from a background thread, you need to wrap it in a call to dispatch_async(dispatch_get_main_queue()) as in MobileMon's answer (Or some other approach that causes the code to be run from the main thread, but this is the preferred way.)

Edit:

To be clear, you say:

The problem I'm running into, is that from a worker thread I'm calling

WKExteralLoader.externView!.hidden = false; WKExteralLoader.webView!.hidden = true;

You can't do UI code from a worker thread. Instead you need to write that code like this:

dispatch_async(dispatch_get_main_queue(), ^{
  WKExteralLoader.externView!.hidden = false;
  WKExteralLoader.webView!.hidden = true;
});

That way, from your worker thread you dispatch your UI code back to the main thread.

You may have other code that's trying to do UIKit calls from a background thread when you're not aware of it. To check, add the following line:

print("isMainThread = \(Thread.current.isMainThread)")

And then check to see what's printing in the console.

Thank you for the detailed overview! – Ajwhiteway Jun 27 '16 at 15:48 — Ajwhiteway, Jun 27 '16 at 15:48
Still facing the issue. And I am unable to resolve it – Faizan Mubasher Feb 14 '18 at 09:24 — Faizan Mubasher, Feb 14 '18 at 09:24

score 3 · Answer 3 · answered Mar 25 '17 at 12:51

3

Swift 3:

DispatchQueue.global(qos: .background).async {
   DispatchQueue.main.async {
   // enter code here
   }
}

answered Mar 25 '17 at 12:51

Martin Allermann Haun

31
2

This fixed my issue of my share extension taking a long time to appear on screen. I was making UI calls in the `completionHandler` of `itemProvider.loadItem`. Using the code above fixed it. – qwertzguy Oct 27 '18 at 06:46

Showing hidden view really slow

3 Answers3

Edit:

Linked