Web page of WKWebView not displaying page inside of dispatch_get_main_queue

Question

I'm trying to create multiple WKWebView views inside of a background process and then add them to a view on the main thread once they are all done loading.

Each of the WKWebView's contains a chart rendered via javascript so the load time takes about a second per WKWebView so I'm trying to offload the processing to the background so the UI isn't blocked.

This works fine when dispatch_get_main_queue is commented out, however the ui is blocked for 5-10 seconds. Only the brown background of the WKWebView shows up, none of the contents from the webpage.

 var webViews : [WKWebView] = []   

 var myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
 dispatch_async(myQueue, {

 for i in 0...10
 {
      var url : NSURL? = NSURL(string:"http://google.com")
      var req = NSURLRequest(URL:url!)

      var webview = WKWebView(frame:CGRectMake(0, height * CGFloat(i), width, height))         
      webview.loadRequest(req)
      webview.backgroundColor = UIColor.brownColor()

      self.webViews.append(webview)
  }

    dispatch_async(dispatch_get_main_queue(),{

      for item in self.webViews
      {
         self.view.addSubview(item)
      }

     });
  });

Answer 1

If I change the WKWebView to UIWebView, there is a crash.

Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread.

Calling a UIKit method from a thread other than main thread is not allowed. WKWebView is also a UIView subclass. So I suggest you move setting frame and addSubView method out of the block, and put it before you call dispatch_get_global_queue, and inside dispatch_get_global_queue block, you load the request one by one.

Edit

To monitor if a request has finished loading, you can implement WKNavigationDelegate's didFinishNavigation function. You can set a counter, make the counter increase by 1 when the function is called, when the counter value is equal to 10, all webviews are fully loaded.

var counter = 0
var globalStart : NSDate?
var globalEnd : NSDate?

And in viewDidLoad.

var start = NSDate()
for i in 0...9
{
    var item = WKWebView()
    item.frame = CGRectMake(0, self.view.bounds.height * CGFloat(i),
        self.view.bounds.width, self.view.bounds.height)
    item.navigationDelegate = self
    self.scrollView.addSubview(item)
    self.webViews.append(item)
}
self.scrollView.contentSize = CGSizeMake(self.view.bounds.width, (self.view.bounds.height - 50.0) * CGFloat(11))
let end = NSDate();
NSLog("creating  webviews  \(end.timeIntervalSinceDate(start))")


dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),{

    self.globalStart = NSDate()

    for item in self.webViews
    {
        var url : NSURL? = NSURL(string:"http://google.com")
        var req = NSURLRequest(URL:url!)
        item.loadRequest(req)
    }
});


func webView(webView: WKWebView, didFinishNavigation navigation: WKNavigation!) {
    counter++
    println("\(counter) \(webView)")
    if counter == 10 {
        globalEnd = NSDate()
        println(globalEnd!.timeIntervalSinceDate(globalStart!))
    }
}

The result is creating webviews 1.85267299413681, while time of loading all requests is more than 8 seconds for me. I didn't find a way to decrease the time of creating webviews, I think it takes that much of time to create those views.