Open Radar

 
Summary:

I have a WKWebView which loads its contents offscreen. Once it loads, it is added to the view hierarchy.

However, I have found that if the WKWebView’s frame is set after the contents load (but while offscreen), once you add it to the view hierarchy it will adjust its contentOffset in an undesired way that makes it scroll 1.5 screens down the page. Setting the contentOffset in  the webView:didFinishNavigation: callback doesn’t help, as the undesired action happens some time after that.

Paste the following code in to a Playground (tested Xcode 7.0) to reproduce. 
==================

import UIKit
import WebKit
import XCPlayground

class WKViewController : UIViewController, WKNavigationDelegate
{
	var web            : WKWebView? = nil
	var loadNavigation : WKNavigation? = nil

	override func loadView()
	{
		super.loadView()

		view.backgroundColor = UIColor.orangeColor()

		web = WKWebView()
		web!.navigationDelegate = self
	}

	override func viewDidLoad()
	{
		let url     = "http://nshipster.com/mirrortype/"
		let nsURL   = NSURL(string:url)!
		let request = NSURLRequest(URL:nsURL)
		loadNavigation = web!.loadRequest(request)

		// NOTE: Uncomment this line to work-around
//		web!.frame = view.bounds
	}

	func webView(webView: WKWebView, didFinishNavigation navigation: WKNavigation!)
	{
		print("finished")

		if navigation == loadNavigation
		{
			view.addSubview( webView )
			webView.frame = view.bounds
			// This line doesn't have any effect; CO gets set after this in some WKWebView magic.
			webView.scrollView.contentOffset = CGPoint.zero
		}
	}

	func webView(webView: WKWebView, didFailNavigation navigation: WKNavigation!, withError error: NSError)
	{
		print("failed")
	}
}

let vc = WKViewController()
XCPShowView( "", view: vc.view )