Swift WKWebView 在 didFailProvisionalNavigation 上崩溃

Question

We are encountering an intermittent (happens on some devices, some of the time) crash that we are having a hard time nailing down, and are unable to reproduce on-demand. This is related to Swift 3 and the WKWebView component, specifically its callback protocol crashes when attempting to get the error code via a switch statement. see below:

func webView(_ webView: WKWebView, didFailProvisionalNavigation navigation: WKNavigation!, withError error: Error) {

  if let err = error as? URLError {

    switch(err.code) {  //  Exception occurs on this line
    case .cancelled:
      Hint(hide: true)

    case .cannotFindHost:
      Hint(hide: false, hint:.CannotFindHost)

    case .notConnectedToInternet:
      Hint(hide: false, hint: .NoInternet)

    case .resourceUnavailable:
      Hint(hide: false)

    case .timedOut:
      Hint(hide: false)

    default:
      Hint(hide: false)
      print("error code: " + String(describing: err.code) + "  does not fall under known failures")
    }
  }
}

func Hint(hide: Bool, hint:SomeCustomEnum = SomeCustomEnum.Default) {
     //Dosomething with ui to let user know something bad happened
}

the error stack indicates:

0 _BridgedStoredNSError.code.getter

1 _BridgedStoredNSError.code.getter

2 specialized WebKitController.webView(WKWebView, didFailProvisionalNavigation : WKNavigation!, withError : Error) -> ()

3 @obj WebKitController.webView(WKWebView, didFailProvisionalNavigation : WKNavigation!, withError : Error) -> ()

...

Reviewing the code it seems as if it should be effectively free from issues since the variable err should be successfully optionally-unwrapped as a valid URLError object by the time the switch statement is invoked. The switch statement at that point should be guaranteed a value in err.code since .code is not optional for URLError.

Attempts to artificially cause an error that might explain the issue have so far not provided much insight. ie. if I create my own custom error without a code property, then attempt to cast that as a URLError, it gracefully falls out of the optional assignment.

Any help or suggestions to revolve or even further troubleshoot is appreciated, in the mean time will continue to try to reproduce on a consistent basis.

Answer 1

When looking into the Swift Bug ( https://bugs.swift.org ) submission site I was able to find a description of the problem, ie. Error cast to URLError can result in the property .code being missing:

https://bugs.swift.org/browse/SR-3879?jql=text%20~%20%22URLError%22

This has a link to the following reference which seems to be the solution (still in process)

https://bugs.swift.org/browse/SR-3881

Effectively URLError is missing two .code definitions:

NSURLErrorAppTransportSecurityRequiresSecureConnection NSURLErrorDataLengthExceedsMaximum

So if your encountering a crash when referencing the .code property of a URLError, you can check for it by casting to NSError and checking against the NSError .code property.

We are mitigating it with a temporary workaround until the bug is resolved (below only addresses the NSURLErrorAppTransportSecurityRequiresSecureConnection (int -1022) type failures):

func webView(_ webView: WKWebView, didFailProvisionalNavigation navigation: WKNavigation!, withError error: Error) {

  let nserr = error as NSError
  if nserr.code == -1022 {
    Hint(hide: false, hint: .NSURLErrorAppTransportSecurityRequiresSecureConnection)

  } else if let err = error as? URLError {

    switch(err.code) {  //  Exception no longer occurs
    case .cancelled:
      Hint(hide: true)

    case .cannotFindHost:
      Hint(hide: false, hint:.CannotFindHost)

    case .notConnectedToInternet:
      Hint(hide: false, hint: .NoInternet)

    case .resourceUnavailable:
      Hint(hide: false)

    case .timedOut:
      Hint(hide: false)

    default:
      Hint(hide: false)
      print("error code: " + String(describing: err.code) + "  does not fall under known failures")
    }
  }
}

func Hint(hide: Bool, hint:SomeCustomEnum = SomeCustomEnum.Default) {
     //Dosomething with ui to let user know something bad happened
}