WKWebView UIAlert Crash

WKWebView를 사용하다보면 종종 아래와 같은 크래시를 발견할 수 있다.

Fatal Exception: NSInternalInconsistencyException Completion handler passed to -[UIViewController webView:runJavaScriptAlertPanelWithMessage:initiatedByFrame:completionHandler:] was not called

원인

• UIAlertController를 네이티브에서 present 된 상태
  1. completionHandler 를 리턴하지 않은 상태에서 UIAlertController 소유한 UIViewController 가 dismiss
  2. completionHandler 가 리턴되지 않은 상태에서 다시 UIAlertController present

WKUIDelegate

이러한 치명적 오류는 아래 코드를 통해 방어할 수 있다.

extension UIViewController: WKUIDelegate {
  func webView(_ webView: WKWebView,
               runJavaScriptAlertPanelWithMessage message: String,
               initiatedByFrame frame: WKFrameInfo,
               completionHandler: @escaping () -> Void) {
    guard webView.url?.host != nil else {
      completionHandler()
      return
    }
    
    let completionHandlerWrapper = CompletionHandlerWrapper(completionHandler: completionHandler, defaultValue: Void())
    let alert = UIAlertController(title: nil,
                                  message: message,
                                  preferredStyle: .alert)
    alert.addAction(UIAlertAction(title: "ok",
                                  style: .default,
                                  handler: { _ in
      completionHandlerWrapper.respondHandler(())
    }))
    
    present(alert, animated: true, completion: nil)
  }
  
  func webView(_ webView: WKWebView,
               runJavaScriptConfirmPanelWithMessage message: String,
               initiatedByFrame frame: WKFrameInfo,
               completionHandler: @escaping (Bool) -> Void) {
    guard webView.url?.host != nil else {
      completionHandler(false)
      return
    }
    
    let completionHandlerWrapper = CompletionHandlerWrapper(completionHandler: completionHandler, defaultValue: false)
    let alert = UIAlertController(title: nil, message: message, preferredStyle: .alert)
    alert.addAction(UIAlertAction(title: "cancel",
                                  style: .cancel,
                                  handler: { _ in
      completionHandlerWrapper.respondHandler(false)
    }))
    
    alert.addAction(UIAlertAction(title: "ok",
                                  style: .default,
                                  handler: { _ in
      completionHandlerWrapper.respondHandler(true)
    }))
    
    present(alert, animated: true, completion: nil)
  }
}

public final class CompletionHandlerWrapper<Element> {
  private var completionHandler: ((Element) -> Void)?
  private let defaultValue: Element
  
  deinit {
    respondHandler(defaultValue)
  }
  
  public init(completionHandler: @escaping ((Element) -> Void), defaultValue: Element) {
    self.completionHandler = completionHandler
    self.defaultValue = defaultValue
  }
  
  public func respondHandler(_ value: Element) {
    completionHandler?(value)
    completionHandler = nil
  }
}