Native側で、JavaScript機能の呼び出し、

func getImages() {
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
            appWebView?.evaluateJavaScript(
            "getLastImages()") { (result, error) -> Void in
                if error != nil {
                    print(error as Any)
                }
                print(result as Any)
            }
        }
    }

JavaScriptからJsonポストして、

getLastImages: function() {
      let message = {
        cmd: 'lastimages',
        count: '1',
        callbackFunc: function(responseAsJSON) {
          let response = JSON.parse(responseAsJSON)
          let image = response['image']
          if(image){
            self.form.image_white = image
          }
        }.toString()
      }
      window.webkit.messageHandlers.native.postMessage(message)
    }

Native側のHandlerは、

func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
    let sentData = message.body as! NSDictionary
    let command = sentData["cmd"] as! String
    var response = Dictionary<String,AnyObject>()
    if command == "lastimages"{
                appWebView?.evaluateJavaScript(
                "document.getElementById('fetchimage').style.display = 'none';") { (result, error) -> Void in
                    if error != nil {
                        print(error as Any)
                    }
                    print(result as Any)
                }
                concurrentQueue.sync {
                    var imageString = [String]()
                    imageString = imgProcessHandler.identifyFetchResult()
                    response["image"] = imageString as AnyObject
                    let callbackString = sentData["callbackFunc"] as? String
                    self.sendResponse(aResponse: response, callback: callbackString)
                }
        }
｝