js 注入时机太迟，做APP时必须提早注入。

[Jockeyvb]

internal async Task OnJavascriptInjectionRequest(string js)
{
if (Control == null || Element == null) return string.Empty;

        var response = string.Empty;

        try
        {
            var script = new WKUserScript(new NSString(js), WKUserScriptInjectionTime.AtDocumentStart, false);
            if (!_contentController.UserScripts.Contains(script)) { 
            _contentController.AddUserScript(script);
            _configuration.UserContentController = _contentController;
            }
            //var obj = await Control.EvaluateJavaScriptAsync(js).ConfigureAwait(false);
            //if (obj != null)
            //    response = obj.ToString();
        }

        catch (Exception) { /* The Webview might not be ready... */ }
        return response;
	}

代码直接写入到WKUserScriptInjectionTime.AtDocumentStart

然后在html 中，这样$(function(){ alert(jsCallAPP));} 则可以响应，如果用Control.EvaluateJavaScriptAsync(js)则在onready 事件中也未注入，所以为undefine 或报错。