How to wait for a return value from executed JavaScript MAUI Android Threading

Question

Hello all

I want get "right Threading way" when I wait for a JavaScript's result inside android webview (unfortunately function EvaluateJavaScriptAsync doesn't work :-(, cause I have own handler – need both directions from and to Javascript-C# include return values)

I have the functional c# code (see below) but code is quite slow and "not threading perfect"

I need REPLACE "while loop" and Thread.Sleep(100); with something "Threading Ingenious" (catch some message?..), I am just "Threading dummy user"

Thanks a lot Threading experts!

W

public class HybridWebViewHandler : ViewHandler<IHybridWebView, Android.Webkit.WebView>
{
...
	VirtualView.RequestEvaluateJavaScript += VirtualView_RequestEvaluateJavaScript;
...
        private void VirtualView_RequestEvaluateJavaScript(object sender, EvaluateJavaScriptAsyncRequest e)
        {
            EvaluateBack evaluateBack = new EvaluateBack();
            sync.Post((o) => PlatformView.EvaluateJavascript(e.Script, evaluateBack), null);
            while(!evaluateBack.Finished)
            {
                Thread.Sleep(100);
            }
            e.SetResult(evaluateBack.JavaScriptReturnValue);
        }

        public class EvaluateBack : Java.Lang.Object, IValueCallback
        {
            public bool Finished = false;
            private string _JavaScriptReturnValue = null;
            public string JavaScriptReturnValue
            {
                get { return _JavaScriptReturnValue; }
                set { _JavaScriptReturnValue = value; }
            }
            public void OnReceiveValue(Java.Lang.Object value)
            {
                _JavaScriptReturnValue = value.ToString();
                Finished = true;
            }
        }
...
}

Call:
EvaluateJavaScriptAsyncRequest request = new EvaluateJavaScriptAsyncRequest(JavaScriptCommand);
await Task.Run(() =>
{
    RequestEvaluateJavaScript?.Invoke(this, request);
});
return request.Task.Result;

Answer 1

I assume you are calling an async javascript routine. an async javascript function when called from c# just gets the promise object converted to a string. you need the javascript function to send a message to the c# on completion. you use the HybridWebView postMessage()

myAsyncFunction(...).then(data => myPostMessage(data)); 

the myPostMessage() should call .postMessage() which differs based on hosting o/s and which webview used. you will also need to implement a custom handler to receive the message. see:

https://github.com/dotnet/maui/discussions/12009

Answer 2

Hello,

WebView provides an out-of-the-box way to get the return value, and you could use EvaluateJavaScriptAsync instead of EvaluateJavaScript.

Please refer to Invoke JavaScript for more detailed information.

Best Regards,

Alec Liu.

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Answer 3

Hallo Yonglun

thanks, I tried something with Send method (thanks for description of difference post/send), but I can't write down right syntax :-(

but I have a some progress!:

I changed Thread.Sleep(100); to Thread.Sleep(5); and it is acceptable solution now (speed quite good), but something inside of my soul says: it is not good clear solution :-)

thanks and regards

W

Answer 4

Hello Thread champions

I found solution apparently without Thread.Sleep here:

https://stackoverflow.com/questions/50998907/android-webview-blocking-evaluatejavascript

but I don't know how to rewrite code from android studio to C# there is some keyword "final", and generated override method inside parameter passed to function etc..

W