How to get a javascript function's return value from android webview to application - Java

Issue

I want to get the return value of a javascript function inside Android webview to be made accessible to the android application and get it converted to a string.

So I put this code:

String fileName = MyWebView.loadUrl("javascript:android.onData(returnFileName())");

But then it shows this error: Incompatible types. Found: 'void', required: 'java.lang.String'.

How to fix this?

Edit: Actually I have an input field where the user can enter the name of a file generated as base64(data:)(which was actually a blob but I converted it). On Firefox and Chrome the downloaded file have the name user entered (I used a.download=document.getElementById.value for that) but on webview while downloading(by setting Download listener) the file I had to process the (data:) to a working URI. Now I would like to add the name entered by User to the downloaded file. For that I need to get the value of input field inside webview to Android application.

Thanks in advance.

Solution

So this is what worked for me after experimenting with the answer by @Introspective; I am posting here because any future visitors may find it helpful. Instead of onPageFinished(), I put evaluateJavascript() inside download listener. A JsonReader was required as the result is a JSON value.

MyWebView.setDownloadListener((url, userAgent, contentDisposition, mimeType, contentLength) -> {
    if (url.startsWith("data:")) {
       MyWebView.evaluateJavascript("returnfileName()", string -> {
           JsonReader jsonReader = new JsonReader(new StringReader(string));
           jsonReader.setLenient(true);//this is needed because I am returning  a single value
    if(jsonReader.peek() != JsonToken.NULL) {
           String fileName = jsonReader.nextString();
           //Now I can use the String fileName as I wish.
           //Rest of the code
   }
}

Javascript function returnFileName()

function returnfileName(){
  var fileName;
  if (document.getElementById("id of input field").value===""){ 
  fileName = "Default file name in case user didn't enter any filename";
  }
  else{
  fileName = document.getElementById("id of input field").value;}
  }
  return fileName;
}

Since evaluateJavascript() is a void, I had to put rest of the code to download the file inside the evaluateJavascript() itself, because otherwise String fileName will not be accessible for them (void doesn't have a return value).

Answered By - ADasGH