Calling functions from WebView to JS

Question

In the java code(container) i have to get some values by calling some javascript functions, for this im using webView.evaluateJavascript, the problem is that i get the callback only when all the code had already been executed. Is there any way to handle this or any other better alternative?

this is the code:

String[] jsonStringTagResult = new String[1];

//trying to get the value returned by patrolTagToInvoke(tagid)
webView.evaluateJavascript("patrolTagToInvoke('"+tagId+"')", new ValueCallback<String>() {
    @Override
    public void onReceiveValue(String value) {
    jsonStringTagResult[0] = value;
    }
});

//the callback from js doesn't arrive in time so jsonStringTagResult will always be null
if (jsonStringTagResult[0] != null || !jsonStringTagResult[0].isEmpty()) {

Answer 1

• the asynchronous nature of evaluateJavascript. The callback you're passing to evaluateJavascript is not guaranteed to be called immediately after the JavaScript code has been executed. This is because the JavaScript code is executed in the context of the WebView, which means it's running on a different thread.

• Add the necessary permissions to your AndroidManifest.xml file

• First you create a Java class that will be used as the JavaScript interface

  public class WebAppInterface {
    private Context context;
  
    public WebAppInterface(Context context) {
      this.context = context;
    }
  
    @JavascriptInterface
    public void sendTagResult(String tagResult) {
      // Handle the tag result here
    }
  }
  

• Then, add the JavaScript interface to your WebView

  webView.addJavascriptInterface(new WebAppInterface(this), "Android");
  

• you can modify your JavaScript code to call the sendTagResult method when you have the result

  function patrolTagToInvoke(tagid) {
    // Your existing code here
    // When you have the result, call the sendTagResult method
    Android.sendTagResult(result);
  }
  

• Finally, in your Java code, you can override the onJsPrompt method of your WebViewClient to handle the prompt from your JavaScript code

  webView.setWebViewClient(new WebViewClient() {
   @Override
   public boolean onJsPrompt(WebView view, String url, String message, String defaultValue, JsPromptResult result) {
       // Check if the message is the tag result
       if (message.startsWith("tagResult:")) {
           // Extract the tag result from the message
           String tagResult = message.substring("tagResult:".length());
           // Handle the tag result here
           // ...
           // Don't forget to call JsPromptResult.confirm to indicate that you've handled the prompt
           result.confirm("");
           // Return true to indicate that you've handled the prompt
           return true;
       }
       // If the message is not a tag result, return false to let the WebView handle the prompt
       return false;
    }
  });
  

• Android Calling JavaScript functions in WebView : Android Calling JavaScript functions in WebView

• Communication with WebView in Android : https://www.techyourchance.com/communication-webview-javascript-android/