android Cannot run Java function from Javascript

Question

In my project I have a WebView which loads a page (HTML). I want to change all images and show a toast when a user clicks on any image.

So I'm adding javascript code which calls Java function:

// code is inside onPageFinished(..) function
JavaScriptInterface jsInterface = new JavaScriptInterface(activity);
webView.getSettings().setJavaScriptEnabled(true);
webView.addJavascriptInterface(jsInterface, "JSInterface");

webView.evaluateJavascript(
  "var imgs = document.getElementsByTagName('img');" +
  "for (var i = 0; i < imgs.length; i++) {" +
  "imgs[i].src = 'file:///android_asset/rules_images_placeholder.png';" +
  "imgs[i].addEventListener('click', function() {" +
  "window.JSInterface.showToast(); " + // <-- isn't called: see logs
  "});" +
  "} " +
  "window.JSInterface.showToast();" // <-- is called
  , null);

JavaScriptInterface class:

public class JavaScriptInterface {
  private Activity activity;
  public JavaScriptInterface(Activity activity) {
    this.activity = activity;
  }
  @JavascriptInterface
  public void showToast() {
    Toast.makeText(activity, "Toast message", Toast.LENGTH_SHORT).show();
  }
}

---

showToast() should be called when

1. page has finished loading

2. user has clicked on image

Problem: showToast() is called only once - when page has finished loading. When the user clicks on image, showToast() isn't called, instead the following log appears:

Uncaught TypeError: Cannot read property 'showToast' of undefined", source:

Question

How to call showToast() on image click?

Answer 1

I am not sure why, but you can fix this problem by adding a JavaScript interface prior to loading your HTML content.

jsInterface = new JavaScriptInterface(activity);
fragmentWebView.getSettings().setJavaScriptEnabled(true);
fragmentWebView.getSettings().setDomStorageEnabled(true);
fragmentWebView.addJavascriptInterface(jsInterface, "JSInterface");

fragmentWebView.loadDataWithBaseURL("file:///android_asset/", fullHtml, "text/html", "utf-8", null);
fragmentWebView.setWebViewClient(new WebViewClient() {
    @Override
    public void onPageFinished(WebView view, String url) {
        super.onPageFinished(view, url);
        view.loadUrl(
            "javascript:var imgs = document.getElementsByTagName('img');" +
            "for (var i = 0; i < imgs.length; i++) {" +
            "imgs[i].src = 'file:///android_asset/rules_images_placeholder.png';" +
            "imgs[i].addEventListener('click', function(e) {" +
            "window.JSInterface.showToast(); " +
            "});" +
            "}" +
            "console.log(window.JSInterface);" +
            "window.JSInterface.showToast(); ");
    }
});

Answer 2

Remove the evaluateJavascript methoad and use loadUrl. Try the below code its working for me:-

webView.getSettings().setJavaScriptEnabled(true);
webView.setWebViewClient(new WebViewClient() {
        @Override
        public void onPageStarted(WebView view, String url, Bitmap favicon) {


        }
        @Override
        public void onPageFinished(final WebView view, String url) {
            Log.e("checking", "MYmsg");

            webView.loadUrl("javascript: var imgs = document.getElementsByTagName('img');" +
                    "for (var i = 0; i < imgs.length; i++) {" +

                    "imgs[i].addEventListener('click', function() {" +
                    "window.CallToAnAndroidFunction.showToast(); " +    
                    "});" +
                    "} " );


        }
    });
    webView.addJavascriptInterface(new WebAppInterface(Main2Activity.this),
            "CallToAnAndroidFunction");

Interface:-

public class WebAppInterface {
    Context mContext;

    /** Instantiate the interface and set the context */
    WebAppInterface(Context c) {
        mContext = c;
    }

    /** Show a toast from the web page */
    @JavascriptInterface
    public void showToast() {
        Toast.makeText(mContext, "Toast message", Toast.LENGTH_SHORT).show();
    }
}