Android Web-View : Inject local Javascript file to Remote Webpage

Question

It has been asked many times before, I browsed through everything, no clear answers yet.

Question simplified: Is it possible to inject local Javascript file (from asset or storage) to remote webpage loaded in an Android Web-View? I know that it is possible to inject such files to local Webpages (Assets HTML) loaded in a Web-View.

Why do I need this to work? : To make browsing experience faster, by avoiding downloading of bigger files such as Js and CSS files every time. I want to avoid Web-View Caching.

Answer 1

There is a way to 'force' the injection of your local Javascript files from local assets (e.g., assets/js/script.js), and to circumvent the 'Not allowed to load local resource : file:///android_assets/js/script.js ...' issue.

It is similar to what described in another thread (Android webview, loading javascript file in assets folder), with additional BASE64 encoding/decoding for representing your Javascript file as a printable string.

I am using an Android 4.4.2, API level 19 Virtual Device.

Here are some code snippets:

[assets/js/script.js]:

    'use strict';

    function test() {
       // ... do something
    }

    // more Javascript

[MainActivity.java]:

    ...

    WebView myWebView = (WebView) findViewById(R.id.webView);
    WebSettings webSettings = myWebView.getSettings();

    webSettings.setJavaScriptEnabled(true);
    webSettings.setAllowUniversalAccessFromFileURLs(true);
    myWebView.setWebViewClient(new WebViewClient() {
       @Override
       public boolean shouldOverrideUrlLoading(WebView view, String url) {
          return false;
       }

       @Override
       public void onPageFinished(WebView view, String url) {
          super.onPageFinished(view, url);

          injectScriptFile(view, "js/script.js"); // see below ...

          // test if the script was loaded
          view.loadUrl("javascript:setTimeout(test(), 500)");
       }

       private void injectScriptFile(WebView view, String scriptFile) {
          InputStream input;
          try {
             input = getAssets().open(scriptFile);
             byte[] buffer = new byte[input.available()];
             input.read(buffer);
             input.close();

             // String-ify the script byte-array using BASE64 encoding !!!
             String encoded = Base64.encodeToString(buffer, Base64.NO_WRAP);
             view.loadUrl("javascript:(function() {" +
                          "var parent = document.getElementsByTagName('head').item(0);" +
                          "var script = document.createElement('script');" +
                          "script.type = 'text/javascript';" +
             // Tell the browser to BASE64-decode the string into your script !!!
                          "script.innerHTML = window.atob('" + encoded + "');" +
                          "parent.appendChild(script)" +
                          "})()");
          } catch (IOException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
          }
       }
    });

    myWebView.loadUrl("http://www.example.com");

    ...