Android Calling JavaScript Functions in Webview

Calling JavaScript from Android

According to CW answer,

You need to wait until your page is loaded

private void helloJs(){
    if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.KITKAT) {
        webView.evaluateJavascript("javascript:hello();", null);
    } else {
        webView.loadUrl("javascript:hello();");
    }
}

webView.setWebViewClient(new WebViewClient() {
    public void onPageFinished(WebView view, String url) {
        helloJs();
    }
});

Run javascript code in Webview

From kitkat onwards use evaluateJavascript method instead loadUrl to call the javascript functions like below

    if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.KITKAT) {
        webView.evaluateJavascript("var FunctionOne = function () {"
            + "  try{document.getElementsByClassName('test')[0].style.color='red';}catch(e){}"
            + "};", null);
    } else {
        webView.loadUrl("javascript:"
            + "var FunctionOne = function () {"
            + "  try{document.getElementsByClassName('test')[0].style.color='red';}catch(e){}"
            + "};");
    }

Enable Javascript for your webview by adding the following line

wb.getSettings().setJavaScriptEnabled(true);

It is possible to call a javascript function in android?

I believe you can use

WebView webView = (WebView) findViewById(R.id.webview);
webView.setWebChromeClient(new MyCustomChromeClient(this));
webView.getSettings().setJavaScriptEnabled(true);
webView.evaluateJavascript("globalJavascriptFunction();", null);

https://developer.android.com/reference/android/webkit/WebView#evaluateJavascript(java.lang.String,%20android.webkit.ValueCallback%3Cjava.lang.String%3E)

The second argument is a callback.

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[edit]

To get a value back from the javascript, you can do something like this:

webView.evaluateJavascript("globalJavascriptFunction();", new ValueCallback<String>() {
    @Override
    public void onReceiveValue(String result) { 
       // result is the return value of globalJavascriptFunction()
       // you'll have to do some testing inside here to figure out exactly how result gets sent back for your usecase
    }
});

How to inject whole javascript function into webview and call it from android

Use the below mentioned method to inject a js file from asset folder

public 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();
    }
}

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