How can I retrieve the Javascript function value from a web page loaded withing a webview component??
You can't, directly.
You can call the Javascript function via loadUrl("javascript:..."), where ... is your function call. However, you cannot get a result this way.
If you inject a Java object into the Web page via addJavascriptInterface(), you could set up another function that called the function you want and returns that value via a call to the injected Java object. That only works if you can modify the Web page, though.
来源：https://stackoverflow.com/questions/3547748/get-the-values-of-the-fields-in-a-form-in-an-android-webview-android