Android Development : Changing theme of Current Activity during runtime (eg: during Button Click event)

Introduction

There may be situations where the entire look of the screen (current activity) needs to be changed as a result of some event. The event may be as simple as a button click, or may be a sensor feedback, or even as a result of streaming data from internet. 

Many articles explain how to set a theme in android manifest file, during design time. However, doing this will result in the application/activity itself starting with the theme and not as a response to an "event".

The Finished Demo

Repository Link

https://github.com/gopalnair/Android-Style-Demo

Key Points On Code

In method MainActivity.onCreate , make sure the setTheme() is called BEFORE call to parent. In this example, when the activity starts for the first time from launcher, there are no extras in the launcher intent. Hence, application start with default theme applied.

However, when user clicks on one of the "Style" buttons, we call the SAME activity (MainActivity), and package the target theme information in intent extras like so:

Notice that after calling startActivity() to start the Main Activity again, we are calling "finish()" method, so that the current instance of Main Activity will be killed, and we will be left with the just "triggered" main activity. If we do not call the "finish", by default, multiple main activities will stack one above other (Unless android manifest file is updated to force the activity to have a single instance).