Let Font Size change in Smartphone Apps

I am experiencing this for Android today but I wonder if does not apply to iPhone or web apps as well. For Android it is a big headache to give users a chance to change font sizes app wide. There are not many google results concerning this topic. How are developers trying to achieve this? The first way is to manipulate or overwrite each and every TextView of an app and set the font size according to user preferences. This assumes that you have all TextViews to manipulate. But in many cases this class is hidden at first glance. You have to find out how to write a custom layout for preferences or dialogs to name a few. Are you sure that you got each text in your application as a TextView? No hidden system font in a hidden TextView?
You can also create your own themes with its special font sizes. But font size is not the only preference witch determines a theme. Imagine you have a dark theme and a light theme. Imagine you have four different font sizes for each theme. You end up with eight themes for only two preferences. Add the next preference and you have much work to do. Since themes are static and can’t be changed at runtime it kills efficiency.
So why not set the font size at OS level? If I were disabled or could not read small fonts, I would try to solve this problem systematically. I still do not know iOS very good. Android has a preference at OS level to set the font size system wide for all apps. You don’t have to set font sizes for each app individually. Accordingly Chrome has a setting to increase the font size for all pages systematically. I am sure iOS will allow the same. Seems to be the best option for each party, users and developers!