/*That's how we do it. In Application class override onConfigurationChanged() like this. If you want different behavior for different activities - override onConfigurationChanged() in Activity.

Don't forget to add manifest tag android:configChanges="fontScale" since you are hadnling this configuration change yourself.*/

@Override
public void onConfigurationChanged(Configuration newConfig) {
    super.onConfigurationChanged(newConfig);

    // In some cases modifying newConfig leads to unexpected behavior,
    // so it's better to edit new instance.
    Configuration configuration = new Configuration(newConfig);
    SystemUtils.adjustFontScale(getApplicationContext(), configuration);
}

//In some helper class we have adjustFontScale() method.

public static void adjustFontScale(Context context, Configuration configuration) {
    if (configuration.fontScale != 1) {
        configuration.fontScale = 1;
        DisplayMetrics metrics = context.getResources().getDisplayMetrics();
        WindowManager wm = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        wm.getDefaultDisplay().getMetrics(metrics);
        metrics.scaledDensity = configuration.fontScale * metrics.density;
        context.getResources().updateConfiguration(configuration, metrics);
    }
}
//WARNING! That will totally ignore Accessibility Font Scale user settings and will prevent your App fonts scaling!