It is clear for
Activities - the method is being called right before the menu is shown. But what about
ActionBar when it is always shown? How to trigger it to be invoked to dynamically update
Use invalidateOptionsMenu() to trigger onPrepareOptionMenu() method as suggested by document.
On Android 3.0 and higher, you must call invalidateOptionsMenu() when you want to update the menu, because the menu is always open. The system will then call onPrepareOptionsMenu() so you can update the menu items.
Note if your min sdk version is less than Honeycomb (aka Android 3.0 aka api level 11), and you are using ActionBarActivity you can call supportInvalidateOptionsMenu() which is the Support library version of invalidateOptionsMenu().
If your menu changes based upon an incoming intent into onCreate(), you can just do the updates in the onPrepareOptionMenu(), if you dynamically change the menu later in the activities life-cycle you will have to call invalideOptionsMenu, or supportInvalidateOptionsMenu depending on your min and target sdk versions.