Here is a very elegant and simple solution on how to add a Spinner (a dropdown) to a toolbar in an Android app.
Add a Spinner as menu item and set the action view class
<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:yourapp="http://schemas.android.com/apk/res-auto" >
<item android:id="@+id/spinner"
yourapp:showAsAction="ifRoom"
yourapp:actionViewClass="android.widget.Spinner" />
</menu>
Then handle the contents in onCreateMenuItem, as usual.
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.menu_layout, menu);
MenuItem item = menu.findItem(R.id.spinner);
Spinner spinner = (Spinner) MenuItemCompat.getActionView(item);
spinner.setAdapter(adapter); // set the adapter to provide layout of rows and content
s.setOnItemSelectedListener(onItemSelectedListener); // set the listener, to perform actions based on item selection
}
Source