title: Is there an equivalent to MFCs OnUpdate in Qt?
body: <p>I have an existing application that uses MFC for the UI, and I'm trying to migrate to Qt. For the most part the migration is straight forward, but I'm not sure how to manage the enabled state of actions (menu and toolbar items).</p>
  
  <p>In MFC you implement a callback with enable/disable logic, and this is called when the item is displayed. In Qt, you only have access to the setEnabled() method.</p>
  
  <p>Is there a built-in or standardized way of connecting an update callback to an action? or do I need to create my solution using timers and registering actions with it? In a large application such as the one I'm working with, the 'should enable' logic can jump all over the place - i.e. certain files on disk must exist, the main display must have a selection, the application's ProcessManager::isProcessing() must be false, etc. It doesn't seem practical to rely on setEnabled() being called on specific actions when there are so many conditions behind the enable/disable logic.</p>