title: how to detect a callout of an annotation is showing on mapview?
body: <p>I have a checkin related app. When user clicks a venue on map, callout will popup and show its name and address. At the same time, if the venue is within 1000m from current location, a check in button will be displayed.</p>
  
  <p>To a venue that can checkin, all I want is,</p>
  
  <p>when callout is popup, check in button shows. when callout is disappear, that button disappears too.</p>
  
  <p>Now show button works well, but I don't know how to detect callout visible status. Is there any callback method that callout show/hide?</p>
  
  <p>Thanks in advance!</p>