In iOS4 applicationWillEnterForeground may get called before applicationDidEnterBackground and it makes problem

Answer 1

This happens to me as well.

Only in my case it sometimes happens when my app receives local notification.

Answer 2

Keep a counter for switches and ignore switches that happen in the wrong order. Something like this:

-(void) handleSwitchToBackground {
  if ( myState == 0 ) { /* do background stuff */ }
  myState += 1;
}

-(void) handleSwitchToForeground {
  myState -= 1;
  if ( myState == 0 ) { /* do foreground stuff */ }
}

If foreground happens before background, neither method does anything.