Question

I have a UISegmentedControl whose “Value changed” event is wired up in Interface Builder to call my controller’s -(IBAction)segmentChangeAction:(id)sender;

When the user taps on the control to change the selected segment, as expected segmentChangeAction is called whether in iOS4 or iOS5.

When I programmatically change the selected segment through segmentedControl.selectedSegmentIndex = newIndex;, on iOS4 segmentChangeAction is called and the segment reflects the new selection. However on iOS5 segmentChangeAction is not called, yet the segment does reflect the new selection.

Is this a change in iOS5? Is there anything I can do to get segmentChangeAction called on iOS5 when I programmatically change the selection?

Answer 1

This is a change in iOS 5 in order for UISegmentedControl to be consistent with all other controls.

The idea is that the action should only fired automatically as a result of user interaction. Prior to iOS 5, UISegmentedControl‘s actions would be fired because of user interaction and programmatic interaction. However, initiating the change programmatically means that you can also do [myControl sendActionsForControlEvents:UIControlEventValueChanged] yourself.

However, you have to be careful with this. Say you do:

[segmentedControl setSelectedSegmentIndex:newIndex];
[segmentedControl sendActionsForControlEvents:UIControlEventValueChanged];

If you build and run this on iOS 5, it works as you expect. If you build and run this on iOS 4, you’ll get your actions fired twice (once when you setSelectedSegmentIndex and again when you sendActions...).

The way around this is to do some sort of guard. This could be a runtime check to indicate that you’re running on an iOS 5+ device, or could even be something more mundane, like this:

// changingIndex is a BOOL ivar
changingIndex = YES;
[segmentedControl setSelectedSegmentIndex:newIndex];
changingIndex = NO;
[segmentedControl sendActionsForControlEvents:UIControlEventValueChanged];

and then in your action method…

- (void)segmentedControlSelectedIndexChanged:(id)sender {
  if (!changingIndex) {
    // your action code here, guaranteed to only run as a result of the sendActions... msg
  }
}