Cocoa - NSSearchFieldDelegate methods is not called

I have simple NSSearchField on view of NSVIewConroller. and connected the NSSearchFIeld's delegate to the view controller. And I have implemented the NSSearcHFieldDelegate Methods as follows:

- (void)searchFieldDidStartSearching:(NSSearchField *)sender NS_AVAILABLE_MAC(10_11);
{
    NSLog(@"search field did start: %@", sender.stringValue);
}
- (void)searchFieldDidEndSearching:(NSSearchField *)sender NS_AVAILABLE_MAC(10_11);
{
    NSLog(@"search field did end: %@", sender.stringValue);

}

These methods are not getting called on start editing and end editing. But If I implement the NSControl default delegates like controlTextDidChange: and control:textShouldBeginEditing: are called on respective events.

Why the NSSearchFieldDelegate methods are not called?..