cocoa-Why there is an IBOutlet and a property that are of the same name? - objective-c

Question

I'm new to objective-c. When I'm reading some source code written by others, I encountered a problem.
I found that there is
IBOutlet NSPopover *popover;
as well as
#property NSPopover *popover;
PopoverViewController.h
#import <Foundation/Foundation.h>
#import <Cocoa/Cocoa.h>
#import "TimerPopoverViewController.h"
#class TimerLogic;
#class TimerInfo;
#interface TimerPopoverDelegate : NSObject <NSPopoverDelegate> {
#private
IBOutlet NSPopover *popover;
IBOutlet NSWindow *detachWindow;
IBOutlet TimerPopoverViewController *viewController;
}
#property NSPopover *popover;
- (void)showPopover:(id)sender timerInfo:(TimerInfo *)timerInfo;
#end
I think they are different variables. However, I can't figure out what do they do?
As far as I'm concerned, the IBOutlet is to show a popover.
But what does the #property does?

Answer 1

This is either very old code or written in a very old (and now discouraged) style. The IBOutlet here is declaring an instance variable (ivar). The #property is declaring a property that is backed by the instance variable. In modern ObjC you should implement it this way:
PopoverViewController.h
#import <Cocoa/Cocoa.h>
#class TimerInfo;
// Things declared here are public
#interface TimerPopoverDelegate : NSObject <NSPopoverDelegate>
// You could leave this here if it is required by other parts of the program,
// but other parts of the program really shouldn't require it. See below.
// #property (nonatomic, readonly, weak) NSPopover *popover;
- (void)showPopover:(id)sender timerInfo:(TimerInfo *)timerInfo;
#end
PopoverViewController.m
// Generally avoid importing local headers into the .h unless you have to.
#import "TimerPopoverViewController.h"
// Things declared here are private. This is much better than the old #private.
#interface TimerPopoverDelegate ()
#property (nonatomic, readwrite, weak) IBOutlet NSPopover *popover;
#property (nonatomic, readwrite, weak) IBOutlet NSWindow *detachWindow;
#property (nonatomic, readwrite, weak) IBOutlet TimerPopoverViewController *viewController;
#end
(Currently popover is public, but you should avoid exposing an IBOutlet that way. Outside objects should not directly touch a view controller's outlets.)