NSTimer is retaind and NSButton action is not retained

Question

My question is why

[myButton addTarget:self action:@selector(myAction) forControlEvents:UIControlEventTouchUpInside];

it is not retain and

mytimer = [NSTimer scheduledTimerWithTimeInterval:1.0f
                                          target:self
                                        selector:@selector(_timerFired:)
                                        userInfo:nil
                                         repeats:YES];

is retaind ,where both have the target self so why one is retain and other is not retaind. @end

Answer 1

The NSTimer created with +[NSTimer scheduledTimerWithTimeInterval:…] is scheduled on the current run loop, and the run loop itself retains the timer.

You can see this if you profile your program with the allocations tool and search for living instances of CFRunLoopTimer. You'll see that under your call to scheduledTimerWithTimeInterval:… there is a call to CFRunLoopAddTimer, and the timer is retained under that call.

So the fact that the timer is retained and the button is not retained has to do with the scheduling of the timer on the run loop and has nothing to do with the target of either object.

You may find the following reference materials helpful:

• About Memory Management
• Automatic Reference Counting
• Run Loop Management
• NSTimer Class Reference

If you're starting a new project you should certainly be using Automatic Reference Counting (and Xcode will have that turned on for you by default, when you create a new project). That system doesn't entirely free you from the responsibility of thinking about your program's memory usage, but it will make things easier.

I included a reference to run loop management, but you probably will not need to do any manual run loop management. It's just good to have a high level understanding of what "the run loop" means, for when it comes up in other documentation.