Retaining IBOutlets

[jeff_lamarche]

Historically speaking, when working with IBOutlets on the Mac, you didn't retain the outlets in your controller class. There's an exception to this in that the root objects in the nib (the ones with an icon in the nib's main window) have to be released since they get created with a retain count of 1, but that is handled for us by the generic controller classes like WindowController, so in most situations, releasing IBOutlets simply wasn't necessary.

However, with the iPhone sample code, I've noticed that they've started declaring properties for the IBOutlets and also releasing the IBOutlets in the Dealloc. I wanted to ask about this change on Cocoa-dev, but the newer Cocoa sample code uses garbage collection, meaning it's only relevant to the iPhone SDK, so I'm sure I'd of gotten slapped down for asking about NDA stuff.

Anyway, here's my guess about what's happening, but haven't been able to confirm this in the documentation:

If you create a property or manually create KVC-compliant accessors and mutators, Interface Builder will use those mutator methods to assign objects to your IBOutlet. If you do not, then it will assign them directly to the instance variable. In the former case, if the mutator sends the object a retain or if you specified the optional retain attribute to our property (Apple does in their sample code), it is then necessary to release the object in your dealloc method.

Does this sound logical or right? Has anybody found a way to confirm or deny this information in the official documentation?

Thanks,
Jeff

[airsource]

Originally Posted by jeff_lamarche

Historically speaking, when working with IBOutlets on the Mac, you didn't retain the outlets in your controller class. There's an exception to this in that the root objects in the nib (the ones with an icon in the nib's main window) have to be released since they get created with a retain count of 1, but that is handled for us by the generic controller classes like WindowController, so in most situations, releasing IBOutlets simply wasn't necessary.

However, with the iPhone sample code, I've noticed that they've started declaring properties for the IBOutlets and also releasing the IBOutlets in the Dealloc. I wanted to ask about this change on Cocoa-dev, but the newer Cocoa sample code uses garbage collection, meaning it's only relevant to the iPhone SDK, so I'm sure I'd of gotten slapped down for asking about NDA stuff.

Anyway, here's my guess about what's happening, but haven't been able to confirm this in the documentation:

If you create a property or manually create KVC-compliant accessors and mutators, Interface Builder will use those mutator methods to assign objects to your IBOutlet. If you do not, then it will assign them directly to the instance variable. In the former case, if the mutator sends the object a retain or if you specified the optional retain attribute to our property (Apple does in their sample code), it is then necessary to release the object in your dealloc method.

Does this sound logical or right? Has anybody found a way to confirm or deny this information in the official documentation?

Thanks,
Jeff

Nearly - the direct assignation case does an retain as well. See
The Airsource - Memory Management and NIBS which has references to the relevant parts of the documentation.

[PhoneyDeveloper]

Jeff,

Outlets are retained by default. If you have retain properties for outlets then they are retained and must be released. If you don't have any properties for outlets then they are retained and must be released. If you have assign properties for outlets then they are not retained.

There is a secondary issue related to memory warnings that makes this all more complicated. In short outlets should be released when a memory warning is received, but this is somewhat complicated due to deficiencies in the UIViewController implementation.

See this thread:

Cocoabuilder - Outlets / IBOutlet declarations (was Re: Interface Builder & Wiring Objects)

[BenClayton]

Sorry to reopen this thread after such a long time, but I've been searching for a simple answer to this question and this was one of the posts that came up in Google.

Essentially if you use interface builder to connect an IBOutlet instance variable to a control, you must always release said IBOutlet in your own -dealloc method.

e.g. in your .h file.

@interface MyView : UIViewController {
IBOutlet UILabel *myLabel;
}
@end

.m file..

-(void) dealloc {
[super dealloc];

// release all IBOutlets..
[myLabel release];
}

You always need to do this, even if you have defined a retain property for the IBOutlet. The reason for this is that loadFromNib will call setValue:withKey for each IBOutlet on your view controller class, which uses the property's setter method or retains the object by default if no setter method is available. There's no system to automatically release things created by the NIB so you need to manually release them yourself.

[Reference: ]Resource Programming Guide: Nib Files. Also see lecture 11 of CS193P at Stanford on iTunes U if it's still available - about 57 minutes in.

Regards,
Ben

[AmanApps]

If you don't release it on dealloc it will raise the memory footprint.

See more detail here with instrument ObjectAlloc graph.