When is the proper time to release an object?

I'm still a little confused on this.

I'm creating an object programmatically in Xcode, let's say a UILa开发者_Python百科bel, which is also going to be a class wide property.

When is the proper time to release the object, in the method in which it is created, or in the dealloc method like normal IBOutlet objects?

Thanks.

This depends on whether your property is set to retain the value or not. Usually you want the accessors (generated by @synthesize) to handle the retain/release when the property is set to a new value. You specify such a property like this:

MyController.h

@interface MyController : UIViewController {
 UILabel *myLabel;
}

@property (nonatomic, retain) UILabel *myLabel;

@end

You can then use @synthesize to generate the default getters and setters. The default setter for a 'retain' property will release the current value and retain the new value. However, nothing is done for you in dealloc. Meaning, that when the controller is destroyed, your reference to you label will leak since release will not be called. For this reason, you need call release on all your 'retain' properties in dealloc, like this:

MyController.m

@implementation MyController

@synthesize myLabel;

-(void) dealloc {
 self.myLabel = nil;

 [super dealloc];
}

@end

Notice that in this case, self.myLabel = nil is almost equivalent to calling [myLabel release] since the setter will call release on the existing value and then call retain on the new value. Since the new value is nil, calling [nil retain] has no effect. I prefer to nil instead of releasing since you are also setting the ivar to nil and avoids dangling pointers.

When you create a property like this programmatically as opposed to from Interface Builder, you don't need to mark it with IBOutlet. In the cases where you do create a control using IB, you should nil all of your IBOutlet references in viewDidUnload. This is because your control could be deallocated along with the view if it wasn't retained. Referencing it afterwards will crash the app so it's a good practice to nil them, like this:

- (void)viewDidUnload {
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;

    self.myIBLabel = nil;
}

Another common mistake that happens when using properties is to omit the 'self' part. If you do not use the self.myIBLabel notation, you are bipassing the getter and setter and working with the ivar directly. This will NOT retain/release the object.

You should release it in the dealloc method, although that depends how you're creating your class property.

If you release it in the method in which you create it, and then use it in some other part of your class (which, since you're making the UILabel a class wide property, I assume you are), you will get a bad access when you try to modify it later on. Note that if you're using a retained property you need to take that into account, in which case you might release the label (because you'll have created it and assigned it to your class property, which will retain it again).

Here's a typical example:

- (void) someMethod {
    UILabel *myLabel = [[UILabel alloc] initWithFrame:myFrame];

    self.textLabel = myLabel;
    [myLabel release];
}


- (void) dealloc {
    [textLabel release];
}