What happens if I don't retain IBOutlet?

问题:

If I do this:

@interface RegisterController : UIViewController <UITextFieldDelegate>
{
    IBOutlet UITextField *usernameField;
}

instead of this:

@interface RegisterController : UIViewController <UITextFieldDelegate>
{
    UITextField *usernameField;
}
@property (nonatomic, retain) IBOutlet UITextField *usernameField;

Will something bad happen? I know in the second case, the field is retained, but does this make a different since the nib owns the field? Will the field go away without the retain? and under what circumstances? The code in the first case works, was wondering whether this is an issue or not in terms of memory management.

回答1:

It is recommended you declare properties for all of your IBOutlets for clarity and consistency. The details are spelled out in the Memory Management Programming Guide. The basic gist is, when your NIB objects are unarchived, the nib loading code will go through and set all of the IBOutlets using setValue:forKey:. When you declare the memory management behavior on the property, there is no mystery as to what is going on. If the view gets unloaded, but you used a property that was declared as retain, you've still got a valid reference to your textfield.

Perhaps a more concrete example would be useful to indicate why you should use a retaining property:

I'm going to make some assumptions about the context in which you're working--I'll assume the UITextField above is a subview of another view that is controlled by a UIViewController. I will assume that at some point, the the view is off the screen (perhaps it is used in the context of a UINavigationController), and that at some point your application gets a memory warning.

So lets say your UIViewController subclass needs to access its view to display it on screen. At this point, the nib file will be loaded and each IBOutlet properties will be set by the nib loading code using setValue:forKey:. The important ones to note here are the top level view that will be set to the UIViewController's view property, (which will retain this top level view) and your UITextField, which will also be retained. If it is simply set, it'll have a retain put on it by the nib loading code, otherwise the property will have retained it. The UITextField will also be a subview of the top level UIView, so it will have an additional retain on it, being in the subviews array of the top level view, so at this point the text field has been retained twice.

At this point if you wanted to switch out the text field programmatically, you could do so. Using the property makes memory management more clear here; you just set the property with a new autoreleased text field. If you had not used the property, you must remember to release it, and optionally retain the new one. At this point it is somewhat ambiguous as to whom owns this new text field, because the memory management semantics are not contained within the setter.

Now let's say a different view controller is pushed on the UINavigation Controller's stack, so that this view is no longer in the foreground. In the case of a memory warning, the view of this offscreen view controller will be unloaded. At this point, the view property of the top level UIView will be nulled out, it will be released and deallocated.

Because the UITextField was set as a property that was retained, the UITextField is not deallocated, as it would have been had its only retain been that of the subviews array of the top level view.

If instead the instance variable for the UITextField not been set via a property, it'd also be around, because the nib loading code had retained it when setting the instance variable.

One interesting point this highlights is that because the UITextField is additionally retained through the property, you'll likely not want to keep it around in case of a memory warning. For this reason you should nil-out the property in the -[UIViewController viewDidUnload] method. This will get rid of the final release on the UITextField and deallocate it as intended. If using the property, you must remember to release it explicitly. While these two actions are functionally equivalent, the intent is different.

If instead of swapping out the text field, you chose to remove it from the view, you might have already removed it from the view hierarchy and set the property to nil, or released the text field. While it is possible to write a correct program in this case, its easy to make the error of over-releasing the text field in the viewDidUnload method. Over-releasing an object is a crash-inducing error; setting a property that is already nil again to nil is not.

My description may have been overly verbose, but I didn't want to leave out any details in the scenario. Simply following the guidelines will help avoid problems as you encounter more complex situations.

It is additionally worth noting that the memory management behavior differs on Mac OS X on the desktop. On the desktop, setting an IBOutlet without a setter does not retain the instance variable; but again uses the setter if available.

回答2:

Declaring something IBOutlet, from a memory management standpoint, does nothing (IBOutlet is literally #defined as nothing). The only reason to include IBOutlet in the declaration is if you intend to connect it in Interface Builder (that's what the IBOutlet declaration is for, a hint to IB).

Now, the only reason to make an @property for an instance variable is if you intend to assign them programatically. If you don't (that is, you're only setting up your UI in IB), it doesn't matter whether you make a property or not. No reason to, IMO.

Back to your question. If you're only setting this ivar (usernameField) up in IB, don't bother with the property, it won't affect anything. If you DO make a property for usernameField (because you're programatically creating it), definitely do make a property for it, and absolutely DO make the property retain if so.

回答3:

In fact there are two models:

THE OLD MODEL

These model was the model before Objective-C 2.0 and inherited from Mac OS X. It still works, but you should not declare properties to modify the ivars. That is:

@interface StrokeWidthController : UIViewController {
    IBOutlet UISlider* slider;
    IBOutlet UILabel* label;
    IBOutlet StrokeDemoView* strokeDemoView;
    CGFloat strokeWidth;
}
@property (assign, nonatomic) CGFloat strokeWidth;
- (IBAction)takeIntValueFrom:(id)sender;
@end

In this model you do not retain IBOutlet ivars, but you have to release them. That is:

- (void)dealloc {
    [slider release];
    [label release];
    [strokeDemoView release];
    [super dealloc];
}

THE NEW MODEL

You have to declare properties for the IBOutlet variables:

@interface StrokeWidthController : UIViewController {
    IBOutlet UISlider* slider;
    IBOutlet UILabel* label;
    IBOutlet StrokeDemoView* strokeDemoView;
    CGFloat strokeWidth;
}
@property (retain, nonatomic) UISlider* slider;
@property (retain, nonatomic) UILabel* label;
@property (retain, nonatomic) StrokeDemoView* strokeDemoView;
@property (assign, nonatomic) CGFloat strokeWidth;
- (IBAction)takeIntValueFrom:(id)sender;
@end

In addition you have to release the variables in dealloc:

- (void)dealloc {
    self.slider = nil;
    self.label = nil;
    self.strokeDemoView = nil;
    [super dealloc];
}

Furthermode, in non-fragile platforms you can remove the ivars:

@interface StrokeWidthController : UIViewController {
    CGFloat strokeWidth;
}
@property (retain, nonatomic) IBOutlet UISlider* slider;
@property (retain, nonatomic) IBOutlet UILabel* label;
@property (retain, nonatomic) IBOutlet StrokeDemoView* strokeDemoView;
@property (assign, nonatomic) CGFloat strokeWidth;
- (IBAction)takeIntValueFrom:(id)sender;
@end

THE WEIRD THING

In both cases, the outlets are setted by calling setValue:forKey:. The runtime internally (in particular _decodeObjectBinary) checks if the setter method exists. If it does not exist (only the ivar exists), it sends an extra retain to the ivar. For this reason, you should not retain the IBOutlet if there is no setter method.

回答4:

There isn't any difference between the way those two interface definitions work until you start using the accessors provided by the property.

In both cases, you'll still need to release and set-to-nil the IBOutlet in your dealloc or viewDidUnload methods.

The IBOutlet points to an object instantiated within a XIB file. That object is owned by the File's Owner object of the XIB file (usually the view controller that the IBOutlet is declared in.

Because the object is created as a result of loading the XIB, it's retain count is 1 and is owned by your File's Owner, as mentioned above. This means that the File's Owner is responsible for releasing it when it's deallocated.

Adding the property declaration with the retain attribute simply specifies that the setter method should retain the object passed in to be set - which is the correct way to do it. If you did not specify retain in the property declaration, the IBOutlet could possibly point to an object that may not exist any more, due to it being released by its owner, or autoreleased at some point in the program's lifecycle. Retaining it prevents that object being deallocated until you're done with it.

回答5:

Objects in the nib file are created with a retain count of 1 and then autoreleased. As it rebuilds the object hierarchy, UIKit reestablishes connections between the objects using setValue:forKey:, which uses the available setter method or retains the object by default if no setter method is available. This means that any object for which you have an outlet remains valid. If there are any top-level objects you do not store in outlets, however, you must retain either the array returned by the loadNibNamed:owner:options: method or the objects inside the array to prevent those objects from being released prematurely.

回答6:

Well, in the second case you're adding a getter/setter method for that particular IBOutlet. Any time you add a getter/setter method you (almost always) want to have it set to retain for memory management issues. I think a better way to have posed you're question would have been this:

@interface RegisterController : UIViewController <UITextFieldDelegate>
{
IBOutlet UITextField *usernameField;
}
@property (nonatomic) IBOutlet UITextField *usernameField;

or

@interface RegisterController : UIViewController <UITextFieldDelegate>
{
IBOutlet UITextField *usernameField;
}
@property (nonatomic, retain) IBOutlet UITextField *usernameField;

In that case, then yes, you would need to add a retain since it will affect memory management. Even though it may not have any effects, if you're programatically adding and removing IBOutlet's, you could potentially run into issues.

As a general rule: always add an @property (with retain) whenever you have an IBOutlet.