CODEWITHSUNDEEP
iosobjective-c
vikmalhotra
vikmalhotra

Reputation: 10071

iOS 5.1: IBOutlet instance variables & @property declarations

While creating a custom iOS table view cell, I created a new .xib file, dragged/dropped some UI elements in interface builder and my .h file looked like this...

#import <UIKit/UIKit.h>

@interface MasterTableViewCell : UITableViewCell
{
    IBOutlet UILabel *cellLabel;
    IBOutlet UIImage *cellImage;
}

@property (nonatomic, retain) IBOutlet UILabel *cellLabel;
@property (nonatomic, retain) IBOutlet UIImage *cellImage;

@end

On some blogs I saw that the instance variables were missing. When do I need to declare instance variables? Are both instance variables and @property declarations not needed for a particular UI object. Also I am creating the app using automatic reference counting, so garbage collection needs aren't there as well. What difference does that make in usage of instance variables & properties?

Upvotes: 4

Views: 3323

Answers (2)

Steve
Steve

Reputation: 1840

There is no garbage collection in iOS. iOS uses reference counting to track ownership of objects. Using ARC does not do away with reference counting, but the compiler takes care of releasing and retaining objects. When using ARC you are not allowed to send a retain, release, or autorelease message to an object, nor are you allowed to call [super dealloc] in a dealloc method. In your code above, since you are using ARC, the 'retain' attributes should be replaced by the 'strong' attribute.

When you use @property, and the corresponding @synthesize in your implementation, you do not need to create a backing instance variable - the compiler does that for you. @property along with @synthesize create your accessor methods (your getters and setters), and also enable you to use dot notation to refer to your objects' properties. You may still write your own accessor methods if you choose.

The above code could be replaced by the following:

#import <UIKit/UIKit.h>

@interface MasterTableViewCell : UITableViewCell

@property (nonatomic, strong) IBOutlet UILabel *cellLabel;
@property (nonatomic, strong) IBOutlet UIImage *cellImage;

@end

In your implementation file you would have:

#import "MasterTableViewCell.h"

@implementation MasterTableViewCell

@synthesize cellLabel;
@synthesize cellImage;

or

@synthesize cellLabel, cellImage;

... remainder of your code

In your code, to ensure that you are using your accessor methods, use 'self' to refer to your properties:

self.cellLabel.text = @"some text";

or

[[self cellLabel] setText:@"some text"];

I hope this helps clarify things a little.

Upvotes: 3

lnafziger
lnafziger

Reputation: 25740

If you don't create the instance variables (iVar's) then they will automatically be created for you if you are using the @synthesize directive (see below) so they really aren't required. If you are using @dynamic or writing your own methods and want to access the iVar directly then you need to declare it yourself.

In the documentation for Declared Properties under the Property Implementation Directives section, it states:

@synthesize You use the @synthesize directive to tell the compiler that it should synthesize the setter and/or getter methods for a property if you do not supply them within the @implementation block. The @synthesize directive also synthesizes an appropriate instance variable if it is not otherwise declared.

Note that this behavior is for "modern" runtimes (2.0 and newer). Before this, the iVar was required to be declared or the @synthesize would generate an error.

Upvotes: 1

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