Question

In a header file such as this, when would an instance variable be used and when would a property be used?

Do they have to have the same name?

#import <UIKit/UIKit.h>

@class BlueViewController;
@class YellowViewController;

@interface SwitchViewController : UIViewController {
    YellowViewController *yellowViewController;
    BlueViewController *blueViewController;
}

@property (retain, nonatomic) YellowViewController *yellowViewController;
@property (retain, nonatomic) BlueViewController *blueViewController;

@end

Answer 1

A @property declaration simply creates accessor methods for an ivar. So properties don’t really have names, only the accessor methods do; and these don’t have to have the same name as the ivar. In fact, they don’t even have to have a corresponding ivar.

You can change the names of the methods using the getter and setter decorators like this:

@property (assign, getter=isValid) BOOL valid;

Now, as I said, a @property declaration simply creates accessor methods for an ivar. So you use properties when you want accessor methods. Here are a few reasons why you might want accessor methods:

1. Encapsulation (a property might be advertised as a different type than the ivar, or might not even have an ivar)

2. Related state changes (change another ivar or invoke a method when an ivar is modified)

3. You can use the retain decorator and @synthesize the property to get much simpler memory management

4. You can use atomic decorator (or simply not use the nonatomic decorator, since properties are atomic by default) to create atomic properties

Here’s an example to demonstrate points 1 and 2:

@interface Foo : NSObject {
    @private
    int flags;
    // Humpty and Dumpty are mutually exclusive
    BOOL humpty;
    BOOL dumpty;
}

@property (nonatomic) BOOL flagA;
@property (nonatomic) BOOL flagB;
@property (nonatomic) BOOL humpty;
@property (nonatomic) BOOL dumpty;
@property (nonatomic, readonly, getter=isClean) BOOL clean;

@end

@implementation Foo

@synthesize humpty, dumpty; // Synthesize the getters, but provide the setters

- (void)setHumpty:(BOOL)value {
    if(value && dumpty)
        dumpty = NO;

    humpty = value;
}

- (void)setDumpty:(BOOL)value {
    if(value && humpty)
        humpty = NO;

    dumpty = value;
}

- (BOOL)flagA {
    return flags & 0x01;
}

- (void)setFlagA:(BOOL)value {
    if(value)
        flags |= 0x01;
    else
        flags &= ~0x01;
}

- (BOOL)flagB {
    return flags & 0x02;
}

- (void)setFlagB:(BOOL)value {
    if(value)
        flags |= 0x02;
    else
        flags &= ~0x02;
}

// Making this a property doesn't really make sense
// but I'm just trying to demonstrate what you can do
// with properties
- (BOOL)isClean {
    return flags == 0;
}

@end