要点一：
在设计类的时候，我们会充分运用属性来封装数据，但应该尽量把对外公布的属性设置为只读，而且只在确有必要时才将属性对外公布。
// 举例:头文件中
#import <Foundation/Foundation.h>
@interface EOCPointOfInterest : NSObject
@property (nonatomic, copy, readonly) NSString *identifier;
@property (nonatomic, copy, readonly) NSString *title;
@property (nonatomic, assign, readonly) float latitude;
@property (nonatomic, assign, readonly) float longitude;
@end
要点二：
有时需要修改封装在对象内部的数据，不想令这些数据为外人所改动。我们可以将属性在对象内部重新声明为readwrite，这一操作可于“class-continuation分类”中完成，在公共接口中声明的属性可于此处重新声明，属性的其他特质必须保持不变，而readonly可以扩展为readwrite。
// 举例：实现文件中
#import "EOCPointOfInterest.h"
@interface EOCPointOfInterest()
@property (nonatomic, copy, readwrite) NSString *identifier;
@property (nonatomic, copy, readwrite) NSString *title;
@property (nonatomic, assign, readwrite) float latitude;
@property (nonatomic, assign, readwrite) float longitude;
@end
@implementation EOCPointOfInterest
@end
要点三：
对象表示的各种集合属性，应该提供一个readonly属性供外界使用，该属性将返回不可变的集合，而该集合是内部那个可变集合的一份拷贝。即不要把可变的集合作为属性公开，而应该提供相关方法，以此修改对象中的可变集合。
// 头文件：
#import <Foundation/Foundation.h>
@interface EOCPerson : NSObject
@property (nonatomic, copy, readonly) NSString *firstName;
@property (nonatomic, copy, readonly) NSString *lastName;
@property (nonatomic, copy, readonly) NSSet *friends;
- (id)initWithFirstName:(NSString *)firstName andLastName:(NSString *)lastName;
- (void)addFriend:(EOCPerson *)person;
- (void)removeFriend:(EOCPerson *)person;
@end
// 实现文件：
#import "EOCPerson.h"
@interface EOCPerson()
@property (nonatomic, copy, readwrite) NSString *firstName;
@property (nonatomic, copy, readwrite) NSString *lastName;
@end
@implementation EOCPerson {
NSMutableSet *_internalFriends;
}
- (NSSet *)friends
{
return [_internalFriends copy];
}
- (void)addFriend:(EOCPerson *)person
{
[_internalFriends addObject:person];
}
- (void)removeFriend:(EOCPerson *)person
{
[_internalFriends removeObject:person];
}
- (id)initWithFirstName:(NSString *)firstName andLastName:(NSString *)lastName
{
if (self = [super init]) {
_firstName = firstName;
_lastName = lastName;
_internalFriends = [NSMutableSet new];
}
return self;
}
@end
