接口与API设计--18：尽量使用不可变对象

• 使用属性时，可将其声明为“readonly”（默认：readwrite）
• 比如通过初始化方法传入的属性，那么单独声明的属性值就可以声明为readonly

#import 
@interface Phone : NSObject
@property (nonatomic,copy,readonly) NSString *name;
@property (nonatomic,assign,readonly) NSInteger price;

- (instancetype)initWithName:(NSString *)name withPrice:(NSInteger)price;
@end

• 如果想修改数据，且不想让外人知道，通常可以在对象内部分类中将其设置为readwrite

#import "Phone.h"
//分类中重新声明
@interface Phone()
@property (nonatomic,copy,readwrite) NSString *name;
@property (nonatomic,assign,readwrite) NSInteger price;
@end

@implementation Phone
- (instancetype)initWithName:(NSString *)name withPrice:(NSInteger)price{
    self = [super init];
    if (self) {
        
        _name = [name copy];
        _price = price;
    }
    return self;
}
@end

• 尽量不要把可变的array、set、dictionary等作为公开属性，应该提供相应的方法，以此修改对象中的可变属性
  为什么不能直接把phoneArray定义成可变的，通过phoneArray来控制？
  比如在添加或者删除时，Phone对象可能要执行其他相关操作，如果直接从底层修改了内部用于存储的phoneArray，在Phone对象不知情时，直接修改phoneArray可能会让对象间各数据不一致

#import 
@interface Phone : NSObject
@property (nonatomic,copy,readonly) NSString *name;
@property (nonatomic,assign,readonly) NSInteger price;
@property (nonatomic,strong,readonly) NSArray *phoneArray;

- (instancetype)initWithName:(NSString *)name withPrice:(NSInteger)price;
- (void)addPhone:(Phone *)phone;
- (void)removePhone:(Phone *)phone;
@end

#import "Phone.h"
@interface Phone()
@property (nonatomic,copy,readwrite) NSString *name;
@property (nonatomic,assign,readwrite) NSInteger price;
@property (nonatomic,strong,readwrite) NSMutableArray *savePhoneArray;
@end

@implementation Phone

- (instancetype)initWithName:(NSString *)name withPrice:(NSInteger)price{
    self = [super init];
    if (self) {
        _name = [name copy];
        _price = price;
        _savePhoneArray = [[NSMutableArray alloc]init];
    }
    return self;
}
- (instancetype)init{
    return [self initWithName:@"iPhone" withPrice:999];
}
-(NSMutableArray *)savePhoneArray{
    if (_savePhoneArray == nil) {
        _savePhoneArray = [NSMutableArray new];
    }
    return _savePhoneArray;
}

- (void)addPhone:(Phone *)phone{
    [self.savePhoneArray addObject:phone];
}
- (NSArray *)phoneArray{
    return [self.savePhoneArray copy];
}
- (void)removePhone:(Phone *)phone{
    [self.savePhoneArray removeObject:phone];
}
@end

参考

Effective+Objective-C 2.0 编写高质量iOS与OS X代码的52个有效方法