OC Property

1、四种写法的区别：

@property (nonatomic, strong) NSArray *array0;
@property (nonatomic, copy) NSArray *array1;
@property (nonatomic, strong) NSMutableArray *array2;
@property (nonatomic, copy) NSMutableArray *array3;

PS：注意以下两点：

1>修饰属性问题，小心闪退：

@property (nonatomic,copy) NSMutableArray *mutableArray;
self.mutableArray = [NSMutableArray arrayWithObject:@"111"];
//    self.immutableArray = [NSArray array];
//    
//    NSLog(@"before mutableArray=%@,immutabxrleArray=%@",self.mutableArray,self.immutableArray);
//    
//    self.immutableArray = self.mutableArray;
[self.mutableArray addObject:@"222"];
    

上面的代码会闪退的。相当于以下代码。

NSMutableArray *arr =  [NSMutableArray arrayWithObject:@"111"];
self.mutableArray = [arr copy];

正确的写法：

@property (nonatomic,copy) NSArray *immutableArray;

@property (nonatomic,strong) NSMutableArray *mutableArray;

2>
NSArray,NSDictionary,NSSet -> copy
NSMutableArray,NSMutableDictionary,NSMutableSet->strong

参考链接：

https://www.jianshu.com/p/18241d9c075c

2、class修饰属性，更方便的实现单例：

@property (nonatomic, class) ViewController *vc;

+ (ViewController *)vc {
    static ViewController *instance = nil;
    static  dispatch_once_t onceToken = 0;
    dispatch_once(&onceToken, ^{
        instance = [[self alloc] init];
    });
    return instance;
}

+ (void)setVc:(ViewController *)vc {
    
}