NSArray, NSDictionary支持下标运算符，使用下标运算符，可以使代码看起来比较优雅，工程师也可以少输入很多字符。那么问题来了，能不能让我们自定义的类也支持下标运算符呢？答案是“可以”，并且实现起来很简单。

如果想支持索引下标，自定义的类需要实现如下两个方法：

- (id)objectAtIndexedSubscript: (NSUInteger)idx;
- (void)setObject: (id)obj atIndexedSubscript: (NSUInteger)idx;

如果想支持键值下标，自定义的类需要实现如下两个方法：

- (id)objectForKeyedSubscript: (id<NSCopying>)key;
- (void)setObject: (id)obj forKeyedSubscript: (id<NSCopying>)key;

例子

头文件

@interface TestSubscripting : NSObject

- (id)objectAtIndexedSubscript: (NSUInteger)idx;
- (void)setObject: (id)obj atIndexedSubscript: (NSUInteger)idx;

- (id)objectForKeyedSubscript: (id<NSCopying>)key;
- (void)setObject: (id)obj forKeyedSubscript: (id<NSCopying>)key;

@end

实现文件

@interface TestSubscripting ()

@property (nonatomic, strong) NSMutableArray *array;
@property (nonatomic, strong) NSMutableDictionary *dictionary;

@end

@implementation TestSubscripting

- (instancetype)init {
    self = [super init];
    if (self) {
        _array = [NSMutableArray array];
        _dictionary = [NSMutableDictionary dictionary];
    }
    
    return self;
}

- (id)objectAtIndexedSubscript:(NSUInteger)idx {
    return self.array[idx];
}

- (void)setObject:(id)obj atIndexedSubscript:(NSUInteger)idx {
    self.array[idx] = obj;
}

- (id)objectForKeyedSubscript:(id<NSCopying>)key {
    return self.dictionary[key];
}

- (void)setObject:(id)obj forKeyedSubscript:(id<NSCopying>)key {
    self.dictionary[key] = obj;
}
@end