ios - 如何初始化从 NSString 到 NSArray 的字典

Question

我正在尝试创建一个从 NSString 到数组(不确定它应该是 NSArray 还是 NSMutableArray)。

属性:

@property(nonatomic, readonly) NSMutableDictionary * categories;

实现:

@synthesize categories = _categories;


- (NSMutableDictionary *)categories{
    if(! _categories) {
        for(PFObject * each in self.products) {
            NSString * currentcategory = [each valueForKey:@"subtitle"];
            NSArray * currentlist = [_categories objectForKey:currentcategory];
            if(! currentlist) {
                currentlist = [[NSArray alloc] init];
            }
            NSMutableArray * newArray = [currentlist mutableCopy];
            [newArray addObject:each];
            NSArray * newlist = [NSArray arrayWithArray:newArray];
            [_categories setObject:newlist forKey:currentcategory];
        }
    }
    NSLog(@"After constructor the value of the dictionary is %d", [_categories count]);
    return _categories;
}

从调试 NSLog 我意识到字典在构建后是空的。这里有什么问题，我该如何更改？

Answer 1

代码行之后

 if(! _categories) {

添加

_categories = [NSMutableDictionary new];