objective-c - 为什么更改字典中包含的可变数组不需要更新字典？

Question

我正在尝试 CS193P 类(class) (Objective-C) 中的一段代码。我注意到编译器的工作方式。一个名为 photos 的 NSMutableArray 被添加到一个 NSMutableDictionary，photosByPhotographer。后来，对 photos 进行了更改，但未对 photosByPhotographer 进行任何更改。当我登录 photosByPhotographer 时，更改会自动应用到它，不需要任何额外的代码行!

我想知道编译器是如何做到这一点的？有什么 Material 可以阅读吗？

代码如下:

- (void)updatePhotosByPhotographer
{
    NSMutableDictionary *photosByPhotographer = [NSMutableDictionary dictionary];
    for (NSDictionary *photo in self.photos) {
        NSString *photographer = [photo objectForKey:FLICKR_PHOTO_OWNER];
        NSMutableArray *photos = [photosByPhotographer objectForKey:photographer];
        if (!photos) {
            photos = [NSMutableArray array];
            [photosByPhotographer setObject:photos forKey:photographer];
            NSLog(@"photosByPhotographer in if: %@", photosByPhotographer);
        }
        [photos addObject:photo];
        NSLog(@"photosByPhotographer after if: %@", photosByPhotographer);
    }
    self.photosByPhotographer = photosByPhotographer;
}

NSLog()结果如下:

2012-07-20 20:05:57.618 Shutterbug[453:f803] photosByPhotographer in if: {
Dowbiggin =     (
);
}

2012-07-20 20:05:57.620 Shutterbug[453:f803] photosByPhotographer after if: {
Dowbiggin =     (
            {
        accuracy = 16;
        context = 0;
        dateupload = 1342836026;
        description =             {
            "_content" = "";
        };
        farm = 9;
        "geo_is_contact" = 0;
        "geo_is_family" = 0;
        "geo_is_friend" = 0;
        "geo_is_public" = 1;
        id = 7612787270;
        isfamily = 0;
        isfriend = 0;
        ispublic = 1;
        latitude = "37.307085";
        longitude = "-121.900395";
        originalformat = jpg;
        originalsecret = 052e70d412;
        owner = "22751315@N05";
        ownername = Dowbiggin;
        "place_id" = cils8sJUV7MeXHwt9A;
        secret = 4437007c99;
        server = 8161;
        tags = "square squareformat iphoneography instagramapp uploaded:by=instagram foursquare:venue=49f13597f964a5209c691fe3";
        title = "My little goofball";
        woeid = 55971033;
    }
);
}

Answer 1

那是因为在 Cocoa 中，您使用的是对象并通过引用传递。

想象一下这段代码:

NSMutableArray *a, *b;

a  = [NSMutableArray new];
[a addObject:@"A"]; // a contains @"A"
b = a; // b is assigned to the same object
[b addObject:@"B"]; // a and b contain @"A", @"B"

NSLog(@"a = %p and b = %p", a, b); // same values

变量a 和b 指向 同一个对象。您还可以通过比较指针值看到这一点。

但是，如果我们执行以下操作:

NSMutableArray *a, *b;

a  = [NSMutableArray new];
[a addObject:@"A"]; // a contains @"A"
b = [[a mutableCopy] autorelease]; // b is assigned to a COPY of the object
[b addObject:@"B"]; // b contains @"A", @"B", while a still contains @"A"

NSLog(@"a = %p and b = %p", a, b); // NOT the same values

b 被分配给 a 的副本(不是原始的)，因此 b 指向另一个对象。您可以通过比较对象的地址来检查。