为什么更改字典中包含的可变数组不需要更新字典？ [英] Why does changing a mutable array contained in a dictionary not require updating the dictionary?

问题描述

我正在尝试一段来自CS193P课程（Objective-C）的代码。我注意到了编译器工作的方式。 NSMutableDictionary ，已将 NSMutableArray code> photosByPhotographer 。稍后，更改照片，但未对 photosByPhotographer 进行任何更改。当我记录 photosByPhotographer 时，更改自动应用到它，它不需要任何额外的代码行！

I was trying a piece of code from CS193P course (Objective-C). I noticed something in the way that the compiler works. An NSMutableArray called photos was added to an NSMutableDictionary, photosByPhotographer. Later on, a change was made to photos without any changes to photosByPhotographer. When I logged photosByPhotographer, the change was automatically applied to it and it did not need any extra lines of code!

我不知道编译器如何使这项工作？任何要阅读的材料？

I wonder how the compiler makes this work? Any materials to read from?

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;
    }
);
}

推荐答案

正在使用对象并通过引用传递。

That's because in Cocoa, you are using objects and pass by reference.

想象下这段代码：

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 b 点到同一对象。

The variables a and b point to the same object. You can also see that by comparing the pointer values.

但是，如果我们执行以下操作：

However, if we do the following:

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

分配给 a （不是原始）的副本，因此 b 指向其他对象。您可以通过比较对象的地址进行检查。

b is assigned to a copy of a (not the original), so b points to an other object. You can check by comparing the addresses of the objects.

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