NSJSONSerialization没有创建可变容器 [英] NSJSONSerialization not creating mutable containers

问题描述

以下是代码：

NSError *parseError;
NSMutableArray *listOfObjects = [NSJSONSerialization JSONObjectWithData:[@"[]" dataUsingEncoding:NSUTF8StringEncoding] options:NSJSONReadingMutableContainers error:&parseError];
NSLog(@"Is mutable? %li", [listOfObjects isKindOfClass:[NSMutableArray class]]);

listOfObjects = [NSJSONSerialization JSONObjectWithData:[@"[[],{}]" dataUsingEncoding:NSUTF8StringEncoding] options:NSJSONReadingMutableContainers error:&parseError];
NSLog(@"Is mutable? %li", [listOfObjects isKindOfClass:[NSMutableArray class]]);

正如你所看到的，我正在调用完全相同的方法来解析JSON两次，一次使用JSON中的空列表，然后是包含内部对象的列表。结果如下：

As you can see, I'm calling exactly the same method for parsing the JSON both times, one with an empty list in the JSON, and then a list with an object inside. Here's the result:

Is mutable? 0
Is mutable? 1 

问题是NSJSONSerialization似乎没有遵循为空创建可变容器的选项名单。对我来说似乎是一个错误，但也许我只是误解了一些事情。

The problem is that the NSJSONSerialization doesn't seem to follow the option to create mutable containers for empty lists. Seems like a bug to me, but maybe I just misunderstanding things.

任何想法？

推荐答案

这与预期一样：

NSString *s = @"{ \"objs\": [ \"a\", \"b\" ] }";    
NSData *d = [NSData dataWithBytes:[s UTF8String] length:[s length]];
id dict = [NSJSONSerialization JSONObjectWithData:d options:NSJSONReadingMutableContainers error:NULL];

NSLog(@"%@", dict);

[[dict objectForKey:@"objs"] addObject:@"c"];

NSLog(@"%@", dict);
NSLog(@"%@", [[dict objectForKey:@"objs"] class]);

这是控制台输出：

2012-03-28 13:49:46.224 ExampleRunner[42526:707] {
    objs =     (
        a,
        b
    );
}
2012-03-28 13:49:46.225 ExampleRunner[42526:707] {
    objs =     (
        a,
        b,
        c
    );
}
2012-03-28 13:49:46.225 ExampleRunner[42526:707] __NSArrayM

编辑

请注意，如果我们将以下行附加到上面的代码中...

Note that if we append the following line to the code above...

NSLog(@"%@", [[dict objectForKey:@"objs"] superclass]);

...我们在控制台上获得以下输出：

...we get the following output on the console:

2012-03-28 18:09:53.770 ExampleRunner[42830:707] NSMutableArray

...以防万一 __ NSArrayM 是 NSMutableArray <的私有子类/ code>，从而证明OP的代码确实按预期工作（除了他的 NSLog 语句）。

...just in case it wasn't clear that __NSArrayM is a private subclass of NSMutableArray, thus proving that the OP's code did indeed work as expected (except for his NSLog statement).

编辑

哦，顺便说一下，以下代码行......

Oh, and by the way, the following line of code...

NSLog(@"%d", [[dict objectForKey:@"objs"] isKindOfClass:[NSMutableArray class]]);

...导致以下控制台输出：

...results in the following console output:

2012-03-28 18:19:19.721 ExampleRunner[42886:707] 1

编辑（回复已更改的问题）

有趣......看起来像个错误。给出以下代码：

Interesting...looks like a bug. Given the following code:

NSData *dictData2 = [@"{ \"foo\": \"bar\" }" dataUsingEncoding:NSUTF8StringEncoding];
id dict2 = [NSJSONSerialization JSONObjectWithData:dictData2 options:NSJSONReadingMutableContainers error:NULL];
NSLog(@"%@", [dict2 class]);
NSLog(@"%@", [dict2 superclass]);
NSLog(@"%d", [dict2 isKindOfClass:[NSMutableDictionary class]]);

// This works...
[dict2 setObject:@"quux" forKey:@"baz"];
NSLog(@"%@", dict2);

NSData *dictData = [@"{}" dataUsingEncoding:NSUTF8StringEncoding];
id emptyDict = [NSJSONSerialization JSONObjectWithData:dictData options:NSJSONReadingMutableContainers error:NULL];
NSLog(@"%@", [emptyDict class]);
NSLog(@"%@", [emptyDict superclass]);
NSLog(@"%d", [emptyDict isKindOfClass:[NSMutableDictionary class]]);

//...but this fails:
[emptyDict setObject:@"quux" forKey:@"baz"];
NSLog(@"%@", emptyDict);

这是控制台输出：

2012-03-29 09:40:52.781 ExampleRunner[43816:707] NSMutableDictionary
2012-03-29 09:40:52.782 ExampleRunner[43816:707] 1
2012-03-29 09:40:52.782 ExampleRunner[43816:707] __NSCFDictionary
2012-03-29 09:40:52.782 ExampleRunner[43816:707] NSMutableDictionary
2012-03-29 09:40:52.783 ExampleRunner[43816:707] 1
2012-03-29 09:40:52.783 ExampleRunner[43816:707] {
    baz = quux;
    foo = bar;
}
2012-03-29 09:40:52.784 ExampleRunner[43816:707] __NSCFDictionary
2012-03-29 09:40:52.784 ExampleRunner[43816:707] NSMutableDictionary
2012-03-29 09:40:52.784 ExampleRunner[43816:707] 1
2012-03-29 09:40:52.785 ExampleRunner[43816:707] NSException: -[__NSCFDictionary setObject:forKey:]: mutating method sent to immutable object

因此以这种方式创建的空数组和字典似乎没有按预期运行。

So empty arrays and dictionaries created this way don't seem to behave as expected.

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