NSJSONSerialization 不创建可变容器

Question

代码如下：

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]]);

如您所见，我两次调用完全相同的方法来解析 JSON，一次在 JSON 中包含一个空列表，然后是一个包含对象的列表。结果如下：

Is mutable? 0
Is mutable? 1 

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

有什么想法吗？

Answer 1

这正如预期的那样工作：

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，如果我们在上面的代码中添加以下行...

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

...我们在控制台上得到以下输出：

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

...以防万一不清楚__NSArrayM 是NSMutableArray 的私有子类，从而证明OP 的代码确实按预期工作（除了他的NSLog 声明）。

编辑

哦，对了，下面这行代码……

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

...导致以下控制台输出：

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

编辑（响应更改的问题）

有趣...看起来像一个错误。给定以下代码：

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。

Answer 2

这是我解决此问题的方法：

#import "NSJSONSerialization+MutableBugFix.h"

@implementation NSJSONSerialization (NSJSONSerialization_MutableBugFix)

+ (id)JSONObjectWithDataFixed:(NSData *)data options:(NSJSONReadingOptions)opt error:(NSError **)error {
    id object = [NSJSONSerialization JSONObjectWithData:data options:opt error:error];

    if (opt & NSJSONReadingMutableContainers) {
        return [self JSONMutableFixObject:object];
    }

    return object;
}

+ (id)JSONMutableFixObject:(id)object {
    if ([object isKindOfClass:[NSDictionary class]]) {
        // NSJSONSerialization creates an immutable container if it's empty (boo!!)
        if ([object count] == 0) {
            object = [object mutableCopy];
        }

        for (NSString *key in [object allKeys]) {
            [object setObject:[self JSONMutableFixObject:[object objectForKey:key]] forKey:key];
        }
    } else if ([object isKindOfClass:[NSArray class]]) {
        // NSJSONSerialization creates an immutable container if it's empty (boo!!)
        if (![object count] == 0) {
            object = [object mutableCopy];
        }

        for (NSUInteger i = 0; i < [object count]; ++i) {
            [object replaceObjectAtIndex:i withObject:[self JSONMutableFixObject:[object objectAtIndex:i]]];
        }
    }

    return object;
}

@end

所以我打电话：

NSDictionary *object = [NSJSONSerialization JSONObjectWithDataFixed:jsonData options:NSJSONReadingMutableContainers error:&err];

而不是通常的：

NSDictionary *object = [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers error:&err];

Answer 3

其他人也认为这是一个错误，请参阅

1. https://github.com/couchbaselabs/TouchDB-iOS/issues/44 或
2. 例如https://github.com/johnlabarge/jlbiosutils/blob/master/jlbiosutils/DynamicProperties.m。

在后一种情况下，您也可以看到空字典的完整解决方法（请参阅DynamicGetter(...) 方法）。

Answer 4

这是我的工作：

BOOL needsWorkaround = YES;
if (needsWorkaround)
{
    NSMutableDictionary* appState2 = 
        (__bridge_transfer NSMutableDictionary*) 
        CFPropertyListCreateDeepCopy (
            kCFAllocatorDefault, (__bridge void*)appState,
            kCFPropertyListMutableContainersAndLeaves
        );
    appState = appState2;
}