我很好奇，由于 json-framework 的帮助，我目前有一个 NSDictionary，其中一些值设置为 NSNull 对象。
目的是去除所有NSNull 值并将其替换为空字符串。
我确定有人在某处做过这个？毫无疑问，它可能是一个四班轮而且很简单，我只是太累了，无法自己解决这个问题。
【问题讨论】：
标签： objective-c nsstring nsdictionary json-framework nsnull
我已经测试了 Stakenborg 解决方案。它工作得很好，但它有以下问题。例如，如果某个对象预期为数字，则将其转换为 NSNull 可能会导致错误。
我创建了一个新方法来直接删除 NSNull 条目。这样你只需要检查对应的键是否存在。
添加NSDictionary+NullReplacement
- (NSDictionary *)dictionaryByRemovingNulls{
const NSMutableDictionary *replaced = [self mutableCopy];
const id nul = [NSNull null];
for (NSString *key in self) {
id object = [self objectForKey:key];
if (object == nul) [replaced removeObjectForKey:key];
else if ([object isKindOfClass:[NSDictionary class]]) [replaced setObject:[object dictionaryByRemovingNulls] forKey:key];
else if ([object isKindOfClass:[NSArray class]]) [replaced setObject:[object arrayByRemovingNulls] forKey:key];
}
return [NSDictionary dictionaryWithDictionary:[replaced copy]];
}
在NSArray+NullReplacement
- (NSArray *)arrayByRemovingNulls {
NSMutableArray *replaced = [self mutableCopy];
const id nul = [NSNull null];
for (int idx = [replaced count]-1; idx >=0; idx--) {
id object = [replaced objectAtIndex:idx];
if (object == nul) [replaced removeObjectAtIndex:idx];
else if ([object isKindOfClass:[NSDictionary class]]) [replaced replaceObjectAtIndex:idx withObject:[object dictionaryByRemovingNulls]];
else if ([object isKindOfClass:[NSArray class]]) [replaced replaceObjectAtIndex:idx withObject:[object arrayByRemovingNulls]];
}
return [replaced copy];
}
【讨论】：
我对 Jacob 的原始答案进行了一些更改，以将其扩展为处理存储在原始字典中的字典和数组。
#import "NSDictionary+NullReplacement.h"
#import "NSArray+NullReplacement.h"
@implementation NSDictionary (NullReplacement)
- (NSDictionary *)dictionaryByReplacingNullsWithBlanks {
const NSMutableDictionary *replaced = [self mutableCopy];
const id nul = [NSNull null];
const NSString *blank = @"";
for (NSString *key in self) {
id object = [self objectForKey:key];
if (object == nul) [replaced setObject:blank forKey:key];
else if ([object isKindOfClass:[NSDictionary class]]) [replaced setObject:[object dictionaryByReplacingNullsWithBlanks] forKey:key];
else if ([object isKindOfClass:[NSArray class]]) [replaced setObject:[object arrayByReplacingNullsWithBlanks] forKey:key];
}
return [NSDictionary dictionaryWithDictionary:[replaced copy]];
}
@end
当然还有一个数组类：
#import "NSArray+NullReplacement.h"
#import "NSDictionary+NullReplacement.h"
@implementation NSArray (NullReplacement)
- (NSArray *)arrayByReplacingNullsWithBlanks {
NSMutableArray *replaced = [self mutableCopy];
const id nul = [NSNull null];
const NSString *blank = @"";
for (int idx = 0; idx < [replaced count]; idx++) {
id object = [replaced objectAtIndex:idx];
if (object == nul) [replaced replaceObjectAtIndex:idx withObject:blank];
else if ([object isKindOfClass:[NSDictionary class]]) [replaced replaceObjectAtIndex:idx withObject:[object dictionaryByReplacingNullsWithBlanks]];
else if ([object isKindOfClass:[NSArray class]]) [replaced replaceObjectAtIndex:idx withObject:[object arrayByReplacingNullsWithBlanks]];
}
return [replaced copy];
}
@end
有了这个，你可以获取任何数组或字典并递归地清除所有 [NSNull null] 实例。
附：为了完整起见，这里是头文件：
@interface NSDictionary (NullReplacement)
- (NSDictionary *)dictionaryByReplacingNullsWithBlanks;
@end
还有数组头：
@interface NSArray (NullReplacement)
- (NSArray *)arrayByReplacingNullsWithBlanks;
@end
【讨论】：
这是我的解决方案：
+ (NSDictionary *)cleanNullInJsonDic:(NSDictionary *)dic
{
if (!dic || (id)dic == [NSNull null])
{
return dic;
}
NSMutableDictionary *mulDic = [[NSMutableDictionary alloc] init];
for (NSString *key in [dic allKeys])
{
NSObject *obj = dic[key];
if (!obj || obj == [NSNull null])
{
// [mulDic setObject:[@"" JSONValue] forKey:key];
}else if ([obj isKindOfClass:[NSDictionary class]])
{
[mulDic setObject:[self cleanNullInJsonDic:(NSDictionary *)obj] forKey:key];
}else if ([obj isKindOfClass:[NSArray class]])
{
NSArray *array = [BasicObject cleanNullInJsonArray:(NSArray *)obj];
[mulDic setObject:array forKey:key];
}else
{
[mulDic setObject:obj forKey:key];
}
}
return mulDic;
}
+ (NSArray *)cleanNullInJsonArray:(NSArray *)array
{
if (!array || (id)array == [NSNull null])
{
return array;
}
NSMutableArray *mulArray = [[NSMutableArray alloc] init];
for (NSObject *obj in array)
{
if (!obj || obj == [NSNull null])
{
// [mulArray addObject:[@"" JSONValue]];
}else if ([obj isKindOfClass:[NSDictionary class]])
{
NSDictionary *dic = [self cleanNullInJsonDic:(NSDictionary *)obj];
[mulArray addObject:dic];
}else if ([obj isKindOfClass:[NSArray class]])
{
NSArray *a = [BasicObject cleanNullInJsonArray:(NSArray *)obj];
[mulArray addObject:a];
}else
{
[mulArray addObject:obj];
}
}
return mulArray;
}
【讨论】：
nsnull 上返回 nil 的类别似乎也有意义，至少对我而言。那里有几个。一个使所有调用都返回 nil，这似乎是有道理的。抱歉没有链接。我猜如果您以后需要使用 nspropertylistserialization，该类别可能不适合您。
【讨论】：
-(NSDictionary*)stripNulls:(NSDictionary*)dict{
NSMutableDictionary *returnDict = [NSMutableDictionary new];
NSArray *allKeys = [dict allKeys];
NSArray *allValues = [dict allValues];
for (int i=0; i<[allValues count]; i++) {
if([allValues objectAtIndex:i] == (NSString*)[NSNull null]){
[returnDict setValue:@"" forKey:[allKeys objectAtIndex:i]];
}
else
[returnDict setValue:[allValues objectAtIndex:i] forKey:[allKeys objectAtIndex:i]];
}
return returnDict;
}
【讨论】：
真的很简单：
@interface NSDictionary (JRAdditions)
- (NSDictionary *)dictionaryByReplacingNullsWithStrings;
@end
@implementation NSDictionary (JRAdditions)
- (NSDictionary *)dictionaryByReplacingNullsWithStrings {
const NSMutableDictionary *replaced = [self mutableCopy];
const id nul = [NSNull null];
const NSString *blank = @"";
for(NSString *key in self) {
const id object = [self objectForKey:key];
if(object == nul) {
//pointer comparison is way faster than -isKindOfClass:
//since [NSNull null] is a singleton, they'll all point to the same
//location in memory.
[replaced setObject:blank
forKey:key];
}
}
return [replaced copy];
}
@end
用法：
NSDictionary *someDictThatHasNulls = ...;
NSDictionary *replacedDict = [someDictThatHasNulls dictionaryByReplacingNullsWithStrings];
【讨论】：
NSNull是单例，你可以直接测试值指针：if([self objectForKey:key]==[NSNull null])。
翻阅字典寻找 NSNull 是解决问题的一种方法，但我采取了一种稍微懒惰的方法。你可以指定一个空字符串，而不是 nil，但原理是一样的。
@interface NSDictionary (CPJSONDictionary)
- (id)jsonObjectForKey:(id)aKey;
@end
@implementation NSDictionary (CPJSONDictionary)
- (id)jsonObjectForKey:(id)aKey {
id object = [self objectForKey:aKey];
if ([object isKindOfClass:[NSNull class]]) {
object = nil;
}
return object;
}
@end
【讨论】：
另一种变化：
NSDictionary * NewDictionaryReplacingNSNullWithEmptyNSString(NSDictionary * dict) {
NSMutableDictionary * const m = [dict mutableCopy];
NSString * const empty = @"";
id const nul = [NSNull null];
NSArray * const keys = [m allKeys];
for (NSUInteger idx = 0, count = [keys count]; idx < count; ++idx) {
id const key = [keys objectAtIndex:idx];
id const obj = [m objectForKey:key];
if (nul == obj) {
[m setObject:empty forKey:key];
}
}
NSDictionary * result = [m copy];
[m release];
return result;
}
结果与 Jacob 的结果相同，看起来几乎相同，但在我进行的测试中，速度和内存要求是二分之一到三分之一（ARC 或 MRC）。当然，你也可以将它作为一个分类方法使用。
【讨论】：