原來 Dictionary 不能放null, 真是長知識了。
NSDictionary and other collections cannot contain nil values. When NSDictionary must store a null, a special value [NSNull null] is stored.
Compare the value at @"My Data" to [NSNull null] to determine if the corresponding value is null or not.
// Since [NSNull null] is a singleton, you can use == instead of isEqual
if ([details objectForKey:@"My Data"] == [NSNull null]) {
// Display the alert
}It looks like you have a dangling or wild pointer.
You can consider Objective-C objects as pointers to structs.
You can then of course compare them with NULL, or with other pointers.
So:
( myDict == NULL )and
( myDict == [ NSNull null ] )are both valid.
The first one will check if the pointer is NULL. NULL is usually defined as a void * with a value of 0.
Note that, for Objective-C objects, we usually use nil. nil is also defined as a void * with a value of 0, so it equals NULL. It’s just here to denote a NULL pointer to an object, rather than a standard pointer.
The second one compares the address of myDict with the singleton instance of the NSNullclass. So you are here comparing two pointers values.
So to quickly resume:
NULL == nil == Nil == 0And as [ NSNull null ] is a valid instance:
NULL != [ NSNull null ]Now about this:
( [ myDict count ] == 0 )It may crash if you have a wild pointer:
NSDictionary * myDict;
[ myDict count ];Unless using ARC, it will surely crash, because the myDict variable has not been initialised, and may actually point to anything.
It may also crash if you have a dangling pointer:
NSDictionary * myDict;
myDict = [ [ NSDictionary alloc ] init ];
[ myDict release ];
[ myDict count ];Then you’ll try to send a message to a deallocated object.
Sending a message to nil/NULL is always valid in Objective-C.
So it depends if you want to check if a dictionary is nil, or if it doesn’t have values (as a valid dictionary instance may be empty).
In the first case, compare with nil. Otherwise, checks if count is 0, and ensure you’re instance is still valid. Maybe you just forgot a retain somewhere.
相關範例：
-(void)CheckDictionary:(NSMutableDictionary *)dic
{
NSArray *Arr = [dic allKeys];
for (int i = 0; i<Arr.count; i++)
{
if ([[dic valueForKey:[Arr objectAtIndex:i]] isKindOfClass:[NSNull class]])
{
[dic setObject:@"" forKey:[Arr objectAtIndex:i]];
}
else if ([[dic valueForKey:[Arr objectAtIndex:i]] isKindOfClass:[NSDictionary class]])
{
NSMutableDictionary *dict = [[dic valueForKey:[Arr objectAtIndex:i]] mutableCopy];
[dic setObject:dict forKey:[Arr objectAtIndex:i]];
[self CheckDictionary:dict];
}
else if ([[dic valueForKey:[Arr objectAtIndex:i]] isKindOfClass:[NSMutableArray class]])
{
NSMutableArray *Arr12 = [dic valueForKey:[Arr objectAtIndex:i]];
for (int j = 0; j<Arr12.count; j++)
{
if ([[Arr12 objectAtIndex:j] isKindOfClass:[NSDictionary class]])
{
NSDictionary *dict123 = [Arr12 objectAtIndex:j];
NSMutableDictionary *dict = [dict123 mutableCopy];
[Arr12 replaceObjectAtIndex:j withObject:dict];
[self CheckDictionary:dict];
}
}
}
}
}And Just pass the whole dictionary when you call this method I hope it would be helpful!!
NSDictionary *dictionary = [NSJSONSerialization JSONObjectWithData:responseObject options: NSJSONReadingMutableContainers error: nil];
if (dictionary)
{
NSMutableDictionary *dict = [dictionary mutableCopy];
[self CheckDictionary:dict];
dictionary = [NSDictionary dictionaryWithDictionary:dict];
}