Assign NSNull Object to NSString

问题

I'm writing an iOS App where i need to get data from a SQL-Database over mobile Services from Azure.

After downloading the data I get a NSDictionary with all attributes from the SQL-Table. If an attribute is empty, the value is NSNull.

Is there a way to pass NSNull to NSString without an IF-Statement (I don't want to have 20 if statements..)?

回答1:

I wrote a category just for dealing with this issue. I used it with Core Data but it should help you, too.

@interface NSDictionary (Extensions)

- (id)NSNullToNilForKey:(NSString *)key;

@end

@implementation NSDictionary (Extensions)

- (id)NSNullToNilForKey:(NSString *)key
{
    id value = [self valueForKey:key];

    return value != [NSNull null] ? value : nil;
}

@end

Sample use:

NSString *value = [dictionary NSNullToNilForKey:@"key"];

回答2:

You can't just assign it, but you can filter out all of the NSNull instances using something like this:

NSDictionary *dictionary = // data from server
NSDictionary *filteredDictionary = [dictionary mutableCopy];

NSSet *keysToRemove = [orig keysOfEntriesPassingTest:^BOOL(id key, id obj, BOOL *stop) {
    if (obj == [NSNull null]) {
        return YES;
    } else {
        return NO;
    }
}];
[filteredDictionary removeObjectsForKeys:[keysToRemove allObjects]];

Now you have the same dictionary except that every key with an NSNull has been removed.

来源：https://stackoverflow.com/questions/21922026/assign-nsnull-object-to-nsstring