objectForKey null检查

Question

尝试检查项目中数据的有效性 （项目为NSDictionary）

我认为这应该有用，但我确实进入了第二个如果并且崩溃了：
unrecognized selector sent to instance 因为galleryArr是(null)

    NSArray *galleryArr = [item objectForKey:@"photos"];

    if (galleryArr != nil ) {
        if ([galleryArr count] != 0) {
            //do something
        }
    }

有什么想法吗？

Answer 1

我用简单的这个简单的Objective-C类别解决了这个问题：

的NSDictionary + NotNull.h

#import <Foundation/Foundation.h>

/*! This category extends NSDictionary to work around an issue with NSNull object.
 */
@interface NSDictionary (NotNull)

/*! @abstract Returns the value associated with a given key, but only if the value is not NSNull.
    @param aKey The key for which to return the corresponding value.
    @return The value associated with the given key, or nil if no value is associated with the key, or the value is NSNull.
 */
- (id)objectOrNilForKey:(id)aKey;

@end

的NSDictionary + NotNull.m

#import "NSDictionary+NotNull.h"

@implementation NSDictionary (NotNull)

- (id)objectOrNilForKey:(id)aKey
{
    id object = [self objectForKey:aKey];
    if (object == [NSNull null]) {
        return nil;
    }
    return object;
}

@end

现在你可以打电话：

NSArray *galleryArr = [item objectOrNilForKey:@"photos"];

Answer 2

添加[gallryArr isKindOfClass:[NSArray class]]。

的支票

Answer 3

也许你得到NSNull？这是一个代表零的单例（[NSNull null]）对象。您可以查看if([gallryArr isKindOfClass:[NSArray class]])。