objectForKey空檢查

Question

試圖在項目 （產品NSDictionary）

我想這應該工作，但我不進入第二，如果和崩潰與檢查數據的有效性：
unrecognized selector sent to instance 監守galleryArr是(null)

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

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

任何想法？

Answer 1

添加支票[gallryArr isKindOfClass:[NSArray class]]。

Answer 2

也許你會得到NSNull？這是一個代表零的單例（[NSNull null]）對象。你可以檢查if([gallryArr isKindOfClass:[NSArray class]])。

Answer 3

我已經解決了這個問題，有一個簡單的這個簡單的Objective-C類：

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 

現在你可以撥打電話：

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