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转载说明：（谢谢）

http://blog.csdn.net/a21064346/article/details/7762034

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在这里说明一点东西：
NSSortDescriptor是一个专门用来排序的。它可以设定关键字（字典中的key），根据对应key的value来进行一个排序。
其中，如果是对array数组进行排序，那么object为字典，并且key对应的value 一定得是string，如果对应的是对象obj，那么系统会报错。
一般用到的地方有NSArray（object 为NSDictionary），core data过滤筛选。

根据字典中关键字，对字典进行排序  
//First create the array of dictionaries  
NSString *LAST = @"lastName";  
NSString *FIRST = @"firstName";  
   
NSMutableArray *array = [NSMutableArray array];  
NSArray *sortedArray;  
   
NSDictionary *dict;  
dict = [NSDictionary dictionaryWithObjectsAndKeys:  
                     @"Jo", FIRST, @"Smith", LAST, nil];  
[array addObject:dict];  
   
dict = [NSDictionary dictionaryWithObjectsAndKeys:  
                     @"Joe", FIRST, @"Smith", LAST, nil];  
[array addObject:dict];  
   
dict = [NSDictionary dictionaryWithObjectsAndKeys:  
                     @"Joe", FIRST, @"Smythe", LAST, nil];  
[array addObject:dict];  
   
dict = [NSDictionary dictionaryWithObjectsAndKeys:  
                     @"Joanne", FIRST, @"Smith", LAST, nil];  
[array addObject:dict];  
   
dict = [NSDictionary dictionaryWithObjectsAndKeys:  
                     @"Robert", FIRST, @"Jones", LAST, nil];  
[array addObject:dict];  
   
//Next we sort the contents of the array by last name then first name  
   
// The results are likely to be shown to a user  
// Note the use of the localizedCaseInsensitiveCompare: selector  
NSSortDescriptor *lastDescriptor =  
    [[[NSSortDescriptor alloc] initWithKey:LAST  
                               ascending:YES  
                               selector:@selector(localizedCaseInsensitiveCompare:)] autorelease];  
NSSortDescriptor *firstDescriptor =  
    [[[NSSortDescriptor alloc] initWithKey:FIRST  
                               ascending:YES  
                               selector:@selector(localizedCaseInsensitiveCompare:)] autorelease];  
   
NSArray *descriptors = [NSArray arrayWithObjects:lastDescriptor, firstDescriptor, nil];  
sortedArray = [array sortedArrayUsingDescriptors:descriptors];