我们以数组的排序为例(也许NSSortDescriptor最常用的地方是NSFetchedResultsController中,但用法大致相同)。
假设要对userArray数组中的对象进行排序,而数组中含有多个User对象(User继承于NSManagedObject),User中有一个属性叫做country。
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1. NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"country" ascending:YES];
2. [userArray sortUsingDescriptors:[NSArray arrayWithObject:sortDescriptor]];
这样,就可以根据每一个User的country来进行升序或降序的排列,sortUsingDescriptors的参数是一个数组,也就是说可以使用多个排序参数。
再比如说,User有一个关系叫做image,image有个属性叫做timestamp,如果要根据User的image关系的timestamp排序,仅需把上面代码中的country改为,image.timestamp。即
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1. NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"image.timestamp" ascending:YES];
2. [userArray sortUsingDescriptors:[NSArray arrayWithObject:sortDescriptor]];
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];
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