NSSet转成NSArray 以及NSSortDescriptor的使用

//如果想排序以后再取，可以这样：
NSSet *users = [groupUser users];

//如果是存的字典，则key后面写的是想按照哪个关键字进行排序 
NSSortDescriptor *sd = [[NSSortDescriptor alloc] initWithKey:@"displayName" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sd, nil];
NSArray *userArray = [users sortedArrayUsingDescriptors:sortDescriptors];
//如果直接取，可以这样：
NSArray *userArray = [users  allObjects];

NSSortDescriptor 指定用于对象数组排序的对象的属性。

如果是Employee对象需要按照name来排序，就生成下面的descriptor

NSSortDescriptor *descriptor = [NSSortDescriptor sortDescriptorWithKey:name ascending:YES];

如果需要多个排序，比如先按name排序，再按入职日期排序。那就创建两个descriptor

NSSortDescriptor *descriptor = [NSSortDescriptor sortDescriptorWithKey:hireDate ascending:YES];

两个descriptor放到数组里一起传给需要排序的数组。

如果对象就是NSString，就是字符串数组排序，那更简单了，sortdescriptor的key直接指定为nil，就直接排序对象，而不是对象的某一个属性了。

    NSSortDescriptor *descriptor = [NSSortDescriptor sortDescriptorWithKey:nil ascending:YES];

    NSArray *descriptors = [NSArray arrayWithObject:descriptor];

    NSArray *myDataArray = [NSArray arrayWithObjects:@"what", @"xero", @"highligth",@"mountain", @"Victory", @"Balance", nil];

    NSArray *resultArray = [myDataArray sortedArrayUsingDescriptors:descriptors];

    NSLog(@"%@", resultArray);

NSArray 使用sortedArrayUsingDescriptors，返回排序好的数组。

NSMutableArray可以直接使用sortUsingDescriptors，对数组本身排序。

关于NSSortDescriptor

NSMutableArray *ma1 = [[NSMutableArray alloc] init];
    [ma1 addObject:@"2"];
    [ma1 addObject:@"1"];
    [ma1 addObject:@"3"];
    
    NSSortDescriptor *sd1 = [NSSortDescriptor sortDescriptorWithKey:@"self" ascending:NO];
    NSArray *arr1 = [ma1 sortedArrayUsingDescriptors:[NSArray arrayWithObjects:sd1, nil]];
    
    for (NSString *str in arr1) {
        NSLog(@"%@", str);
    }
    
    NSMutableArray *ma2 = [[NSMutableArray alloc] init];
    [ma2 addObject:@"b"];
    [ma2 addObject:@"c"];
    [ma2 addObject:@"a"];    
    
    NSSortDescriptor *sd2 = [NSSortDescriptor sortDescriptorWithKey:@"self" ascending:YES];
    NSArray *arr2 = [ma2 sortedArrayUsingDescriptors:[NSArray arrayWithObjects:sd2, nil]];
    
    for (NSString *str in arr2) {
        NSLog(@"%@", str);
    }
    
    NSMutableArray *ma3 = [[NSMutableArray alloc] init];
    
    NSMutableDictionary *md1 = [[NSMutableDictionary alloc] init];
    [md1 setObject:@"e" forKey:@"name"];
    
    NSMutableDictionary *md2 = [[NSMutableDictionary alloc] init];
    [md2 setObject:@"d" forKey:@"name"];
    
    NSMutableDictionary *md3 = [[NSMutableDictionary alloc] init];
    [md3 setObject:@"f" forKey:@"name"];
    
    [ma3 addObject:md1];
    [ma3 addObject:md2];
    [ma3 addObject:md3];
    
    [md1 release];
    [md2 release];
    [md3 release];    
    
    NSSortDescriptor *sd3 = [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES];
    NSArray *arr3 = [ma3 sortedArrayUsingDescriptors:[NSArray arrayWithObjects:sd3, nil]];
    
    for (NSMutableDictionary *md in arr3) {
        NSLog(@"%@", [md objectForKey:@"name"]);
    }

引用自：http://blog.sina.com.cn/s/blog_8c87ba3b0101phr5.html

http://www.cnblogs.com/zhw511006/archive/2011/06/16/2082787.html

http://blog.csdn.net/zhangkongzhongyun/article/details/7904055