关于NSSortDescriptor

NSSortDescriptor由下述参数组成 ：
键：对于一个给定的集合，对应值的键位将对集合中的每个对象进行排序。
升序：指定一个集合是否按照升序（YES）还是降序（NO）进行排序的布尔值。

另外NSSortDescriptor还有一个涉及到排序的值之间的比较的第三个可选参数。默认情况下，这是一个简单的相等性检查，但它的行为可以通过传递一个选择器（SEL）或者比较器（NSComparator）而发生改变。

任何时候当你在为面向用户的字符串排序时，一定要加入localizedStandardCompare:选择器，它将根据当前语言环境的语言规则进行排序（语言环境可能会根据大小写，变音符号等等的顺序而发生改变）。

有的集合（比如NSArray和NSSet）有以sortDescriptors作为参数，可以返回排过序的数组的方法。排序描述符按顺序应用，所以如果两个元素碰巧被捆绑在同一个特定的排序标准，束缚将被后续的任意描述符所打破。

NSSortDescriptor在Foundation和其他系统框架中随处可见，它在Core Data中发挥着极其重要的作用。如果你自己的类需要定义排序顺序，请按照惯例合理设置sortDescriptors参数。

因为，在现实中，你应该以商业逻辑来思考它，而不是用数学公式或者映射化简函数来考量它。在这一方面，NSSortDescriptor是一个大灌篮，以至于每当你走出Objective-C和Cocoa的世界时都会思念它。

//Person.h
#import<Foundation/Foundation.h>

@interface Person : NSObject
@property(strong) NSString *firstName;
@property(strong) NSString *lastName;
@property(assign) int age;

- (id)initWithFirstName:(NSSrting *)fName lastName:(NSString *)lName andAge:(int)a;
- (void)reportState;

@end

//Person.m
#import "Person.h"

@implementation Person
@synthesize firstName, lastName, age;

- (id)initWithFirstName:(NSString *)fName lastName:(NSString *)lName andAge:(int)a
{
	self = [super init];
	if(self){
		self.firstName = fName;
		self.lastName  = lName;
		self.age       = a;
	}
	return self;
}

- (void)reportState
{
	NSLog(@"This person's name is %@ %@ who is %i years old", 

firstName,lastName,age);
}

@end 

#import <Foundation/Foundation.h>
#import "Person.h"

int main (int argc, const char * argv[])
{
	@autoreleasepool {
		Person *p1 = [[Person alloc] initWithFirstName:@"Rebecca" lastName:@"Smith" andAge:33];
		Person *p2 = [[Person alloc] initWithFirstName:@"Albert" lastName:@"Case" andAge:24];
		Person *p3 = [[Person alloc] initWithFirstName:@"Anton" lastName:@"Belfey" andAge:45];
		Person *p4 = [[Person alloc] initWithFirstName:@"Tom" lastName:@"Gun" andAge:17];
		Person *p5 = [[Person alloc] initWithFirstName:@"Cindy" lastName:@"Lou" andAge:6];
		Person *p6 = [[Person alloc] initWithFirstName:@"Yanno" lastName:@"Dirst" andAge:76];

		NSArray *listOfObjects = [NSArray arrayWithObjects:p1, p2, p3, p4, p5, p6, nil];

		NSLog(@"PRINT OUT ARRAY UNSORTED");
		[listOfObjects makeObjectsPerformSelector:@selector(reportState)];

		NSSortDescriptor *sd1 = [NSSortDescriptor sortDescriptorWithKey:@"age" ascending:YES];
		NSSortDescriptor *sd2 = [NSSortDescriptor sortDescriptorWithKey:@"lastName" ascending:YES];
		NSSortDescriptor *sd3 = [NSSortDescriptor sortDescriptorWithKey:@"firstName" ascending:NO];

		NSArray *sdArray1 = [NSArray arrayWithObjects:sd1, sd2, sd3, nil];

		NSLog(@"PRINT OUT SORTED ARRAY (AGE, LASTNAME, FIRSTNAME)");
		NSArray *sortedArray1 = [listOfObjects sortedArrayUsingDescriptors:sdArray1];

		[sortedArray1 makeObjectsPerformSelector:@selector(reportState)];
		NSArray *sdArray2 = [NSArray arrayWithObjects:sd2, sd1, sd3, nil];
		NSArray *sortedArray2 = [listOfObjects sortedArrayUsingDescriptors:sdArray2];
		NSLog(@"PRINT OUT SORTED ARRAY (LASTNAME, FIRSTNAME, AGE)");
		[sortedArray2 makeObjectsPerformSelector:@selector(reportState)];
	}
	return 0;
}