iOS 使用NSSortDescriptor创建排序规则

比较规则的抽象，描述如何基于所有对象共有的属性对对象集合排序。
@interface NSSortDescriptor : NSObject <NSSecureCoding, NSCopying> {
@private
    NSUInteger _sortDescriptorFlags;
    NSString *_key;
    SEL _selector;
    id _selectorOrBlock;
一、初始化方法
+ (instancetype)sortDescriptorWithKey:(nullable NSString *)key ascending:(BOOL)ascending API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0));
+ (instancetype)sortDescriptorWithKey:(nullable NSString *)key ascending:(BOOL)ascending selector:(nullable SEL)selector API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0));
+ (instancetype)sortDescriptorWithKey:(nullable NSString *)key ascending:(BOOL)ascending comparator:(NSComparator)cmptr API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0));
- (instancetype)initWithKey:(nullable NSString *)key ascending:(BOOL)ascending;
- (instancetype)initWithKey:(nullable NSString *)key ascending:(BOOL)ascending selector:(nullable SEL)selector;
- (instancetype)initWithKey:(nullable NSString *)key ascending:(BOOL)ascending comparator:(NSComparator)cmptr API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0));
key：执行比较的关键路径，即需要比较的属性。
ascending：如果按升序排序，则为YES，否则为NO。
comparator：一个比较规则的block。
selector：一个函数，为比较对象属性时使用的方法。
二、sortedArrayUsingDescriptors: 方法
- (NSArray<ObjectType> *)sortedArrayUsingDescriptors:(NSArray<NSSortDescriptor *> *)sortDescriptors;
参数传入一个NSArray<NSSortDescriptor *>，作为一组排序规则，组内的规则优先级从高到低排列，第一个规则优先级最高。
三、简单示例
举例一个学生类，学生有属性年龄，姓名，考试成绩，对其进行排序
Student *s1 = [[Student alloc] initWithName:@"zhangsna" age:@"19" score:@"90"];
Student *s2 = [[Student alloc] initWithName:@"lisi" age:@"20" score:@"87"];
Student *s3 = [[Student alloc] initWithName:@"wangwu" age:@"21" score:@"93"];
NSArray *student = @[s1, s2, s3];
NSSortDescriptor *age = [NSSortDescriptor sortDescriptorWithKey:@"age" ascending:YES];
NSSortDescriptor *score=[NSSortDescriptor sortDescriptorWithKey:@"score" ascending:YES];
NSArray *descriptors = [NSArray arrayWithObjects: age, score]; //学生先按关键字age(年龄)再按score(成绩)排序
student = [[student sortedArrayUsingDescriptors: descriptors];
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