看源码

/*
 These constants are used to indicate how items in a request are ordered, from the first one given in a method invocation or function call to the last (that is, left to right in code).
 
 Given the function:
   NSComparisonResult f(int a, int b)
 
 If:
    a < b   then return NSOrderedAscending. The left operand is smaller than the right operand.
    a > b   then return NSOrderedDescending. The left operand is greater than the right operand.
    a == b  then return NSOrderedSame. The operands are equal.
*/
// 注意！！ 这里的升序降序指的是 按照 index 升序降序
typedef NS_CLOSED_ENUM(NSInteger, NSComparisonResult) {
    NSOrderedAscending = -1L,// 升序排列
    NSOrderedSame,// 相等
    NSOrderedDescending// 降序排列
};

typedef NSComparisonResult (^NSComparator)(id obj1, id obj2);