数组内对象根据指定属性排序

参考：https://www.jianshu.com/p/732461f2439a

[arr sortUsingComparator:^NSComparisonResult(id  _Nonnull obj1, id  _Nonnull obj2) {

            BTItemModel *item1 = (BTItemModel *)obj1;

            BTItemModel *item2 = (BTItemModel *)obj2;

            return [item1.rank compare:item2.rank];

}];

错误方式，结果是无序的：

 [arr sortUsingComparator:^NSComparisonResult(id  _Nonnull obj1, id  _Nonnull obj2) {

            BTItemModel *item1 = (BTItemModel *)obj1;

            BTItemModel *item2 = (BTItemModel *)obj2;

            if (item1.rank >  item2.rank) {

                return NSOrderedDescending;

            } else {

                return NSOrderedAscending;

            }

            return 0;

        }];

规范方式：

NSArray *sortedArray = [array sortedArrayUsingComparator: ^(id obj1, id obj2) {

    if ([obj1 integerValue] > [obj2 integerValue]) {
        return (NSComparisonResult)NSOrderedDescending;
    }

    if ([obj1 integerValue] < [obj2 integerValue]) {
        return (NSComparisonResult)NSOrderedAscending;
    }
    return (NSComparisonResult)NSOrderedSame;
}];