RACIndexSetSequence作为RACSequence的子类，通过名字可以知道该类与NSIndexSet类有关系。

如果对NSIndexSet不了解的，可以先看下这篇文章。

完整测试用例在这里。

接下来对.m中的方法进行分析。

+ (instancetype)sequenceWithIndexSet:(NSIndexSet *)indexSet {
    NSUInteger count = indexSet.count;

    if (count == 0) return self.empty;

    NSUInteger sizeInBytes = sizeof(NSUInteger) * count;

    NSMutableData *data = [[NSMutableData alloc] initWithCapacity:sizeInBytes];
    [indexSet getIndexes:data.mutableBytes maxCount:count inIndexRange:NULL];

    RACIndexSetSequence *seq = [[self alloc] init];
    seq->_data = data;
    seq->_indexes = data.bytes;
    seq->_count = count;
    return seq;
}

初始化方法，根据indexSet的count来判断返回空序列还是indexSet序列。如果返回indexSet序列，通过_indexes保存indexSet的信息，完成初始化工作。

测试用例：

- (void)test_sequenceWithIndexSet
{
    RACIndexSetSequence *sequence = [RACIndexSetSequence sequenceWithIndexSet:[NSIndexSet indexSetWithIndex:1]];
    NSLog(@"sequenceWithIndexSet -- %@", sequence);
    NSLog(@"sequenceWithIndexSet -- %@", sequence.head);
    NSLog(@"sequenceWithIndexSet -- %@", sequence.tail.head);

    // 打印日志；
    /*
     2018-08-16 17:58:09.695653+0800 TestRACIndexSetSequence[18761:17127607] sequenceWithIndexSet -- <RACIndexSetSequence: 0x604000250f50>{ name = , indexes = 1 }
     2018-08-16 17:58:09.696250+0800 TestRACIndexSetSequence[18761:17127607] sequenceWithIndexSet -- 1
     2018-08-16 17:58:09.696531+0800 TestRACIndexSetSequence[18761:17127607] sequenceWithIndexSet -- (null)
     */
}

+ (instancetype)sequenceWithIndexSetSequence:(RACIndexSetSequence *)indexSetSequence offset:(NSUInteger)offset {
    NSCParameterAssert(offset < indexSetSequence.count);

    RACIndexSetSequence *seq = [[self alloc] init];
    seq->_data = indexSetSequence.data;
    seq->_indexes = indexSetSequence.indexes + offset;
    seq->_count = indexSetSequence.count - offset;
    return seq;
}

也是一个初始化方法，通过参数indexSetSequence offset获取一个新的序列。

测试用例：

- (void)test_sequenceWithIndexSetSequence
{
    NSIndexSet *set = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, 3)];
    RACIndexSetSequence *sequence1 = [RACIndexSetSequence sequenceWithIndexSet:set];
    RACIndexSetSequence *sequence2 = [RACIndexSetSequence sequenceWithIndexSetSequence:sequence1 offset:2];
    NSLog(@"sequenceWithIndexSetSequence -- %@ -- %@ -- %@", sequence1, sequence1.head, sequence1.tail);
    NSLog(@"sequenceWithIndexSetSequence -- %@ -- %@ -- %@", sequence2, sequence2.head, sequence2.tail);

    // 打印日志：
    /*
     2018-08-16 18:03:26.545363+0800 TestRACIndexSetSequence[18992:17143794] sequenceWithIndexSetSequence -- <RACIndexSetSequence: 0x60400044d350>{ name = , indexes = 0, 1, 2 } -- 0 -- <RACIndexSetSequence: 0x60400044d260>{ name = , indexes = 1, 2 }
     2018-08-16 18:03:26.547246+0800 TestRACIndexSetSequence[18992:17143794] sequenceWithIndexSetSequence -- <RACIndexSetSequence: 0x60400044d200>{ name = , indexes = 2 } -- 2 -- <RACEmptySequence: 0x60400001e7c0>{ name =  }

     */
}

- (id)head {
    return @(self.indexes[0]);
}

由于indexes存放着indexSet的索引信息，然后通过0获取到第一个索引值。

测试用例：

- (void)test_head
{
    NSIndexSet *set = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, 3)];
    RACIndexSetSequence *sequence = [RACIndexSetSequence sequenceWithIndexSet:set];
    NSLog(@"head -- %@ -- %@", sequence, sequence.head);

    // 打印日志：
    /*
     2018-08-16 18:05:13.159825+0800 TestRACIndexSetSequence[19078:17149555] head -- <RACIndexSetSequence: 0x604000255150>{ name = , indexes = 0, 1, 2 } -- 0
     */
}

- (RACSequence *)tail {
    if (self.count <= 1) return [RACSequence empty];

    return [self.class sequenceWithIndexSetSequence:self offset:1];
}

如果索引值最多只有一个，就返回一个空序列；否则调用sequenceWithIndexSetSequence:offset:并指定offset为1。

测试用例：

- (void)test_tail
{
    NSIndexSet *set = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, 3)];
    RACIndexSetSequence *sequence = [RACIndexSetSequence sequenceWithIndexSet:set];
    NSLog(@"tail -- %@ -- %@", sequence, sequence.tail);

    // 打印日志：
    /*
     2018-08-16 18:06:40.614784+0800 TestRACIndexSetSequence[19156:17154407] tail -- <RACIndexSetSequence: 0x604000256cb0>{ name = , indexes = 0, 1, 2 } -- <RACIndexSetSequence: 0x604000257460>{ name = , indexes = 1, 2 }
     */
}

关于 遍历、格式化日志的方法不再分析。

综上，这个类的作用是将NSIndexSet存储的索引值可以用序列的方法处理。