RACSignalSequence作为RACSequence的子类，提供了一个方法通过RACSignal初始化一个RACSequence。

完整测试用例在这里。

查看.m中的方法：

+ (RACSequence *)sequenceWithSignal:(RACSignal *)signal {
	RACSignalSequence *seq = [[self alloc] init];

	RACReplaySubject *subject = [RACReplaySubject subject];
	[signal subscribeNext:^(id value) {
		[subject sendNext:value];
	} error:^(NSError *error) {
		[subject sendError:error];
	} completed:^{
		[subject sendCompleted];
	}];

	seq->_subject = subject;
	return seq;
}

方法中初始化RACSignalSequence RACReplaySubject对象，并对参数signal完成了订阅，此时通过RACReplaySubject保存信号的信息。所以，通过seq可以获取signal的所有信息。

测试用例：

- (void)test_sequenceWithSignal
{
    RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        
        [subscriber sendNext:@"1"];
        [subscriber sendCompleted];
        
        return nil;
    }];
    
    RACSequence *sequence = [RACSignalSequence sequenceWithSignal:signal];
    
    NSLog(@"sequenceWithSignal -- %@", sequence);
    
    // 打印日志：
    /*
     2018-08-17 17:08:33.093062+0800 TestRACSignalSequence[50146:18295650] sequenceWithSignal -- <RACSignalSequence: 0x60400023fe80>{ name = , values = (
     1
     ) … }
     */
}

---

- (id)head {
	id value = [self.subject firstOrDefault:self];

	if (value == self) {
		return nil;
	} else {
		return value ?: NSNull.null;
	}
}

通过firstOrDefault:获取subject的第一个值，作为head值。

测试用例：

- (void)test_head
{
    RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        
        [subscriber sendNext:@"1"];
        [subscriber sendCompleted];
        
        return nil;
    }];
    
    RACSequence *sequence = [RACSignalSequence sequenceWithSignal:signal];
    
    NSLog(@"head -- %@", sequence.head);
    
    // 打印日志：
    /*
     2018-08-17 17:10:39.537477+0800 TestRACSignalSequence[50245:18302711] head -- 1
     */
}

---

- (RACSequence *)tail {
	RACSequence *sequence = [self.class sequenceWithSignal:[self.subject skip:1]];
	sequence.name = self.name;
	return sequence;
}

通过skip:1获取信号除去第一个值之后的信号，并通过sequenceWithSignal:生成一个RACSequence对象作为tail值。

测试用例：

- (void)test_tail
{
    RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        
        [subscriber sendNext:@"1"];
        [subscriber sendNext:@"2"];
        [subscriber sendCompleted];
        
        return nil;
    }];
    
    RACSequence *sequence = [RACSignalSequence sequenceWithSignal:signal];
    
    NSLog(@"tail -- %@", sequence.tail);
    
    // 打印日志：
    /*
     2018-08-17 17:12:54.839794+0800 TestRACSignalSequence[50350:18310055] tail -- <RACSignalSequence: 0x60400023c820>{ name = , values = (
     2
     ) … }
     */
}

---

- (NSArray *)array {
	return self.subject.toArray;
}

通过toArray获取到信号值组成的数组。

测试用例：

- (void)test_array
{
    RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        
        [subscriber sendNext:@"1"];
        [subscriber sendNext:@"2"];
        [subscriber sendCompleted];
        
        return nil;
    }];
    
    RACSequence *sequence = [RACSignalSequence sequenceWithSignal:signal];
    
    NSLog(@"array -- %@", sequence.array);
    
    // 打印日志：
    /*
     2018-08-17 17:17:30.637284+0800 TestRACSignalSequence[50521:18323552] array -- (
     1,
     2
     )
     */
}

---

格式化日志方法不再分析。

所以，这个类的功能就是将一个信号转换成一个序列，通过序列的方法获取信号的值。