RACSignal *signal = [RACSignal createSignal:^RACDisposable * _Nullable(id<RACSubscriber>  _Nonnull subscriber) {
    NSLog(@"-----");
   [subscriber sendNext:@"test"];
   return [RACDisposable disposableWithBlock:^{
       NSLog(@"disposabled");
   }];
}];

RACMulticastConnection *connection = [signal publish];
[connection.signal subscribeNext:^(id  _Nullable x) {
    NSLog(@"1--%@",x);
}];
[connection.signal subscribeNext:^(id  _Nullable x) {
    NSLog(@"2--%@",x);
}];
[connection connect];

打印結(jié)果：
ReactiveCocoa[6757:757221] -----
ReactiveCocoa[6757:757221] 1--test
ReactiveCocoa[6757:757221] 2--test
ReactiveCocoa[6757:757221] disposabled

[signal publish]的實(shí)現(xiàn)：

- (RACMulticastConnection *)publish {
    RACSubject *subject = [[RACSubject subject] setNameWithFormat:@"[%@] -publish", self.name];
    RACMulticastConnection *connection = [self multicast:subject];
    return connection;
}

- (RACMulticastConnection *)multicast:(RACSubject *)subject {
    [subject setNameWithFormat:@"[%@] -multicast: %@", self.name, subject.name];
    RACMulticastConnection *connection = [[RACMulticastConnection alloc] initWithSourceSignal:self subject:subject];
    return connection;
}

- (instancetype)initWithSourceSignal:(RACSignal *)source subject:(RACSubject *)subject {
    NSCParameterAssert(source != nil);
    NSCParameterAssert(subject != nil);

    self = [super init];
    //sourceSignal存放的是最開(kāi)始創(chuàng)建
    _sourceSignal = source;
    _serialDisposable = [[RACSerialDisposable alloc] init];
    // RACMulticastConnection的signal是publish方法創(chuàng)建的subject
    _signal = subject;
    
    return self;
}

signal調(diào)用publish 方法創(chuàng)建了一個(gè)RACSubject對(duì)象，然后將signal和新建的subject作為參數(shù)創(chuàng)建了RACMulticastConnection對(duì)象，connection對(duì)象的兩個(gè)成員變量_sourceSignal和_signal分別接收之前創(chuàng)建的signal和subject對(duì)象。

所以[connection.signal subscribeNext:]本質(zhì)上是調(diào)用RACSubject的subscribeNext:方法將訂閱者添加到屬性subscribers中。

最后查看connect方法的實(shí)現(xiàn)：

- (RACDisposable *)connect {
   BOOL shouldConnect = OSAtomicCompareAndSwap32Barrier(0, 1, &_hasConnected);

   if (shouldConnect) {
       self.serialDisposable.disposable = [self.sourceSignal subscribe:_signal];
   }

   return self.serialDisposable;
}

connect方法實(shí)現(xiàn)就是調(diào)用最初創(chuàng)建的signal的subscribe:方法，而參數(shù)subscriber則是publish方法中創(chuàng)建的subject。所以調(diào)用connect方法會(huì)觸發(fā)signal的block，subscriber會(huì)發(fā)生消息，因?yàn)閟ubscriber是subject對(duì)象，所以會(huì)遍歷subscribers給每個(gè)訂閱者發(fā)送消息，所以只打印了一次'-----'。