RACMulticastConnection: NSObject

RACMulticastConnection用于信号的一对多传播

@property (nonatomic, strong, readonly) RACSignal<ValueType> *signal;
{
RACSubject *_signal;
}
@property (nonatomic, readonly, strong) RACSignal *sourceSignal;

这个signal属性即为对外开发的信号，其实现是使用RACSubject对象，而sourceSignal即为订阅的原信号

在初始化方法中初始化这两个对象:

- (instancetype)initWithSourceSignal:(RACSignal *)source subject:(RACSubject *)subject { 
    _sourceSignal = source;
    _signal = subject;
}

使用

我们不应该直接初始化RACMulticastConnection对象 使用RACSignal的实例化方法初始化这个实例对象:

- (RACMulticastConnection *)publish {
    RACSubject *subject = [RACSubject subject];
    RACMulticastConnection *connection = [self multicast:subject];
    return connection;
}

- (RACMulticastConnection *)multicast:(RACSubject *)subject {
    RACMulticastConnection *connection = [[RACMulticastConnection alloc] initWithSourceSignal:self subject:subject];
    return connection;
}

可以很简单的看出来这两个方法只是简单的声明了RACSubject对象，并初始化一个RACMulticastConnection对象并返回

connect && autoconnect

具体信号的订阅是在connect方法中，只有执行了connect方法 才会订阅该信号 才能接收信号

- (RACDisposable *)connect {
    self.serialDisposable.disposable = [self.sourceSignal subscribe:_signal];
}

autoconnect方法返回一个信号 当订阅了该信号时就会执行connect方法 订阅sourceSignal从而收到信号值

- (RACSignal *)autoconnect {

    return [RACSignal
        createSignal:^(id<RACSubscriber> subscriber) {
            RACDisposable *subscriptionDisposable = [self.signal subscribe:subscriber];
            [self connect];
            }];
        }]
}

补充

当使用publish方法返回的RACMulticastConnection对象时，只有当connect之前订阅的才能收到消息，而在connect之后订阅的对象并不能收到消息

因此此时我们可以使用RACReplaySubject对象，可以通过[RACSignal replay]方法实现

- (RACSignal *)replay {
    RACReplaySubject *subject = [RACReplaySubject subject];
    RACMulticastConnection *connection = [self multicast:subject];
    [connection connect];
    return connection.signal;
}

除此之外 还有

Capacity:1 的ReplaySubject

- (RACSignal *)replayLast {
    RACReplaySubject *subject = [RACReplaySubject replaySubjectWithCapacity:1];
    RACMulticastConnection *connection = [self multicast:subject];
    [connection connect];
    return connection.signal;
}

只有在返回的信号在被第一次订阅时 才会执行connect方法

- (RACSignal *)replayLazily {
    RACMulticastConnection *connection = [self multicast:[RACReplaySubject subject]];
    return [RACSignal
        defer:^{
            [connection connect];
            return connection.signal;
        }];
}