ReactiveCocoa 是一个FRP的思想在Objective-C中的实现框架，目前在美团的项目中被广泛使用。对于ReactiveCocoa的基本用法，网上有很多相关的资料，本文不再讨论。RACSignal是ReactiveCocoa中一个非常重要的概念，而本文主要关注RACSignal的实现原理。在阅读之前，你需要基本掌握 RACSignal的基本用法
本文主要包含2个部分，前半部分主要分析RACSignal的subscription过程，后半部分是对前半部分的深入，在subscription过程的基础上分析ReactiveCocoa中比较难理解的两个操作：multicast && replay。
PS：为了解释清楚，我们下面只讨论next，不讨论error以及completed，这二者与next类似。本文基于ReactiveCocoa 2.x版本。
我们先刨析RACSignal的subscription过程
RACSignal的常见用法
-(RACSignal *)signInSignal { // part 1:[RACSignal createSignal]来获得signal return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) { [self.signInService signInWithUsername:self.usernameTextField.text password:self.passwordTextField.text complete:^(BOOL success) { // part 3: 进入didSubscribe，通过[subscriber sendNext:]来执行next block [subscriber sendNext:@(success)]; [subscriber sendCompleted]; }]; return nil; }]; } // part 2 : [signal subscribeNext:]来获得subscriber，然后进行subscription [[self signInSignal] subscribeNext:^(id x) { NSLog(@"Sign in result: %@", x); }]; Subscription过程概括
RACSignal的Subscription过程概括起来可以分为三个步骤：
- [RACSignal createSignal]来获得signal
- [signal subscribeNext:]来获得subscriber，然后进行subscription
- 进入didSubscribe，通过[subscriber sendNext:]来执行next block
步骤一：[RACSignal createSignal]来获得signal
RACSignal.m中： + ( RACSignal *)createSignal:( RACDisposable * (^)( id < RACSubscriber > subscriber))didSubscribe { return [ RACDynamicSignal createSignal :didSubscribe]; } RACDynamicSignal.m中 + ( RACSignal *)createSignal:( RACDisposable * (^)( id < RACSubscriber > subscriber))didSubscribe { RACDynamicSignal *signal = [[ self alloc ] init ]; signal-> _didSubscribe = [didSubscribe copy ]; return [signal setNameWithFormat : @"+createSignal:" ]; }[RACSignal createSignal]会调用子类RACDynamicSignal的createSignal来返回一个signal，并在signal中保存后面的 didSubscribe这个block
步骤二：[signal subscribeNext:]来获得subscriber，然后进行subscription
RACSignal.m中： - ( RACDisposable *)subscribeNext:( void (^)( id x))nextBlock { RACSubscriber *o = [ RACSubscriber subscriberWithNext :nextBlock error : NULL completed : NULL ]; return [ self subscribe :o]; } RACSubscriber.m中： + ( instancetype )subscriberWithNext:( void (^)( id x))next error:( void (^)( NSError *error))error completed:( void (^)( void ))completed { RACSubscriber *subscriber = [[ self alloc ] init ]; subscriber-> _next = [next copy ]; subscriber-> _error = [error copy ]; subscriber-> _completed = [completed copy ]; return subscriber; } RACDynamicSignal.m中： - (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber { RACCompoundDisposable *disposable = [RACCompoundDisposable compoundDisposable]; subscriber = [[RACPassthroughSubscriber alloc] initWithSubscriber:subscriber signal:self disposable:disposable]; if (self.didSubscribe != NULL) { RACDisposable *schedulingDisposable = [RACScheduler.subscriptionScheduler schedule:^{ RACDisposable *innerDisposable = self.didSubscribe(subscriber); [disposable addDisposable:innerDisposable]; }]; [disposable addDisposable:schedulingDisposable]; } return disposable; } - [signal subscribeNext]先会获得一个subscriber，这个subscriber中保存了nextBlock、errorBlock、completedBlock
- 由于这个signal其实是RACDynamicSignal类型的，这个[self subscribe]方法会调用步骤一中保存的didSubscribe，参数就是1中的subscriber
步骤三：进入didSubscribe，通过[subscriber sendNext:]来执行next block
RACSubscriber.m中： - (void)sendNext:(id)value { @synchronized (self) { void (^nextBlock)(id) = [self.next copy]; if (nextBlock == nil) return; nextBlock(value); } }任何时候这个[subscriber sendNext:],就直接调用nextBlock
signal的subscription过程回顾
从上面的三个步骤，我们看出：
- 先通过createSignal和subscribeNext这两个调用，
声明了流中value到来时的处理方式 - didSubscribe block块中异步处理完毕之后，subscriber进行sendNext，
自动处理
搞清楚了RAC的subscription过程，接着在此基础上我们讨论一个RACSignal中比较容易混淆的两个操作：multicast和replay。
为什么要清楚这两者的原理
RACSignal+Operation.h中 - (RACMulticastConnection *)publish; - (RACMulticastConnection *)multicast:(RACSubject *)subject; - (RACSignal *)replay; - (RACSignal *)replayLast; - (RACSignal *)replayLazily;- 在RACSignal+Operation.h中，连续定义了5个跟我们这个主题有关的RACSignal的操作，这几个操作的区别很细微，但用错的话很容易出问题。只有理解了原理之后，才明白它们之间的细微区别
- 很多时候我们意识不到需要用这些操作，这就可能因为side effects执行多次而导致程序bug
multicast && replay的应用场景
"Side effects occur for each subscription by default, but there are certain situations where side effects should only occur once – for example, a network request typically should not be repeated when a new subscriber is added."
// 引用ReactiveCocoa源码的Documentation目录下的一个例子 // This signal starts a new request on each subscription. RACSignal *networkRequest = [RACSignal createSignal:^(id<RACSubscriber> subscriber) { AFHTTPRequestOperation *operation = [client HTTPRequestOperationWithRequest:request success:^(AFHTTPRequestOperation *operation, id response) { [subscriber sendNext:response]; [subscriber sendCompleted]; } failure:^(AFHTTPRequestOperation *operation, NSError *error) { [subscriber sendError:error]; }]; [client enqueueHTTPRequestOperation:operation]; return [RACDisposable disposableWithBlock:^{ [operation cancel]; }]; }]; // Starts a single request, no matter how many subscriptions `connection.signal` // gets. This is equivalent to the -replay operator, or similar to // +startEagerlyWithScheduler:block:. RACMulticastConnection *connection = [networkRequest multicast:[RACReplaySubject subject]]; [connection connect]; [connection.signal subscribeNext:^(id response) { NSLog(@"subscriber one: %@", response); }]; [connection.signal subscribeNext:^(id response) { NSLog(@"subscriber two: %@", response); }]; - 在上面的例子中，如果我们不用RACMulticastConnection的话，那就会因为执行了两次subscription而导致发了两次网络请求。
- 从上面的例子中，我们可以看到对一个Signal进行multicast之后，我们是对connection.signal进行subscription而不是原来的networkRequest。这点是"side effects should only occur once"的关键，我们将在后面解释
multicast原理分析
replay是multicast的一个特殊case而已，而multicast的整个过程可以拆分成两个步骤，下面进行详细讨论
multicast的机制Part 1:
RACMulticastConnection.m中： - (id)initWithSourceSignal:(RACSignal *)source subject:(RACSubject *)subject { NSCParameterAssert(source != nil); NSCParameterAssert(subject != nil); self = [super init]; if (self == nil) return nil; _sourceSignal = source; _serialDisposable = [[RACSerialDisposable alloc] init]; _signal = subject; return self; } - 结合上面的例子来看，RACMulticastConnection的init是以networkRequest作为sourceSignal，而最终connnection.signal指的是[RACReplaySubject subject]
RACMulticastConnection.m中： - (RACDisposable *)connect { BOOL shouldConnect = OSAtomicCompareAndSwap32Barrier(0, 1, &_hasConnected); if (shouldConnect) { self.serialDisposable.disposable = [self.sourceSignal subscribe:_signal]; } return self.serialDisposable; }- 结合上面的RACSignal分析的Subscription过程，[self.sourceSignal subscribe:_signal]会执行self.sourceSignal的didSubscribe这个block。再结合上面的例子，也就是说会把_signal作为subscriber，发网络请求，success的时候，_signal会sendNext，这里的这个signal就是[RACReplaySubject subject]。可以看出，一旦进入到这个didSubscribe中，后续的不管是sendNext还是subscription，都是对这个[RACReplaySubject subject]进行的，与原来的sourceSignal彻底无关了。这就解释了为什么"side effects only occur once"。
multicast的机制Part 2:
在进行multicast的步骤二之前，需要介绍一下RACSubject以及RACReplaySubject
---------------------恼人的分隔线 start------------------
RACSubject
"A subject can be thought of as a signal that you can manually control by sending next, completed, and error."
RACSubject的一个用法如下：
RACSubject *letters = [RACSubject subject]; // Outputs: A B [letters subscribeNext:^(id x) { NSLog(@"%@ ", x); }]; [letters sendNext:@"A"]; [letters sendNext:@"B"];接下来分析RACSubject的原理
RACSubject.m中： - (id)init { self = [super init]; if (self == nil) return nil; _disposable = [RACCompoundDisposable compoundDisposable]; _subscribers = [[NSMutableArray alloc] initWithCapacity:1]; return self; } - RACSubject中有一个subscribers数组
RACSubject.m中： - (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber { NSCParameterAssert(subscriber != nil); RACCompoundDisposable *disposable = [RACCompoundDisposable compoundDisposable]; subscriber = [[RACPassthroughSubscriber alloc] initWithSubscriber:subscriber signal:self disposable:disposable]; NSMutableArray *subscribers = self.subscribers; @synchronized (subscribers) { [subscribers addObject:subscriber]; } return [RACDisposable disposableWithBlock:^{ @synchronized (subscribers) { // Since newer subscribers are generally shorter-lived, search // starting from the end of the list. NSUInteger index = [subscribers indexOfObjectWithOptions:NSEnumerationReverse passingTest:^ BOOL (id<RACSubscriber> obj, NSUInteger index, BOOL *stop) { return obj == subscriber; }]; if (index != NSNotFound) [subscribers removeObjectAtIndex:index]; } }]; } - 从subscribe:的实现可以看出，对RACSubject对象的每次subscription，都是将这个subscriber加到subscribers数组中而已
RACSubject.m中： - (void)sendNext:(id)value { [self enumerateSubscribersUsingBlock:^(id<RACSubscriber> subscriber) { [subscriber sendNext:value]; }]; }- 从sendNext:的实现可以看出，每次RACSubject对象sendNext，都会对其中保留的subscribers进行sendNext,如果这个subscriber是RACSignal的话，就会执行Signal的next block。
RACReplaySubject
"A replay subject saves the values it is sent (up to its defined capacity) and resends those to new subscribers."，可以看出，replaySubject是可以对它send next（error，completed）的东西进行buffer的。RACReplaySubject是继承自RACSubject的，它的内部的实现例如subscribe:、sendNext:的实现也会调用super的实现
RACReplaySubject.m中： - (instancetype)initWithCapacity:(NSUInteger)capacity { self = [super init]; if (self == nil) return nil; _capacity = capacity; _valuesReceived = (capacity == RACReplaySubjectUnlimitedCapacity ? [NSMutableArray array] : [NSMutableArray arrayWithCapacity:capacity]); return self; } - 从init中我们看出，RACReplaySubject对象持有capacity变量（用于决定valuesReceived缓存多少个sendNext：出来的value，这在区分replay和replayLast的时候特别有用）以及valuesReceived数组（用来保存sendNext:出来的value），这二者接下来会重点涉及到
RACReplaySubject.m中： - (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber { RACCompoundDisposable *compoundDisposable = [RACCompoundDisposable compoundDisposable]; RACDisposable *schedulingDisposable = [RACScheduler.subscriptionScheduler schedule:^{ @synchronized (self) { for (id value in self.valuesReceived) { if (compoundDisposable.disposed) return; [subscriber sendNext:(value == RACTupleNil.tupleNil ? nil : value)]; } if (compoundDisposable.disposed) return; if (self.hasCompleted) { [subscriber sendCompleted]; } else if (self.hasError) { [subscriber sendError:self.error]; } else { RACDisposable *subscriptionDisposable = [super subscribe:subscriber]; [compoundDisposable addDisposable:subscriptionDisposable]; } } }]; [compoundDisposable addDisposable:schedulingDisposable]; return compoundDisposable; } - 从subscribe:可以看出，RACReplaySubject对象每次subscription，都会把之前valuesReceived中buffer的value重新sendNext一遍，然后调用super把当前的subscriber加入到subscribers数组中
RACReplaySubject.m中： - (void)sendNext:(id)value { @synchronized (self) { [self.valuesReceived addObject:value ?: RACTupleNil.tupleNil]; [super sendNext:value]; if (self.capacity != RACReplaySubjectUnlimitedCapacity && self.valuesReceived.count > self.capacity) { [self.valuesReceived removeObjectsInRange:NSMakeRange(0, self.valuesReceived.count - self.capacity)]; } } } 从sendNext:可以看出，RACReplaySubject对象会buffer每次sendNext的value，然后会调用super，对subscribers中的每个subscriber，调用sendNext。buffer的数量是根据self.capacity来决定的
---------------------恼人的分隔线 end------------------
介绍完了RACReplaySubject之后，我们继续进行multicast的part 2部分。在上面的例子中，我们对connection.signal进行了两次subscription，结合上面的RACReplaySubject的subscription的subscribe:，我们得到以下过程：
- [RACReplaySubject subject]会将这两次subscription过程中的subscriber都保存在subscribers数组中
- 当网络请求success后，会[subscriber sendNext:response]，前面已经讲过这个subscriber就是[RACReplaySubject subject]，这样，就会把sendNext：的value保存在valuesReceived数组中，供后续subscription使用（不知道你是否注意到RACReplaySubject的subscribe：中有个for循环），然后对subscribers中保存的每个subscriber执行sendNext。
后续思考
- 上面讨论的是RACReplaySubject对象先进行subscription，再进行sendNext，如果是先sendNext，再subscription呢？其实魅力就在于RACReplaySubject的subscribe：中的for循环。具体过程留作思考
- 在RACSignal+Operation中关于multicast && replay的，一共有5个操作：publish、multicast、replay、replayLast、replayLazily，他们之间有什么细微的差别呢？相信在我上面内容的基础上，他们之间的细微差别不难理解，这里推荐一篇帮助大家理解的 blog
![[HBLOG]公众号](http://www.liuhaihua.cn/img/qrcode_gzh.jpg)
