RACSignal的Subscription深入

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

参考资料

ReactiveCocoa github主页
ReactiveCocoa Documentation
ReactiveCocoa raywenderlich上的资料

转载自：http://tech.meituan.com/RACSignalSubscription.html