电子商城网站开发项目描述,网站建设的收获体会,怎么能加强门户网站建设,quark搜索引擎入口ReactiveCocoa是一个FRP的思想在Objective-C中的实现框架#xff0c;目前在美团的项目中被广泛使用。对于ReactiveCocoa的基本用法#xff0c;网上有很多相关的资料#xff0c;本文不再讨论。RACSignal是ReactiveCocoa中一个非常重要的概念#xff0c;而本文主要关注RACSig… 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signalreturn [RACSignal createSignal:^RACDisposable *(idRACSubscriber subscriber) {[self.signInServicesignInWithUsername:self.usernameTextField.textpassword:self.passwordTextField.textcomplete:^(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:(idRACSubscriber)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 RACSignalOperation.h中
- (RACMulticastConnection *)publish;- (RACMulticastConnection *)multicast:(RACSubject *)subject;- (RACSignal *)replay;- (RACSignal *)replayLast;- (RACSignal *)replayLazily;在RACSignalOperation.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bugmulticast 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:^(idRACSubscriber subscriber) {AFHTTPRequestOperation *operation [clientHTTPRequestOperationWithRequest:requestsuccess:^(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 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:(idRACSubscriber)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 (idRACSubscriber 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:^(idRACSubscriber 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 nexterrorcompleted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:(idRACSubscriber)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。 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RACSignalOperation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
