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https://www.cnblogs.com/wanyakun/p/6472752.html

通过下面一张图理解RACSignal的调用过程：

创建signale

RACSignal通过子类[RACDynamicSignal createSignal:]方法获得Signal，并将disSubscribe这个block保存在Signal中。

+ (RACSignal *)createSignal:(RACDisposable * (^)(id subscriber))didSubscribe {return[RACDynamicSignal createSignal:didSubscribe];}

+ (RACSignal *)createSignal:(RACDisposable * (^)(id subscriber))didSubscribe {    RACDynamicSignal *signal = [[selfalloc] init];    signal->_didSubscribe = [didSubscribecopy];return[signal setNameWithFormat:@"+createSignal:"];}

创建subscriber

signal通过调用subscribeNext方法生成subscriber，并将next、error、completed block保存在subscriber中

- (RACDisposable *)subscribeNext:(void(^)(idx))nextBlock {NSCParameterAssert(nextBlock !=NULL);        RACSubscriber *o = [RACSubscriber subscriberWithNext:nextBlock error:NULLcompleted:NULL];return[selfsubscribe:o];}

+ (instancetype)subscriberWithNext:(void(^)(idx))next error:(void(^)(NSError*error))error completed:(void(^)(void))completed {    RACSubscriber *subscriber = [[selfalloc] init];    subscriber->_next = [nextcopy];    subscriber->_error = [errorcopy];    subscriber->_completed = [completedcopy];returnsubscriber;}

进行subscribe

第二步创建subscriber之后调用signal的subscribe方法，并将创建的subscriber作为参数。

这一步会生成RACCompoundDisposable和RACPassthroughSubscriber对象。

RACCompoundDisposable：RACDisposable的子类，可以加入多个RACDisposable对象。当RACCompoundDisposable对象被dispose的时候，会dispose容器内的所有RACDisposable对象。

RACPassthroughSubscriber：分别保存对RACSignal，RACSubscriber，RACCompoundDisposable的引用。通过RACPassthroughSubscriber对象来转发给真正的Subscriber。

- (RACDisposable *)subscribe:(id)subscriber {NSCParameterAssert(subscriber !=nil);    RACCompoundDisposable *disposable = [RACCompoundDisposable compoundDisposable];    subscriber = [[RACPassthroughSubscriber alloc] initWithSubscriber:subscriber signal:selfdisposable:disposable];if(self.didSubscribe !=NULL) {        RACDisposable *schedulingDisposable = [RACScheduler.subscriptionScheduler schedule:^{            RACDisposable *innerDisposable =self.didSubscribe(subscriber);            [disposable addDisposable:innerDisposable];        }];        [disposable addDisposable:schedulingDisposable];    }returndisposable;}

执行disSubscribe block

RACSignal通过RACScheduler.subscriptionScheduler来执行闭包，disSubscribe真正被调用的的位置就是上一步的RACDisposable *innerDisposable = self.didSubscribe(subscriber);

- (RACDisposable *)schedule:(void(^)(void))block {NSCParameterAssert(block !=NULL);if(RACScheduler.currentScheduler ==nil)return[self.backgroundScheduler schedule:block];    block();returnnil;}

调用sendNext sendError sendCompleted

进入didSubscribe闭包后，调用sendNext:、sendError:、sendCompleted。由于第三步中将subscriber替换为RACPassthroughSubscriber对象，真正的subscriber被存储在RACPassthroughSubscriber对象中，即innerSubscriber，所以这一步的各种send方法其实是一个转发过程。

- (void)sendNext:(id)value {if(self.disposable.disposed)return;if(RACSIGNAL_NEXT_ENABLED()) {        RACSIGNAL_NEXT(cleanedSignalDescription(self.signal), cleanedDTraceString(self.innerSubscriber.description), cleanedDTraceString([value description]));    }    [self.innerSubscriber sendNext:value];}- (void)sendError:(NSError*)error {if(self.disposable.disposed)return;if(RACSIGNAL_ERROR_ENABLED()) {        RACSIGNAL_ERROR(cleanedSignalDescription(self.signal), cleanedDTraceString(self.innerSubscriber.description), cleanedDTraceString(error.description));    }    [self.innerSubscriber sendError:error];}- (void)sendCompleted {if(self.disposable.disposed)return;if(RACSIGNAL_COMPLETED_ENABLED()) {        RACSIGNAL_COMPLETED(cleanedSignalDescription(self.signal), cleanedDTraceString(self.innerSubscriber.description));    }    [self.innerSubscriber sendCompleted];}

执行next error completed闭包

通过调用innerSubscriber的sendNext:、sendError、和sendCompleted方法执行真正的subscriber中的next error completed闭包

- (void)sendNext:(id)value {@synchronized(self) {void(^nextBlock)(id) = [self.nextcopy];if(nextBlock ==nil)return;        nextBlock(value);    }}- (void)sendError:(NSError*)e {@synchronized(self) {void(^errorBlock)(NSError*) = [self.errorcopy];        [self.disposable dispose];if(errorBlock ==nil)return;        errorBlock(e);    }}- (void)sendCompleted {@synchronized(self) {void(^completedBlock)(void) = [self.completedcopy];        [self.disposable dispose];if(completedBlock ==nil)return;        completedBlock();    }}

过程回顾

去掉中间的繁杂细节，大致过程如下：

1.通过createSignal生成信号

2.通过subscribeNext确定信号内容到来时的处理方式

3.didSubscribe block块中异步处理完毕之后，进行sendNext、sendError和sendCompleted自动处理