一张图理解RACSignal的Subscription过程

通过下面一张图理解RACSignal的调用过程：

RAC

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<RACSubscriber> subscriber))didSubscribe {
    return [RACDynamicSignal createSignal:didSubscribe];
}

+ (RACSignal *)createSignal:(RACDisposable * (^)(id<RACSubscriber> subscriber))didSubscribe {
    RACDynamicSignal *signal = [[self alloc] init];
    signal->_didSubscribe = [didSubscribe copy];
    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 (^)(id x))nextBlock {
    NSCParameterAssert(nextBlock != NULL);
    
    RACSubscriber *o = [RACSubscriber subscriberWithNext:nextBlock error:NULL completed:NULL];
    return [self subscribe:o];
}

+ (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;
}

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<RACSubscriber>)subscriber {
    NSCParameterAssert(subscriber != nil);

    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;
}

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();
    return nil;
}

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.next copy];
        if (nextBlock == nil) return;

        nextBlock(value);
    }
}

- (void)sendError:(NSError *)e {
    @synchronized (self) {
        void (^errorBlock)(NSError *) = [self.error copy];
        [self.disposable dispose];

        if (errorBlock == nil) return;
        errorBlock(e);
    }
}

- (void)sendCompleted {
    @synchronized (self) {
        void (^completedBlock)(void) = [self.completed copy];
        [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