RACSignal创建流程

 //1、创建信号
    RACSignal * signal = [RACSignal createSignal:^RACDisposable * _Nullable(id<RACSubscriber>  _Nonnull subscriber) {
        //3、发送信号
        [subscriber sendNext:@"LGCooci"];
        return nil;
    }];
   
    //2、订阅信号
    [signal subscribeNext:^(id  _Nullable x) {
        NSLog(@"订阅到了%@",x);
    }];

• 创建信号 - createSignal()
• 订阅信号 - subscribeNext()
• 发送信号 - sendNext() sendCompleted() sendError()

核心逻辑源码分析

创建信号

+ (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:"];
}

1. createSignal 带入一个block:didSubscribe
2. RACDynamicSignal创建信号，并保存block：_didSubscribe

订阅信号

- (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;
}

1. RACSubscriber创建订阅者o, 传入block:nextBlock
2. RACSubscriber内部保存blcok:nextBlock的_next,_error,_completed
3. 调用[self subscribe:o],此时的self即为：RACDynamicSignal对象

进入RACDynamicSignal的subscribe内部：

- (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;
}

4. RACPassthroughSubscriber(接受了RACSubscriber协议)传入subscriber重新初始化了新的subscriber
5. 调用self.didSubscribe(subscriber), 这里的didSubscribe即为创建信号时候保存的block, 这里就直接就把subscriber回调了回去

发送信号

//3、发布信号
[subscriber sendNext:@"LGCooci"];
[subscriber sendCompleted];

- (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();
    }
}

1. 这里的subscriber即为订阅信号调用didSubscribe回调传出来的RACPassthroughSubscriber对象, 当调用sendNext,sendError,sendCompleted的时候，就会通过保存的.next, .error, .completed block进行回调，最后就完成了整个流程