Signal

RAC的核心是signal，RAC世界所有的消息都是通过信号的方式传递的。

• 创建信号

RACSignal *signal = [RACSignal createSignal:^RACDisposable * _Nullable(id<RACSubscriber>  _Nonnull subscriber) {
        [subscriber sendNext:@1];
        [subscriber sendNext:@2];
        [subscriber sendCompleted];
        return nil;
    }];

它的内部实现是创建一个RACDynamicSignal，同时将创建信号的block传递给RACDynamicSignal实例。

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

• 订阅信号
  信号创建之后如果不被订阅，block不会被执行

[signal subscribeNext:^(id  _Nullable x) {
        NSLog(@"signal: %@", x);
    }];

订阅的内部实现其实也是函数式编程思想，block传递

• 创建订阅者

RACSubscriber *o = [RACSubscriber subscriberWithNext:nextBlock error:NULL completed:NULL];
[self subscribe:o];

• 执行创建信号时的block

RACDisposable *schedulingDisposable = [RACScheduler.subscriptionScheduler schedule:^{
            RACDisposable *innerDisposable = self.didSubscribe(subscriber);
            [disposable addDisposable:innerDisposable];
        }];

Bind

信号的绑定是个相对复杂的操作，其内部是创建了一个新的信号

RACSignal *bindSignal = [signal bind:^RACSignalBindBlock _Nonnull{
        return ^(NSNumber *value, BOOL *stop) {
            value = @(value.integerValue * value.integerValue);
            return [RACSignal return:value];
        };
    }];

除去RACDisposable处理过程，bind内部可简化成

- (RACSignal *)bind:(RACSignalBindBlock (^)(void))block {
    return [[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
        RACSignalBindBlock bindingBlock = block();
        return [self subscribeNext:^(id x) {
            BOOL stop = NO;
            id signal = bindingBlock(x, &stop);
            if (signal) {
                [signal subscribeNext:^(id x) {
                    [subscriber sendNext:x];
                } error:^(NSError *error) {
                    [subscriber sendError:error];
                } completed:^{
                    [subscriber sendCompleted];
                }];
            }
            if (signal == nil || stop) {
                [subscriber sendCompleted];
            }
        } error:^(NSError *error) {
            [subscriber sendError:error];
        } completed:^{
            [subscriber sendCompleted];
        }];
    }] setNameWithFormat:@"[%@] -bind:", self.name];
}

因为`RACReturnSignal订阅过程实现了sendNext

[RACScheduler.subscriptionScheduler schedule:^{
        [subscriber sendNext:self.value];
        [subscriber sendCompleted];
    }];

当bindSignal被订阅时

[bindSignal subscribeNext:^(id  _Nullable x) {
    NSLog(@"bindSignal: %@", x);
}];

bindingBlock返回的returnSignal会被订阅，进而触发[subscriber sendNext:x]