Signal

RAC的核心是signal，RAC世界所有的消息都是通過信號的方式傳遞的。

• 創建信號

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

它的內部實現是創建一個RACDynamicSignal，同時將創建信號的block傳遞給RACDynamicSignal實例。

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

• 訂閱信號
  信號創建之后如果不被訂閱，block不會被執行

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

訂閱的內部實現其實也是函數式編程思想，block傳遞

• 創建訂閱者

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

• 執行創建信號時的block

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

Bind

信號的綁定是個相對復雜的操作，其內部是創建了一個新的信號

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

除去RACDisposable處理過程，bind內部可簡化成

- (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];
}

因為`RACReturnSignal訂閱過程實現了sendNext

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

當bindSignal被訂閱時

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

bindingBlock返回的returnSignal會被訂閱，進而觸發[subscriber sendNext:x]