ReactiveCocoa 源码阅读记录。

1：RACSingle 需要订阅信号

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

     [signal subscribeNext:^(id x) {

     } error:^(NSError * _Nullable error) {

     } completed:^{

     }];
 /*************    对应的源码    **************/
 // 生成RACDynamicSignal。即生成RACSignal的子类
 + (RACSignal *)createSignal:(RACDisposable * (^)(id<RACSubscriber> subscriber))didSubscribe {
     return [RACDynamicSignal createSignal:didSubscribe];
 }

 // 在子类中保存 didSubscribe block, 每一次订阅都会执行
 + (RACSignal *)createSignal:(RACDisposable * (^)(id<RACSubscriber> subscriber))didSubscribe {
     RACDynamicSignal *signal = [[self alloc] init];
     signal->_didSubscribe = [didSubscribe copy];
     return [signal setNameWithFormat:@"+createSignal:"];
 }

 /// 在订阅时执行保存在RACDynamicSignal中的didSubscribe block
 - (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;
 }

 // 生成订阅者。并调用RACDynamicSignal中的订阅方法，执行didSubscribe block
 - (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock {
     NSCParameterAssert(nextBlock != NULL);

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

 // 生成的订阅者中保存 nextBlock， errorBlock， completedBlock.
 + (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;
 }

 // 当订阅者收到sendNext方法时，就会执行nextBlock
 - (void)sendNext:(id)value {
     @synchronized (self) {
         void (^nextBlock)(id) = [self.next copy];
         if (nextBlock == nil) return;

         nextBlock(value);
     }
 }

2 RACSubject 既能订阅信号又能发送信号。 必须先订阅在发送。不然由于在sendNext时还没有订阅者会无法触发。

    /// 事例
     RACSubject *sub = [[RACSubject alloc] init];
     [sub subscribeNext:^(id  _Nullable x) {
         NSLog(@"%@", a);
     }];

     [sub sendNext:@"a"];

 //调用时和RACSignal有区别的方法
 // 使用 subscribers 保存 订阅者。（RACDynamicSignal 还要执行didSubscrbe Block）
 - (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber {
     NSCParameterAssert(subscriber != nil);

     RACCompoundDisposable *disposable = [RACCompoundDisposable compoundDisposable];
     subscriber = [[RACPassthroughSubscriber alloc] initWithSubscriber:subscriber signal:self disposable:disposable];

     NSMutableArray *subscribers = self.subscribers;
     @synchronized (subscribers) {
         [subscribers addObject:subscriber];
     }

     [disposable addDisposable:[RACDisposable disposableWithBlock:^{
         @synchronized (subscribers) {
             // Since newer subscribers are generally shorter-lived, search
             // starting from the end of the list.
             NSUInteger index = [subscribers indexOfObjectWithOptions:NSEnumerationReverse passingTest:^ BOOL (id<RACSubscriber> obj, NSUInteger index, BOOL *stop) {
                 return obj == subscriber;
             }];

             if (index != NSNotFound) [subscribers removeObjectAtIndex:index];
         }
     }]];

     return disposable;
 }

 // 找到需要的订阅者
 - (void)enumerateSubscribersUsingBlock:(void (^)(id<RACSubscriber> subscriber))block {
     NSArray *subscribers;
     @synchronized (self.subscribers) {
         subscribers = [self.subscribers copy];
     }

     for (id<RACSubscriber> subscriber in subscribers) {
         block(subscriber);
     }
 }

 // 这行nextBlock。
 - (void)sendNext:(id)value {
     [self enumerateSubscribersUsingBlock:^(id<RACSubscriber> subscriber) {
         [subscriber sendNext:value];
     }];
 }

3 bind bind是RAC中map，merge， flatten， flattenMap的基础

flattenMap 实际上就是调用bind。 经过flattenMap映射后，subscribe的是block(value)即block里面的信号

　 map 调用的flattenMap 经过Map映射后 subscribe的是[self return:block(value)]的信号假定为wrapSignal.而不是block(value)的信号假定为子信号。

flatten 调用的是flattenMap，subscribe的子信号。调用flatten订阅到的值必须是一个Signal。

     RACSignal *returnSig = [RACSignal return:@""];
     RACSignal *bindSig = [returnSig bind:^RACSignalBindBlock {
         return ^RACSignal * (id value, BOOL *bl) {
             return [RACSignal return:value];
         };
     }];

     [bindSig subscribeNext:^(id  _Nullable x) {
         NSLog(@"%@", x);
     }];
     /*
      * -bind: should:
      * 订阅原始信号，并将订阅到的原始信号值，赋值给binding block。如果binding block返回了新的信号。订阅它, 之后用生成的中转信号的订阅者，执行返回的信号的sendNext方法和 sendComplete方法
      * 1. Subscribe to the original signal of values.
      * 2. Any time the original signal sends a value, transform it using the binding block.
      * 3. If the binding block returns a signal, subscribe to it, and pass all of its values through to the subscriber as they're received.
      * 4. If the binding block asks the bind to terminate, complete the _original_ signal.
      * 5. When _all_ signals complete, send completed to the subscriber.
      *
      * If any signal sends an error at any point, send that to the subscriber.
      */

     /*
     - (RACSignal *)bind:(RACSignalBindBlock (^)(void))block {
         NSCParameterAssert(block != NULL);

         return [[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
             RACSignalBindBlock bindingBlock = block();

             __block volatile int32_t signalCount = 1;   // indicates self

             RACCompoundDisposable *compoundDisposable = [RACCompoundDisposable compoundDisposable];

             void (^completeSignal)(RACDisposable *) = ^(RACDisposable *finishedDisposable) {
                 if (OSAtomicDecrement32Barrier(&signalCount) == 0) {
                     [subscriber sendCompleted];
                     [compoundDisposable dispose];
                 } else {
                     [compoundDisposable removeDisposable:finishedDisposable];
                 }
             };
             // signal 是bindingBlock返回的信号。订阅这个信号。由于在Block中有这个信号的sendNext方法故会直接执行他。之后接收到的值发送出去。
             void (^addSignal)(RACSignal *) = ^(RACSignal *signal) {
                 OSAtomicIncrement32Barrier(&signalCount);

                 RACSerialDisposable *selfDisposable = [[RACSerialDisposable alloc] init];
                 [compoundDisposable addDisposable:selfDisposable];

                 RACDisposable *disposable = [signal subscribeNext:^(id x) {
                     [subscriber sendNext:x];
                 } error:^(NSError *error) {
                     [compoundDisposable dispose];
                     [subscriber sendError:error];
                 } completed:^{
                     @autoreleasepool {
                         completeSignal(selfDisposable);
                     }
                 }];

                 selfDisposable.disposable = disposable;
             };

             @autoreleasepool {
                 RACSerialDisposable *selfDisposable = [[RACSerialDisposable alloc] init];
                 [compoundDisposable addDisposable:selfDisposable];
                 // 订阅初始的信号（上面的returnSig）， 执行bindingBLock，
                 RACDisposable *bindingDisposable = [self subscribeNext:^(id x) {
                     // Manually check disposal to handle synchronous errors.
                     if (compoundDisposable.disposed) return;

                     BOOL stop = NO;
                     id signal = bindingBlock(x, &stop);

                     @autoreleasepool {
                         if (signal != nil) addSignal(signal);
                         if (signal == nil || stop) {
                             [selfDisposable dispose];
                             completeSignal(selfDisposable);
                         }
                     }
                 } error:^(NSError *error) {
                     [compoundDisposable dispose];
                     [subscriber sendError:error];
                 } completed:^{
                     @autoreleasepool {
                         completeSignal(selfDisposable);
                     }
                 }];

                 selfDisposable.disposable = bindingDisposable;
             }

             return compoundDisposable;
         }] setNameWithFormat:@"[%@] -bind:", self.name];
     }

     */