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:@"1"];
        [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:@"8"];
    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];
    }

    */