[摘要]本文是對ReactiveCocoa源碼解讀（一）的講解，對學習IOS蘋果軟件開發有所幫助,與大家分享。

本著飲水思源的想法，面對ReactiveCocoa的強大功能，按捺不住心中的好奇心，於是走進其源碼之中，一探ReactiveCocoa的魅力所在。雖然，耳聞其強大功能的核心是：信號，但一直不知道這個信號是如何產生、如何傳遞，又是如何被處理的。曾經以為信號傳遞是通知，但是真正讀了源碼後，才發現之前的想法有多不妥，而人家的實現又是多巧妙。

本文主要從ReactiveCocoa的主要類入手，通過剖析其整個應用過程中，信號的生命周期來領略其編程之美。

一、RACSignal

1. 應用

// 1.創建信號
RACSignal *siganl = [RACSignal createSignal:^RACDisposable *(id subscriber) {
    // 注：block在此僅僅是個參數，未被調用，
    //當有訂閱者訂閱信號時會調用block。

    // 2.發送信號
    [subscriber sendNext:@1];
    // 如果不在發送數據，最好發送信號完成，內部會自動調用[RACDisposable disposable]取消訂閱信號。
    return nil;
}];

// 3.訂閱信號,才會激活信號.
[siganl subscribeNext:^(id x) {
    // block調用時刻：每當有信號發出數據，就會調用block.
    NSLog(@"接收到數據:%@",x);
}];

2.源碼實現

創建信號

+(RACSignal *)createSignal:(RACDisposable * (^)(id<RACSubscriber> subscriber))didSubscribe;

// RACDynamicSignal.m

+ (RACSignal *)createSignal:(RACDisposable * (^)(id subscriber))didSubscribe {
    //創建了一個RACDynamicSignal類的信號
    RACDynamicSignal *signal = [[self alloc] init];
    //將代碼塊保存到信號裡面（但此時僅僅是保存，沒有調用），所以信號還是冷信號
    signal->_didSubscribe = [didSubscribe copy];
    return [signal setNameWithFormat:@"+createSignal:"];
}

訂閱信號

(RACDisposable *)subscribeNext:(void (^ )(id x))nextBlock；

// RACSignal.m
- (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock {
    NSCParameterAssert(nextBlock != NULL);
    // 內部創建了RACSubscriber（訂閱者）類的實例對象o，並且將nextBlock保存到o中，在返回值出執行o,實際也是執行了nextBlock。
    RACSubscriber *o = [RACSubscriber subscriberWithNext:nextBlock error:NULL completed:NULL];
    return [self subscribe:o];
}

// RACSubscriber.m
+ (instancetype)subscriberWithNext:(void (^)(id x))next error:(void (^)(NSError *error))error completed:(void (^)(void))completed {
    RACSubscriber *subscriber = [[self alloc] init];
    // 將block保存到subscriber中
    subscriber->_next = [next copy];
    subscriber->_error = [error copy];
    subscriber->_completed = [completed copy];

    return subscriber;
}

// RACDynamicSignal.m
- (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber {
    NSCParameterAssert(subscriber != nil);

    RACCompoundDisposable *disposable = [RACCompoundDisposable compoundDisposable];
    subscriber = [[RACPassthroughSubscriber alloc] initWithSubscriber:subscriber signal:self disposable:disposable];
    //判斷有無self.didSubscribe,有則執行該self.didSubscribe，意味著將訂閱者subscriber發送過去
    if (self.didSubscribe != NULL) {
        RACDisposable *schedulingDisposable = [RACScheduler.subscriptionScheduler schedule:^{
            RACDisposable *innerDisposable = self.didSubscribe(subscriber);
            [disposable addDisposable:innerDisposable];
        }];

        [disposable addDisposable:schedulingDisposable];
    }
    
    return disposable;
}

// RACPassthroughSubscriber.m
- (instancetype)initWithSubscriber:(id)subscriber signal:(RACSignal *)signal disposable:(RACCompoundDisposable *)disposable {
    NSCParameterAssert(subscriber != nil);

    self = [super init];
    if (self == nil) return nil;
    // 保存訂閱者，信號，處理操作
    _innerSubscriber = subscriber;
    _signal = signal;
    _disposable = disposable;

    [self.innerSubscriber didSubscribeWithDisposable:self.disposable];
    return self;
}

發送信號

[subscriber sendNext:@1]

// RACPassthroughSubscriber.m
- (void)sendNext:(id)value {
    if (self.disposable.disposed) return;

    if (RACSIGNAL_NEXT_ENABLED()) {
        RACSIGNAL_NEXT(cleanedSignalDescription(self.signal), cleanedDTraceString(self.innerSubscriber.description), cleanedDTraceString([value description]));
    }

    [self.innerSubscriber sendNext:value];
}

// RACSubscriber.m
- (void)sendNext:(id)value {
    @synchronized (self) {
        void (^nextBlock)(id) = [self.next copy];
        if (nextBlock == nil) return;
        // 名為next的block是返回值為void，參數為id類型的value，在sendNext:內部，將next復制給nextBlock，執行該方法後，subscribeNext:的block參數才會被調用。
        nextBlock(value);
    }
}

3.流程圖

4.總結

先創建信號，然後訂閱信號，最後執行didSubscribe內部的方法，順序是不能變的

RACSignal底層實現

* 1.創建信號，首先把didSubscribe保存到信號中，還不會觸發。
* 2.當信號被訂閱，也就是調用signal的subscribeNext:nextBlock
    2.1 subscribeNext內部會創建訂閱者subscriber，並且把nextBlock保存到subscriber中。
    2.2 subscribeNext內部會調用siganl的didSubscribe
* 3.siganl的didSubscribe中調用[subscriber sendNext:@1];
    3.1 sendNext底層其實就是執行subscriber的nextBlock

二、RACSubject

1. 應用

// 創建信號
RACSubject *subject = [RACSubject subject];

// 訂閱
[subject subscribeNext:^(id x) {
    NSLog(@"第一個訂閱者：%@", x);
}];

// 發送信號
[subject sendNext:@"1"];

2.源碼實現

創建信號

// RACSubject.m
+ (instancetype)subject {
    return [[self alloc] init];
}

- (id)init {
    self = [super init];
    if (self == nil) return nil;

    _disposable = [RACCompoundDisposable compoundDisposable];
    _subscribers = [[NSMutableArray alloc] initWithCapacity:1];

    return self;
}

訂閱信號

RACSubject訂閱信號的實質就是將內部創建的訂閱者保存在訂閱者數組self.subscribers中，僅此而已。訂閱者對象有一個名為nextBlock的block參數

// RACSignal.m
- (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock {
    NSCParameterAssert(nextBlock != NULL);

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

// RACSubscriber.m
+ (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;
}

// RACSubject.m
- (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];
    }
    
    return [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];
        }
    }];
}

發送信號

底層實現：
1. 先遍歷訂閱者數組中的訂閱者;
2. 後執行訂閱者中的nextBlock;
3. 最後讓訂閱者發送信號

// RACSubject.m
- (void)sendNext:(id)value {
    [self enumerateSubscribersUsingBlock:^(id subscriber) {
        [subscriber sendNext:value];
    }];
}

- (void)enumerateSubscribersUsingBlock:(void (^)(id subscriber))block {
    NSArray *subscribers;
    @synchronized (self.subscribers) {
        subscribers = [self.subscribers copy];
    }

    for (id subscriber in subscribers) {
        block(subscriber);
    }
}

3.流程圖

4.總結

RACSubscriber:表示訂閱者的意思，用於發送信號，這是一個協議，不是一個類，只要遵守這個協議，並且實現方法才能成為訂閱者。通過create創建的信號，都有一個訂閱者，幫助他發送數據。

RACDisposable:用於取消訂閱或者清理資源，當信號發送完成或者發送錯誤的時候，就會自動觸發它。

RACSubject的底層與RACSignal不一樣：

調用subscribeNext訂閱信號，只是把訂閱者保存起來，並且訂閱者的nextBlock已經賦值了 調用sendNext發送信號，遍歷剛剛保存的所有訂閱者，一個一個調用訂閱者的nextBlock 由於本質是將訂閱者保存到數組中，所以可以有多個訂閱者訂閱信息。

必須先訂閱，後發送信息。訂閱信號就是創建訂閱者的過程，如果不先訂閱，數組中就沒有訂閱者對象，那就通過訂閱者發送消息

三、RACReplaySubject

1.應用

RACReplaySubject即可以先訂閱後發送信號，也可以反過來

RACReplaySubject *subject = [RACReplaySubject subject];

// 第一次訂閱
[subject subscribeNext:^(id x) {
    NSLog(@"%@", x);
}];

// 發送信號
[subject sendNext:@"1"];

// 第一次訂閱
[subject subscribeNext:^(id x) {
    NSLog(@"%@", x);
}];

2.源碼實現

創建信號

// RACSubject.m
+ (instancetype)subject {
    return [[self alloc] init];
}

// RACReplaySubject.m
- (instancetype)init {
    return [self initWithCapacity:RACReplaySubjectUnlimitedCapacity];
}

- (instancetype)initWithCapacity:(NSUInteger)capacity {
    self = [super init];
    if (self == nil) return nil;

    _capacity = capacity;
    // 會用這個數組保存值value
    _valuesReceived = (capacity == RACReplaySubjectUnlimitedCapacity ? [NSMutableArray array] : [NSMutableArray arrayWithCapacity:capacity]);

    return self;
}

訂閱信號

// RACSignal.m
- (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock {
    NSCParameterAssert(nextBlock != NULL);

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

// RACReplaySubject.m
- (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber {
    RACCompoundDisposable *compoundDisposable = [RACCompoundDisposable compoundDisposable];

    RACDisposable *schedulingDisposable = [RACScheduler.subscriptionScheduler schedule:^{
        @synchronized (self) {
            for (id value in self.valuesReceived) {
                if (compoundDisposable.disposed) return;

                [subscriber sendNext:(value == RACTupleNil.tupleNil ? nil : value)];
            }

            if (compoundDisposable.disposed) return;

            if (self.hasCompleted) {
                [subscriber sendCompleted];
            } else if (self.hasError) {
                [subscriber sendError:self.error];
            } else {
                RACDisposable *subscriptionDisposable = [super subscribe:subscriber];
                [compoundDisposable addDisposable:subscriptionDisposable];
            }
        }
    }];

    [compoundDisposable addDisposable:schedulingDisposable];

    return compoundDisposable;
}

發送信號

// RACReplaySubject.m
- (void)sendNext:(id)value {
    @synchronized (self) {
        //重點：發送信號的時候，會先將值value保存到數組中
        [self.valuesReceived addObject:value ?: RACTupleNil.tupleNil];
        //調用父類發送（先遍歷訂閱者，然後發送值value）
        [super sendNext:value];

        if (self.capacity != RACReplaySubjectUnlimitedCapacity && self.valuesReceived.count > self.capacity) {
            [self.valuesReceived removeObjectsInRange:NSMakeRange(0, self.valuesReceived.count - self.capacity)];
        }
    }
}

3.原理圖

4.總結

RACReplaySubject是RACSubject的子類
由於每次發送信號時，會先保存nextBlock，然後調用父類的sendNext方法，遍歷訂閱者，執行信號；而每次訂閱信號時，會從valuesReceived中取值，然後調用sendNext方法，遍歷訂閱者，執行信號。所以，訂閱和發送沒有先後順序。

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未完待續 ......