一、RACChannel的入口

RACChannel 可以被理解为一个双向的连接，这个连接的两端都是 RACSignal 实例，它们可以向彼此发送消息，如果我们在视图和模型之间通过 RACChannel 建立这样的连接：

    RACChannelTo(self.view, backgroundColor) = RACChannelTo(self.webView, backgroundColor);

等价于

    RACKVOChannel *a = [[RACKVOChannel alloc] initWithTarget:self.view keyPath:@"backgroundColor" nilValue:nil];
    RACKVOChannel *b = [[RACKVOChannel alloc] initWithTarget:self.webView keyPath:@"backgroundColor" nilValue:nil];
    a[@"followingTerminal"] = b[@"followingTerminal"];

因为RACKVOChannel重写了下标方法，所以接下去会触发对象a的方法：

- (void)setObject:(RACChannelTerminal *)otherTerminal forKeyedSubscript:(NSString *)key {
    NSCParameterAssert(otherTerminal != nil);

    RACChannelTerminal *selfTerminal = [self objectForKeyedSubscript:key];
    [otherTerminal subscribe:selfTerminal];
    [[selfTerminal skip:1] subscribe:otherTerminal];
}

等价于

    [b.followingTerminal subscribe:a.followingTerminal];
    [[a.followingTerminal skip:1] subscribe:b.followingTerminal];

从源码可以看出，数据绑定的双方通过内部的followingTerminal来进行数据交换

image.png

二、followingTerminal信号的具体实现

_followingTerminal = [[[RACChannelTerminal alloc] initWithValues:followingSubject otherTerminal:leadingSubject] setNameWithFormat:@"followingTerminal"];

- (instancetype)initWithValues:(RACSignal *)values otherTerminal:(id<RACSubscriber>)otherTerminal {
    NSCParameterAssert(values != nil);
    NSCParameterAssert(otherTerminal != nil);

    self = [super init];

    _values = values;
    _otherTerminal = otherTerminal;

    return self;
}

- (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber {
    return [self.values subscribe:subscriber];
}

- (void)sendNext:(id)value {
    [self.otherTerminal sendNext:value];
}
@end

结合第一步中的源码，可以将代码进行展开

// 1. 源码
[b.followingTerminal subscribe:a.followingTerminal];
[[a.followingTerminal skip:1] subscribe:b.followingTerminal];
// 2. 变形
[b.followingTerminal.values subscribe:a.followingTerminal];
[a.followingTerminal.values subscribe:b.followingTerminal];
// 3. 变形
[b.followingSubject subscribe:a.followingTerminal];
[a.followingSubject subscribe:b.followingTerminal];

image.png

到这里可以看到，RACChannel通过followingSubject作为发送端，将数据传播出去。

三、属性变化的监听

    RACDisposable *observationDisposable = [strongTarget rac_observeKeyPath:keyPath options:NSKeyValueObservingOptionInitial observer:nil block:^(id value, NSDictionary *change, BOOL causedByDealloc, BOOL affectedOnlyLastComponent) {
        // If the change wasn't triggered by deallocation, only affects the last
        // path component, and ignoreNextUpdate is set, then it was triggered by
        // this channel and should not be forwarded.
        if (!causedByDealloc && affectedOnlyLastComponent && self.currentThreadData.ignoreNextUpdate) {
            [self destroyCurrentThreadData];
            return;
        }

        [self.leadingTerminal sendNext:value];
    }];

可以看到，源码中通过KVO监听属性变化，并通过leadingTerminal将数据发送出去。
leadingTerminal与followingTerminal相反，所以

[self.leadingTerminal sendNext:value];

等价于

[followingSubject sendNext:value];

结合第二节的内容，数据的发送就可以串联起来。

image.png

四、数据变化的接收

RACKVOChannel初始化时，会订阅leadingTerminal信号

    [[self.leadingTerminal
        finally:^{
            [observationDisposable dispose];
        }]
        subscribeNext:^(id x) {
            // Check the value of the second to last key path component. Since the
            // channel can only update the value of a property on an object, and not
            // update intermediate objects, it can only update the value of the whole
            // key path if this object is not nil.
            NSObject *object = (keyPathComponentsCount > 1 ? [self.target valueForKeyPath:keyPathByDeletingLastKeyPathComponent] : self.target);
            if (object == nil) return;

            // Set the ignoreNextUpdate flag before setting the value so this channel
            // ignores the value in the subsequent -didChangeValueForKey: callback.
            [self createCurrentThreadData];
            self.currentThreadData.ignoreNextUpdate = YES;

            [object setValue:x ?: nilValue forKey:lastKeyPathComponent];
        } error:^(NSError *error) {
            NSCAssert(NO, @"Received error in %@: %@", self, error);

            // Log the error if we're running with assertions disabled.
            NSLog(@"Received error in %@: %@", self, error);
        }];

RACChannelTerminal内部重写了订阅方法

- (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber {
    return [self.values subscribe:subscriber];
}

所以订阅leadingTerminal信号等价于订阅了leadingSubject信号。
根据前几节的分析，可以得知，当数据改变时会通过信息传播到对方的followingTerminal中，而followingTerminal中重写消息发送的方法

- (void)sendNext:(id)value {
    [self.otherTerminal sendNext:value];
}

- (void)sendError:(NSError *)error {
    [self.otherTerminal sendError:error];
}

- (void)sendCompleted {
    [self.otherTerminal sendCompleted];
}

所以当触发followingTerminal的sendNext方法时，等价于调用了leadingSubject的sendNext方法。
然后在leadingSubject的订阅中通过KVC修改了自身属性。

image.png

五、总结

在RACKVOChannel中总共包含了4个信号，leadingTerminal、followingTerminal、leadingSubject、followingSubject。
而leadingTerminal、followingTerminal只是对leadingSubject、followingSubject的封装，内部通过转发分别将订阅、数据发送转发到两个不同的信号中，而leadingSubject、followingSubject才是实际数据的操作者，followingSubject负责将数据改变传播到外部，leadingSubject负责接收外部的数据来改变自身。

重点: 上述内容只是分析了如果发送和接收数据，但是使用KVC修改属性会触发KVO，所以会导致双向绑定陷入死循环，所以源码中在线程字典中保存了一个标志位，用来判断是否要忽略下一次KVO回调来避免产生死循环