ReactiveCocoa源码解读（二）

上一篇解读了ReactiveCocoa的三个重要的类的底层实现，本篇继续。

一、RACMulticastConnection

1.应用

RACMulticastConnection： 用于当一个信号被多次订阅时，为了保证创建信号时，避免多次调用创建信号的block造成副作用，可以使用该类处理，保证创建信号的block执行一次。

// 创建信号
RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id subscriber) {
    NSLog(@"发送请求");
    [subscriber sendNext:@1];
    return nil;
}];

// 创建连接
RACMulticastConnection *connect = [signal publish];

// 订阅连接的信号
[connect.signal subscribeNext:^(id x) {
    NSLog(@"connect 第一次订阅信号: %@", x);
}];

[connect.signal subscribeNext:^(id x) {
    NSLog(@"connect 第二次订阅信号: %@", x);
}];

// 连接
[connect connect];

2.源码实现

• 底层原理

1.创建connect，connect.sourceSignal -> RACSignal(原始信号) connect.signal -> RACSubject
2.订阅connect.signal，会调用RACSubject的subscribeNext，创建订阅者，而且把订阅者保存起来，不会执行block。
3.[connect connect]内部会订阅RACSignal(原始信号)，并且订阅者是RACSubject
    3.1.订阅原始信号，就会调用原始信号中的didSubscribe
    3.2 didSubscribe，拿到订阅者调用sendNext，其实是调用RACSubject的sendNext
4.RACSubject的sendNext,会遍历RACSubject所有订阅者发送信号。
    4.1 因为刚刚第二步，都是在订阅RACSubject，因此会拿到第二步所有的订阅者，调用他们的nextBlock

• 创建信号

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

// RACDynamicSignal.m
+ (RACSignal *)createSignal:(RACDisposable * (^)(id subscriber))didSubscribe {
    RACDynamicSignal *signal = [[self alloc] init];
    //将代码块保存到信号里面（但此时仅仅是保存，没有调用），所以信号还是冷信号
    signal->_didSubscribe = [didSubscribe copy];
    return [signal setNameWithFormat:@"+createSignal:"];
}

• 创建连接

[signal publish]

// RACSignal+Operations.m
- (RACMulticastConnection *)publish {
    // 创建订阅者
    RACSubject *subject = [[RACSubject subject] setNameWithFormat:@"[%@] -publish", self.name];
    // 创建connection，参数是刚才创建的订阅者
    RACMulticastConnection *connection = [self multicast:subject];
    return connection;
}

- (RACMulticastConnection *)multicast:(RACSubject *)subject {
    [subject setNameWithFormat:@"[%@] -multicast: %@", self.name, subject.name];
    RACMulticastConnection *connection = [[RACMulticastConnection alloc] initWithSourceSignal:self subject:subject];
    return connection;
}

// RACMulticastConnection.m
- (id)initWithSourceSignal:(RACSignal *)source subject:(RACSubject *)subject {
    NSCParameterAssert(source != nil);
    NSCParameterAssert(subject != nil);

    self = [super init];
    if (self == nil) return nil;
    // 保存原始信号
    _sourceSignal = source;
    _serialDisposable = [[RACSerialDisposable alloc] init];
    // 保存订阅者，即_signal是RACSubject对象
    _signal = subject;

    return self;
}

• 订阅信号

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

// 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];
        }
    }];
}

• 连接信号

[connect connect];

// RACMulticastConnection.m
- (RACDisposable *)connect {
    BOOL shouldConnect = OSAtomicCompareAndSwap32Barrier(0, 1, &_hasConnected);

    if (shouldConnect) {
        // 订阅原生信号
        self.serialDisposable.disposable = [self.sourceSignal subscribe:_signal];
    }

    return self.serialDisposable;
}

// RACDynamicSignal.m
- (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;
}

// RACSubject.m
- (void)sendNext:(id)value {
    // 遍历_subscribers数组，执行nextBlock
    [self enumerateSubscribersUsingBlock:^(id subscriber) {
        [subscriber sendNext:value];
    }];
}

3.流程图

RACMulticastConnection

4.总结

RACMulticastConnection利用RACSubject实现了创建信号的block只执行一次的功能。对于需要对此订阅信号，但是不希望多次创建信号的应用场合，可以RACMulticastConnection解决。

二、RACCommand

1.应用

RACCommand类用来表示动作的执行, 是对动作触发后的连锁事件的封装。常用在封装网络请求，按钮点击等等场合。

RACCommand *command = [[RACCommand alloc] initWithSignalBlock:^RACSignal *(id input) {
    return [RACSignal createSignal:^RACDisposable *(id subscriber) {

        if (/* DISABLES CODE */ (YES)) {
        // 正常发送数据，必须发送完成信号
            [subscriber sendNext:@"Smile"];
            [subscriber sendCompleted];
        } else {
            // 发送错误信号
            [subscriber sendError:[NSError errorWithDomain:@"Network failed" code:0005 userInfo:nil]];
        }

        // 信号被销毁前，做一些清理的工作；如果不需要，可以 return nil
        return [RACDisposable disposableWithBlock:^{
            NSLog(@"信号被销毁了");
        }];
    }];
}];

// 执行信号并订阅
[[command execute:nil] subscribeNext:^(id x) {
    NSLog(@"receive data: %@", x);
}];

2.源码实现

RACCommand底层实现

1. 创建命令，保存signalBlock
2. 执行命令
* 2.1 调用signalBlock
* 2.2 创建connect，传入RACReplaySubject对象，然后连接信号
3. 订阅信号
* 3.1 创建订阅者，保存到RACReplaySubject对象的_subscribers数组中
* 3.2 遍历valuesReceived数组，调用订阅者发送数据

• 创建command

- (id)initWithSignalBlock:(RACSignal * (^)(id input))signalBlock

// RACCommand.m
- (id)initWithSignalBlock:(RACSignal * (^)(id input))signalBlock {
    return [self initWithEnabled:nil signalBlock:signalBlock];
}

- (id)initWithEnabled:(RACSignal *)enabledSignal signalBlock:(RACSignal * (^)(id input))signalBlock {
    NSCParameterAssert(signalBlock != nil);

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

    _activeExecutionSignals = [[NSMutableArray alloc] init];
    // 保存创建信号的block
    _signalBlock = [signalBlock copy];
    ......
}

• 执行command

- (RACSignal *)execute:(id)input

// RACCommand.m
- (RACSignal *)execute:(id)input {
    // `immediateEnabled` is guaranteed to send a value upon subscription, so
    // -first is acceptable here.
    BOOL enabled = [[self.immediateEnabled first] boolValue];
    if (!enabled) {
        NSError *error = [NSError errorWithDomain:RACCommandErrorDomain code:RACCommandErrorNotEnabled userInfo:@{
            NSLocalizedDescriptionKey: NSLocalizedString(@"The command is disabled and cannot be executed", nil),
            RACUnderlyingCommandErrorKey: self
        }];

        return [RACSignal error:error];
    }

    RACSignal *signal = self.signalBlock(input);
    ......
    // 创建连接，用RACReplaySubject作为订阅者
    RACMulticastConnection *connection = [[signal
    subscribeOn:RACScheduler.mainThreadScheduler]
    multicast:[RACReplaySubject subject]];

    ......
    // 连接信号
    [connection connect];
        return [connection.signal setNameWithFormat:@"%@ -execute: %@", self, [input rac_description]];
}

// RACMulticastConnection.m
- (RACDisposable *)connect {
    BOOL shouldConnect = OSAtomicCompareAndSwap32Barrier(0, 1, &_hasConnected);

    if (shouldConnect) {
        // 执行创建信号的block
        self.serialDisposable.disposable = [self.sourceSignal subscribe:_signal];
    }

    return self.serialDisposable;
}

• 订阅command

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

// 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;
                // 调用订阅者，发送数据 "Smile"
                [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 {
                // 调用父类方法，保存订阅者到_subscribers数组
                RACDisposable *subscriptionDisposable = [super subscribe:subscriber];
                [compoundDisposable addDisposable:subscriptionDisposable];
            }
        }
    }];

    [compoundDisposable addDisposable:schedulingDisposable];

    return compoundDisposable;
}

3.流程图

RACCommand

4.总结

RACCommand用来封装事件时，还可以订阅信号（executionSignals）、订阅最新信号(switchToLatest)、跳过几次信号（skip）或信号是否正在执行（executing），在执行信号时，还可以监听错误信号和完成信号，请参考demo例子。

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ReactiveCocoa框架的源码分析暂告一段落，如有分析不足之处，欢迎互相交流。

Demo地址：

RACDemo