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 -> RACSubject2.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sendNext4.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

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signalBlock2. 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

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。

---

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