ReactiveCocoa源码解读（一）

本着饮水思源的想法，面对ReactiveCocoa的强大功能，按捺不住心中的好奇心，于是走进其源码之中，一探ReactiveCocoa的魅力所在。虽然，耳闻其强大功能的核心是：信号，但一直不知道这个信号是如何产生、如何传递，又是如何被处理的。曾经以为信号传递是通知，但是真正读了源码后，才发现之前的想法有多不妥，而人家的实现又是多巧妙。

本文主要从ReactiveCocoa的主要类入手，通过剖析其整个应用过程中，信号的生命周期来领略其编程之美。

一、RACSignal

1. 应用

// 1.创建信号
RACSignal *siganl = [RACSignal createSignal:^RACDisposable *(id subscriber) {
	// 注：block在此仅仅是个参数，未被调用，
	//当有订阅者订阅信号时会调用block。

	// 2.发送信号
	[subscriber sendNext:@1];
	// 如果不在发送数据，最好发送信号完成，内部会自动调用[RACDisposable disposable]取消订阅信号。
	return nil;
}];

// 3.订阅信号,才会激活信号.
[siganl subscribeNext:^(id x) {
	// block调用时刻：每当有信号发出数据，就会调用block.
	NSLog(@"接收到数据:%@",x);
}];

2.源码实现

• 创建信号

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

// RACDynamicSignal.m

+ (RACSignal *)createSignal:(RACDisposable * (^)(id subscriber))didSubscribe {
	//创建了一个RACDynamicSignal类的信号
	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:"];
}

• 订阅信号

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

// RACSignal.m
- (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock {
	NSCParameterAssert(nextBlock != NULL);
	// 内部创建了RACSubscriber（订阅者）类的实例对象o，并且将nextBlock保存到o中，在返回值出执行o,实际也是执行了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];
	// 将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];
	//判断有无self.didSubscribe,有则执行该self.didSubscribe，意味着将订阅者subscriber发送过去
	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;
	// 保存订阅者，信号，处理操作
	_innerSubscriber = subscriber;
	_signal = signal;
	_disposable = disposable;

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

• 发送信号

[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;
		// 名为next的block是返回值为void，参数为id类型的value，在sendNext:内部，将next复制给nextBlock，执行该方法后，subscribeNext:的block参数才会被调用。
		nextBlock(value);
	}
}

3.流程图

4.总结

先创建信号，然后订阅信号，最后执行didSubscribe内部的方法，顺序是不能变的

RACSignal底层实现

* 1.创建信号，首先把didSubscribe保存到信号中，还不会触发。
* 2.当信号被订阅，也就是调用signal的subscribeNext:nextBlock
	2.1 subscribeNext内部会创建订阅者subscriber，并且把nextBlock保存到subscriber中。
	2.2 subscribeNext内部会调用siganl的didSubscribe
* 3.siganl的didSubscribe中调用[subscriber sendNext:@1];
	3.1 sendNext底层其实就是执行subscriber的nextBlock

二、RACSubject

1. 应用

// 创建信号
RACSubject *subject = [RACSubject subject];

// 订阅
[subject subscribeNext:^(id x) {
	NSLog(@"第一个订阅者：%@", x);
}];

// 发送信号
[subject sendNext:@"1"];

2.源码实现

• 创建信号

// 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;
}

• 订阅信号

RACSubject订阅信号的实质就是将内部创建的订阅者保存在订阅者数组self.subscribers中，仅此而已。订阅者对象有一个名为nextBlock的block参数

// 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];
		}
	}];
}

• 发送信号

底层实现：
1. 先遍历订阅者数组中的订阅者;
2. 后执行订阅者中的nextBlock;
3. 最后让订阅者发送信号

// 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.流程图

4.总结

RACSubscriber:表示订阅者的意思，用于发送信号，这是一个协议，不是一个类，只要遵守这个协议，并且实现方法才能成为订阅者。通过create创建的信号，都有一个订阅者，帮助他发送数据。

RACDisposable:用于取消订阅或者清理资源，当信号发送完成或者发送错误的时候，就会自动触发它。

RACSubject的底层与RACSignal不一样：

• 调用subscribeNext订阅信号，只是把订阅者保存起来，并且订阅者的nextBlock已经赋值了
• 调用sendNext发送信号，遍历刚刚保存的所有订阅者，一个一个调用订阅者的nextBlock
• 由于本质是将订阅者保存到数组中，所以可以有多个订阅者订阅信息。

必须先订阅，后发送信息。订阅信号就是创建订阅者的过程，如果不先订阅，数组中就没有订阅者对象，那就通过订阅者发送消息

三、RACReplaySubject

1.应用

RACReplaySubject即可以先订阅后发送信号，也可以反过来

RACReplaySubject *subject = [RACReplaySubject subject];

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

// 发送信号
[subject sendNext:@"1"];

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

2.源码实现

• 创建信号

// 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;
	// 会用这个数组保存值value
	_valuesReceived = (capacity == RACReplaySubjectUnlimitedCapacity ? [NSMutableArray array] : [NSMutableArray arrayWithCapacity:capacity]);

	return self;
}

• 订阅信号

// 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;
}

• 发送信号

// RACReplaySubject.m
- (void)sendNext:(id)value {
	@synchronized (self) {
		//重点：发送信号的时候，会先将值value保存到数组中
		[self.valuesReceived addObject:value ?: RACTupleNil.tupleNil];
		//调用父类发送（先遍历订阅者，然后发送值value）
		[super sendNext:value];

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

3.原理图

4.总结

RACReplaySubject是RACSubject的子类

由于每次发送信号时，会先保存nextBlock，然后调用父类的sendNext方法，遍历订阅者，执行信号；而每次订阅信号时，会从valuesReceived中取值，然后调用sendNext方法，遍历订阅者，执行信号。所以，订阅和发送没有先后顺序。

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未完待续 ......