初识 Reactive Cocoa

ReactiveCocoa 是一个符合 FRP（Functional Reactive Programming）范式的，操作和转换数据流的框架。

它可以解决三个问题：

• 开发过程中关于界面的状态信息太多，不易维护。
• 统一各种消息传递的方式。
• 绑定功能可以配合 MVVM 使用。

基本概念

简而言之，Singal 以 stream 的方式将 event 发送给 Observer。

Event

Event 代表值发生变化，或者有用户操作，分为 4 种：

• Next：signal 产生了新的值。
• Failed：signal 执行发生了错误，不再产生新的值。
• Completed：signal 执行结束，不再产生新的值。
• Interrupted：signal 被中止，不再产生新的值。

Singal

Signal 类似于一个 event 源头，event 通过 signal 传递。用户只能通过 subscribe 来开始有序地、被动地获取 signal 里面的值。一一个 signal 的生命周期通常是一个或多个 Next event，然后以 Failed、Completed 和 Interrupted 中的一种 event 结束，不受 subscriber 的影响。

用户可以使用基本操作符对 Signal 进行一些操作，如过滤（filter）、映射（map）等。

Observer

通过 subscribe 一个 signal 来接受 event 的对象。

RACCommand

通常用来执行一个 UI 事件发生后要处理的任务。

RACSubject

一个 signal 的子类，是一个可以手动发送 Next、Failed 等事件的 signal。通常用来将 non-RAC 的代码 bridge 成 RAC 的代码。

基本操作

添加副作用

• -subscribe...:

用来订阅一个 signal 的 Next、Failed、Completed 和 Interrupted 事件中的一种或几种：

RACSignal *letters = [@"A B C D E F G H I" componentsSeparatedByString:@" "].rac_sequence.signal;

// Outputs: A B C D E F G H I
[letters subscribeNext:^(NSString *x) {
    NSLog(@"%@", x);
}];

• -do...:

用来添加副作用，当 Next、Failed、Completed 和 Interrupted 事件中的一种发生时执行 do... 中的 block。

__block unsigned subscriptions = 0;

RACSignal *loggingSignal = [RACSignal createSignal:^ RACDisposable * (id<RACSubscriber> subscriber) {
    subscriptions++;
    [subscriber sendCompleted];
    return nil;
}];

// Does not output anything yet
loggingSignal = [loggingSignal doCompleted:^{
    NSLog(@"about to complete subscription %u", subscriptions);
}];

// Outputs:
// about to complete subscription 1
// subscription 1
[loggingSignal subscribeCompleted:^{
    NSLog(@"subscription %u", subscriptions);
}];

转换 stream

• -map:

将一个 stream 中的值转换成其他的值，组成一个新的 stream。

RACSequence *letters = [@"A B C D E F G H I" componentsSeparatedByString:@" "].rac_sequence;

// Contains: AA BB CC DD EE FF GG HH II
RACSequence *mapped = [letters map:^(NSString *value) {
    return [value stringByAppendingString:value];
}];

• -filter:

用来过滤 stream 中的值，传入的 block 返回一个 BOOL，来决定是否将值放入新的 stream 中。

RACSequence *numbers = [@"1 2 3 4 5 6 7 8 9" componentsSeparatedByString:@" "].rac_sequence;

// Contains: 2 4 6 8
RACSequence *filtered = [numbers filter:^ BOOL (NSString *value) {
    return (value.intValue % 2) == 0;
}];

组合 stream

• -concat:

将一个 stream 的值追加到另一个 stream 的值后面。

RACSequence *letters = [@"A B C D E F G H I" componentsSeparatedByString:@" "].rac_sequence;
RACSequence *numbers = [@"1 2 3 4 5 6 7 8 9" componentsSeparatedByString:@" "].rac_sequence;

// Contains: A B C D E F G H I 1 2 3 4 5 6 7 8 9
RACSequence *concatenated = [letters concat:numbers];

• -flatten:

如果一个 signal 的 stream 中的值也是一个 stream，-flatten: 会将子 stream 中的值取出，组成一个新的 stream。组合的效果可能是 concat：

RACSequence *letters = [@"A B C D E F G H I" componentsSeparatedByString:@" "].rac_sequence;
RACSequence *numbers = [@"1 2 3 4 5 6 7 8 9" componentsSeparatedByString:@" "].rac_sequence;
RACSequence *sequenceOfSequences = @[ letters, numbers ].rac_sequence;

// Contains: A B C D E F G H I 1 2 3 4 5 6 7 8 9
RACSequence *flattened = [sequenceOfSequences flatten];

或者 merge：

RACSubject *letters = [RACSubject subject];
RACSubject *numbers = [RACSubject subject];
RACSignal *signalOfSignals = [RACSignal createSignal:^ RACDisposable * (id<RACSubscriber> subscriber) {
    [subscriber sendNext:letters];
    [subscriber sendNext:numbers];
    [subscriber sendCompleted];
    return nil;
}];

RACSignal *flattened = [signalOfSignals flatten];

// Outputs: A 1 B C 2
[flattened subscribeNext:^(NSString *x) {
    NSLog(@"%@", x);
}];

[letters sendNext:@"A"];
[numbers sendNext:@"1"];
[letters sendNext:@"B"];
[letters sendNext:@"C"];
[numbers sendNext:@"2"];

• -flattenMap:

当一个 stream 的值也是 stream 时，将子 stream 中的值转换后，组成一个新的 stream。

-flattenMap: = -map: + -flatten:

RACSequence *numbers = [@"1 2 3 4 5 6 7 8 9" componentsSeparatedByString:@" "].rac_sequence;

// Contains: 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9
RACSequence *extended = [numbers flattenMap:^(NSString *num) {
    return @[ num, num ].rac_sequence;
}];

// Contains: 1_ 3_ 5_ 7_ 9_
RACSequence *edited = [numbers flattenMap:^(NSString *num) {
    if (num.intValue % 2 == 0) {
        return [RACSequence empty];
    } else {
        NSString *newNum = [num stringByAppendingString:@"_"];
        return [RACSequence return:newNum]; 
    }
}];

组合 signal

• -then:

当一个 signal 执行完成之后，subscriber 后续只会收到新返回的 signal 的值，如同订阅了新的 signal 一样。

RACSignal *letters = [@"A B C D E F G H I" componentsSeparatedByString:@" "].rac_sequence.signal;

// The new signal only contains: 1 2 3 4 5 6 7 8 9
//
// But when subscribed to, it also outputs: A B C D E F G H I
RACSignal *sequenced = [[letters
    doNext:^(NSString *letter) {
        NSLog(@"%@", letter);
    }]
    then:^{
        return [@"1 2 3 4 5 6 7 8 9" componentsSeparatedByString:@" "].rac_sequence.signal;
    }];

• +merge:

将多个 signal 合并为一个，一旦任一合并前的 signal 中产生新的值，新的 signal 立即产生同样的值。

RACSubject *letters = [RACSubject subject];
RACSubject *numbers = [RACSubject subject];
RACSignal *merged = [RACSignal merge:@[ letters, numbers ]];

// Outputs: A 1 B C 2
[merged subscribeNext:^(NSString *x) {
    NSLog(@"%@", x);
}];

[letters sendNext:@"A"];
[numbers sendNext:@"1"];
[letters sendNext:@"B"];
[letters sendNext:@"C"];
[numbers sendNext:@"2"];

• +combineLatest: 和 +combineLatest:reduce:

将多个 signal 合并成一个。与 +merge: 不同的是，这两个方法产生的新的 signal 只有在所有的合并前 signal 至少产生一个新的值时，才将所有的值（以元组的方式）返回。

RACSubject *letters = [RACSubject subject];
RACSubject *numbers = [RACSubject subject];
RACSignal *combined = [RACSignal
    combineLatest:@[ letters, numbers ]
    reduce:^(NSString *letter, NSString *number) {
        return [letter stringByAppendingString:number];
    }];

// Outputs: B1 B2 C2 C3
[combined subscribeNext:^(id x) {
    NSLog(@"%@", x);
}];

[letters sendNext:@"A"];
[letters sendNext:@"B"];
[numbers sendNext:@"1"];
[numbers sendNext:@"2"];
[letters sendNext:@"C"];
[numbers sendNext:@"3"];

• -switchToLatest

用来处理由 signal 组成的 signal （signal-of-signals），总是返回最新的外层 signal 中的最新的 signal 的值。

RACSubject *letters = [RACSubject subject];
RACSubject *numbers = [RACSubject subject];
RACSubject *signalOfSignals = [RACSubject subject];

RACSignal *switched = [signalOfSignals switchToLatest];

// Outputs: A B 1 D
[switched subscribeNext:^(NSString *x) {
    NSLog(@"%@", x);
}];

[signalOfSignals sendNext:letters];
[letters sendNext:@"A"];
[letters sendNext:@"B"];

[signalOfSignals sendNext:numbers];
[letters sendNext:@"C"];
[numbers sendNext:@"1"];

[signalOfSignals sendNext:letters];
[numbers sendNext:@"2"];
[letters sendNext:@"D"];

其他

• take:(NSUInteger)n

仅仅值的前 n 次的变化会继续传递下去。

• distinctUntilChanged

仅当 signal 的值与上一次不同时才会继续传递下去。

• throttle:(NSTimeInterval)interval

在 interval 的时间内发生的值的变化仅传递最后一个。

统一消息传递机制

ReactiveCocoa 统一了 iOS 中几乎所有的消息传递方式：

• Blocks
• Delegates
• Notifications
• Errors
• Target-Action
• KVO
• Method Overriding

// Blocks -> Signal

// Groups code that happens on subscriptions, returns a single value when block
// executes. defer is a general pattern to turn a function into a signal.
//
// This can be used to enclose a long-running function into an
// async signal.

[RACSignal defer:^ {
  // Test for sending a value and completing
  return [RACSignal return:@(arc4random())];
}];

// The same thing as above can be done (more explicitly)
// with createSignal

[RACSignal createSignal:^(id(<RACSubscriber> subscriber) {
  // Perform per-subscription side effects.
  [subscription sendNext:@(arc4random())];
  [subscription sendCompleted];
  return nil;
}];

// Blocks -> Signal

// AFNetworking Example:

[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
  [manager GET:URLString parameters:params
    success:^(AFHTTPRequestOperation *op, id response) {
      [subscriber sendNext:response];
      [subscriber sendCompleted];
    }
    failure:^(AFHTTPRequestOperation *op, NSError *e) {
      [subscriber:sendError:e];
    }];
}];

// Using disposables w/ operations to cancel

[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
  NSOperation *operation = [manager GET:URLString parameters:params
    success:^(AFHTTPRequestOperation *op, id response) {
      [subscriber sendNext:response];
      [subscriber sendCompleted];
    } failure:^(AFHTTPRequestOperation *op, NSError *error) {
      [subscriber sendError:e];
    }];
  return [RACDisposable disposableWithBlock:^ {
    [operation cancel];
  }];
}];


// Core Data : switchToLatest on a search request : automatically cancel old requests.

[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
  RACDisposable *disposable = [RACDisposable new];

  [managedObjectContext performBlock:^ {
    if (disposable.disposed) return;

    NSError *error;
    NSArray *results = [moc performFetch:fetchRequest error:&error];

    if (results != nil) {
      [subscriber sendNext:results];
      [subscriber sendCompleted];
    } else {
      [subscriber sendError:error];
    }
  }];
  return disposable;
}];


// Delegates : creating a signal version of a delegate.

// Shows the general pattern of wrapping delegate callbacks into
// signals. Also how to perform side-effect actions based on the
// subscriber count. In this example, CL is turned on when the
// first subscriber subscribes and turned off after all subscribers
// have unsubscribed.
//
// Notice we are setting `self` as the CL delegate but the
// delegate methods are not implemented - rather subscribed
// to via rac_signalForSelector. This translates
// delegate callbacks into signal values.

// reduceEach is like map. In this case, we use it to return only
// the values in the tuple that matter (the locations / error object).

// flattenMap takes a value and creates a signal from it.

CLLocationManager *locationManager = ...
locationManager.delegate = self; //
static volatile int32_t subscriberCount = 0;

[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
  RACSignal *locations = [[self rac_signalForSeletor:(@selector(...didUpdateLocations:)
    fromProtocol:@protocol(CLLocationManagerDelegate)]
    reduceEach^(id _, NSArray *locations) {
      return locations;
    }];

  RACSignal *error = [[self rac_signalForSeletor:(@selector(...didFailWithError:)
    fromProtocol:@protocol(CLLocationManagerDelegate)]
    reduceEach^(id _, NSError *error) {
      return error;
    }]
    filter:^BOOL (NSError *error) {
      // Documentation says CL will keep trying after kCLErrorLocationUnknown
      return error.code != kCLErrorLocationUnknown;
    }]
    flattenMap:^(NSError *error){
      return [RACSignal error:error]; // create a new signal that will send error.
    }];

    RACDisposable *disposable = [[RACSignal
      merge:@[ locations, error ]]
      subscribe:subscriber];

    // manage side effects if you have multiple subscribers
    if (OSAtomicIncrement32(&subscriberCount) == 1) {
      [locationManager startUpdatingLocation];
    } else {
      [subscriber sendNext:locationManager.location];
    }

    return [RACDisposable disposableWithBlock:^{
      [disposable dispose];
      if (OSAtomicDecrement32(&subscriberCount) == 0) {
        [locationManager stopUpdateLocation];
      }
    }];
}];



// KVO

RACSignal *isReachable = [RACObserve(reachabilityManager, networkReachabilityStatus)
  map:^(NSNumber *networkReachabilityStatus) {
    switch (networkReachabilityStatus.intValue) {
      case AFNetworkReachabilityStatusReachableViaWWAN:
      case AFNetworkReachabilityStatusReachableViaWiFi:
        return @YES;
    }
    return @NO;
  }];

// Notifications

RACSignal *isForeground = [RACSignal merge:@[
  [[defaultCenter rac_addObserverForName:WillEnterForeground ...]
    mapReplace:@YES]
  [[defaultCenter rac_addObserverForName:DidEnterBackground ...]
    mapReplace:@NO]
]];


// Listens to the foreground. When isForeground == @YES, sends values from
// the "didBecomeActive" signal. You can use this from a VM
// to *not* bind to UIApplicationDelegate
RACSignal *hasLaunchedActive = [RACSignal
  if:isForeground
  then:[defaultCenter rac_addObserverForName:DidBecomeActive]
  else:[RACSignal empty]];

UIView Categories

ReactiveCocoa 提供了许多 UIView 的 Category 用来快速将 UI 的事件、属性转换成 signal。例如 rac_textSignal 是 UITextField 的 text 属性转换成的信号；rac_buttonClickedSignal 是 UIAlertView 的按钮点击的信号。

我们订阅这些信号就可以摆脱各种 Delegate。

UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"" message:@"Alert" delegate:nil cancelButtonTitle:@"YES" otherButtonTitles:@"NO", nil];
[[alertView rac_buttonClickedSignal] subscribeNext:^(NSNumber *indexNumber) {
    if ([indexNumber intValue] == 1) {
        NSLog(@"NO");
    } else {
        NSLog(@"YES");
    }
}];
[alertView show];

宏

ReactiveCocoa 提供了一些非常方便的宏。

• RACObserve(self, status)：将 status 转换成一个 signal，当其值发生变化时触发事件。作用与 KVO 相同。
• RAC(self.textField, text)：将某个对象的某个属性绑定到一个 signal 的值上。
• @weakify(self) 和 @strongify(self) 用于避免循环引用。

Ref

• http://www.infoq.com/cn/articles/functional-reactive-programming
• https://github.com/ReactiveCocoa/ReactiveCocoa
• http://www.sprynthesis.com/2014/06/15/why-reactivecocoa/
• http://www.infoq.com/cn/articles/reactivecocoa-ios-new-develop-framework
• https://medium.com/@syshen/reactivecocoa-in-practice-4f04119efc68#.z0m1z8h1q