RAC3创建Signal

项目中添加ReactiveCocoa最简单的方式是使用Carthage,创建如下的Cartfile即可：

github "ReactiveCocoa/ReactiveCocoa" "v3.0-beta.1"

在项目路径下执行carthage update

ReactiveCocoa3.0包含全新的Swift-API，同时也支持Objective-C，详情参考Change Log。ReactiveCocoa3.0存在两种不同类型的Signal,一种是旧的Obj-C风格的RACSignal，另一种是新的Swift风格的Signal.
Swift版signal有一个非常重要的功能是使用的泛型

class Signal<T, E: ErrorType> { 
  ...
}

类型参数T表示signal发出的next事件所带的参数类型，参数E表示signal发出的error事件所带的参数类型，并且必须实现ErrorType协议。
创建Swift版signal和Objective-C版的方式类型。如下是创建一个信号，每秒会发送一个next事件

    func createSignal() -> Signal<String, NSError> {
        var count = 0
        return Signal {
            sink in
            NSTimer.schedule(repeatInterval: 1.0) {
                timer in
                sink.sendNext("tick#\(count++)")
            }
            return nil
        }
    }

Signal的构造函数参数是一个generator:ReactiveCocoa.Observer<Value, Error>，这里使用闭包来构造一个Observer传入Signal的构造函数。闭包中传入的参数就是之前传入的Observer

其中NSTimer.schedule是扩展方法，详情请见NSTimer+Closure.swift

观察Singal

有多种方式来观察或者订阅signal。最简单的方式是使用observe方法
下面是使用observe观察signal

let signal = createSignal()
signal.observeNext{print($0)}

输出：

tick #0
tick #1
tick #2
tick #3
tick #4

另一种方式，可以提供一个观察signal发送事件的对象

createSignal().observe({
   event in
   switch event {
         case let .Next(data):
             print(data)
         default:
             break
   }
})

闭包中的event是Event类型，该类型是一个枚举类型

public enum Event<Value, Error : ErrorType> {
    /// A value provided by the signal.
    case Next(Value)
    /// The signal terminated because of an error. No further events will be
    /// received.
    case Failed(Error)
    /// The signal successfully terminated. No further events will be received.
    case Completed
    /// Event production on the signal has been interrupted. No further events
    /// will be received.
    case Interrupted
    /// Whether this event indicates signal termination (i.e., that no further
    /// events will be received).
    public var isTerminating: Bool { get }
    /// Lifts the given function over the event's value.
    public func map<U>(f: Value -> U) -> ReactiveCocoa.Event<U, Error>
    /// Lifts the given function over the event's error.
    public func mapError<F>(f: Error -> F) -> ReactiveCocoa.Event<Value, F>
    /// Unwraps the contained `Next` value.
    public var value: Value? { get }
    /// Unwraps the contained `Error` value.
    public var error: Error? { get }
}