RxSwift概念讲解

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RxSwift 是 ReactiveX 的 Swift 和 iOS/OSX 平台，是 Rx 的 Swift 版本。跟 Rx 一样，可以简化异步操作和事件/数据流。

便利函数(SupportCode)

在进入正题之前，先看下项目里的 SupportCode.swift ，主要为 playground 提供了两个便利函数。

一个是 example 函数，专门用来写示例代码的，统一输出 log 便于标记浏览，同时还能保持变量不污染全局：

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public func example(description: String, action: () -> ()) { 
    print("\n--- \(description) example ---") 
    action() 
}

另一个是 delay 函数，通过 dispatch_after 用来演示延时的：

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public func delay(delay:Double, closure:()->()) { 
    dispatch_after( 
        dispatch_time( 
            DISPATCH_TIME_NOW, 
            Int64(delay * Double(NSEC_PER_SEC)) 
        ), 
        dispatch_get_main_queue(), closure) 
}

前言(Introduction)

主要介绍了 Rx 的基础： Observable 。
Observable 是观察者模式中被观察的对象，相当于一个事件序列 (GeneratorType) ，会向订阅者发送新产生的事件信息。

事件信息分为三种：

  • Next(value) 表示新的事件数据。
  • Completed 表示事件序列的完结。
  • Error 同样表示完结，但是代表异常导致的完结。

被观察者(Observable)

empty

empty 是一个空的序列，它只发送 .Completed 消息。

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example("empty") { 
    let emptySequence: Observable<Int> = empty() 
 
    let subscription = emptySequence 
        .subscribe { event in 
            print(event) 
        } 
} 
 
--- empty example --- 
Completed

never

never 是没有任何元素、也不会发送任何事件的空序列。

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example("never") { 
    let neverSequence: Observable<String> = never() 
 
    let subscription = neverSequence 
        .subscribe { _ in 
            print("This block is never called.") 
        } 
} 
 
--- never example ---

just

just 是只包含一个元素的序列，它会先发送 .Next(value) ，然后发送 .Completed 。

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example("just") { 
    let singleElementSequence = just(32) 
 
    let subscription = singleElementSequence 
        .subscribe { event in 
            print(event) 
        } 
} 
 
--- just example --- 
Next(32) 
Completed

sequenceOf

sequenceOf 可以把一系列元素转换成事件序列。

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example("sequenceOf") { 
    let sequenceOfElements/* : Observable<Int> */ = sequenceOf(0, 1, 2, 3) 
 
    let subscription = sequenceOfElements 
        .subscribe { event in 
            print(event) 
        } 
} 
 
--- sequenceOf example --- 
Next(0) 
Next(1) 
Next(2) 
Next(3) 
Completed

form

form 是通过 asObservable() 方法把 Swift 中的序列 (SequenceType) 转换成事件序列。

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example("from") { 
    let sequenceFromArray = [1, 2, 3, 4, 5].asObservable() 
 
    let subscription = sequenceFromArray 
        .subscribe { event in 
            print(event) 
        } 
} 
 
--- from example --- 
Next(1) 
Next(2) 
Next(3) 
Next(4) 
Next(5) 
Completed

create

create 可以通过闭包创建序列，通过 .on(e: Event) 添加事件。

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example("create") { 
    let myJust = { (singleElement: Int) -> Observable<Int> in 
        return create { observer in 
            observer.on(.Next(singleElement)) 
            observer.on(.Completed) 
 
            return NopDisposable.instance 
        } 
    } 
 
    let subscription = myJust(5) 
        .subscribe { event in 
            print(event) 
        } 
} 
 
--- create example --- 
Next(5) 
Completed

failWith

failWith 创建一个没有元素的序列，只会发送失败 (.Error) 事件。

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example("failWith") { 
    let error = NSError(domain: "Test", code: -1, userInfo: nil) 
 
    let erroredSequence: Observable<Int> = failWith(error) 
 
    let subscription = erroredSequence 
        .subscribe { event in 
            print(event) 
        } 
} 
 
--- failWith example --- 
Error(Error Domain=Test Code=-1 "The operation couldn’t be completed. (Test error -1.)")

deferred

deferred 会等到有订阅者的时候再通过工厂方法创建 Observable 对象，每个订阅者订阅的对象都是内容相同而完全独立的序列。

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example("deferred") { 
    let deferredSequence: Observable<Int> = deferred { 
        print("creating") 
        return create { observer in 
            print("emmiting") 
            observer.on(.Next(0)) 
            observer.on(.Next(1)) 
            observer.on(.Next(2)) 
 
            return NopDisposable.instance 
        } 
    } 
 
    print("go") 
 
    deferredSequence 
        .subscribe { event in 
            print(event) 
    } 
 
    deferredSequence 
        .subscribe { event in 
            print(event) 
        } 
} 
 
--- deferred example --- 
go 
creating 
emmiting 
Next(0) 
Next(1) 
Next(2) 
creating 
emmiting 
Next(0) 
Next(1) 
Next(2)

为什么需要 defferd 这样一个奇怪的家伙呢？其实这相当于是一种延时加载，因为在添加监听的时候数据未必加载完毕，例如下面这个例子：

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example("TestDeferred") { 
    var value: String? = nil 
    var subscription: Observable<String?> = just(value) 
 
    // got value 
    value = "Hello!" 
 
    subscription.subscribe { event in 
        print(event) 
    } 
} 
 
--- TestDeferred example --- 
Next(nil) 
Completed

如果使用 deffered 则可以正常显示想要的数据：

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example("TestDeferred") { 
    var value: String? = nil 
    var subscription: Observable<String?> = deferred { 
        return just(value) 
    } 
 
    // got value 
    value = "Hello!" 
 
    subscription.subscribe { event in 
        print(event) 
    } 
 
} 
 
--- TestDeferred example --- 
Next(Optional("Hello!")) 
Completed

代理和桥梁(Subjects)

接下来是关于 Subject 的内容。 Subject 可以看做是一种代理和桥梁。它既是订阅者又是订阅源，这意味着它既可以订阅其他 Observable 对象，同时又可以对它的订阅者们发送事件。

如果把 Observable 理解成不断输出事件的水管，那 Subject 就是套在上面的水龙头。它既怼着一根不断出水的水管，同时也向外面输送着新鲜水源。如果你直接用水杯接着水管的水，那可能导出来什么王水胶水完全把持不住；如果你在水龙头下面接着水，那你可以随心所欲的调成你想要的水速和水温。

（好吧上面一段文档里没有，是我瞎掰的，如果理解错了还望打脸(￣ε(#￣)☆╰╮(￣▽￣///))

在开始下面的代码之前，先定义一个辅助函数用于输出数据：

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func writeSequenceToConsole<O: ObservableType>(name: String, sequence: O) { 
    sequence 
        .subscribe { e in 
            print("Subscription: \(name), event: \(e)") 
        } 
}

PublishSubject

PublishSubject 会发送订阅者从订阅之后的事件序列。

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example("PublishSubject") { 
    let subject = PublishSubject<String>() 
    writeSequenceToConsole("1", sequence: subject) 
    subject.on(.Next("a")) 
    subject.on(.Next("b")) 
    writeSequenceToConsole("2", sequence: subject) 
    subject.on(.Next("c")) 
    subject.on(.Next("d")) 
} 
 
 
--- PublishSubject example --- 
Subscription: 1, event: Next(a) 
Subscription: 1, event: Next(b) 
Subscription: 1, event: Next(c) 
Subscription: 2, event: Next(c) 
Subscription: 1, event: Next(d) 
Subscription: 2, event: Next(d)

ReplaySubject

ReplaySubject 在新的订阅对象订阅的时候会补发所有已经发送过的数据队列， bufferSize 是缓冲区的大小，决定了补发队列的最大值。
如果 bufferSize 是1，那么新的订阅者出现的时候就会补发上一个事件，如果是2，则补两个，以此类推。

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example("ReplaySubject") { 
    let subject = ReplaySubject<String>.create(bufferSize: 1) 
 
    writeSequenceToConsole("1", sequence: subject) 
    subject.on(.Next("a")) 
    subject.on(.Next("b")) 
    writeSequenceToConsole("2", sequence: subject) 
    subject.on(.Next("c")) 
    subject.on(.Next("d")) 
} 
 
--- ReplaySubject example --- 
Subscription: 1, event: Next(a) 
Subscription: 1, event: Next(b) 
Subscription: 2, event: Next(b) // 补了一个 b 
Subscription: 1, event: Next(c) 
Subscription: 2, event: Next(c) 
Subscription: 1, event: Next(d) 
Subscription: 2, event: Next(d)

BehaviorSubject

BehaviorSubject 在新的订阅对象订阅的时候会发送最近发送的事件，如果没有则发送一个默认值。

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example("BehaviorSubject") { 
    let subject = BehaviorSubject(value: "z") 
    writeSequenceToConsole("1", sequence: subject) 
    subject.on(.Next("a")) 
    subject.on(.Next("b")) 
    writeSequenceToConsole("2", sequence: subject) 
    subject.on(.Next("c")) 
    subject.on(.Completed) 
} 
 
--- BehaviorSubject example --- 
Subscription: 1, event: Next(z) 
Subscription: 1, event: Next(a) 
Subscription: 1, event: Next(b) 
Subscription: 2, event: Next(b) 
Subscription: 1, event: Next(c) 
Subscription: 2, event: Next(c) 
Subscription: 1, event: Completed 
Subscription: 2, event: Completed

Variable

Variable 是基于 BehaviorSubject 的一层封装，它的优势是：不会被显式终结。即：不会收到 .Completed 和 .Error 这类的终结事件，它会主动在析构的时候发送 .Complete。

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example("Variable") { 
    let variable = Variable("z") 
    writeSequenceToConsole("1", sequence: variable) 
    variable.value = "a" 
    variable.value = "b 
    writeSequenceToConsole("2", sequence: variable) 
    variable.value = "c" 
} 
 
--- Variable example --- 
Subscription: 1, event: Next(z) 
Subscription: 1, event: Next(a) 
Subscription: 1, event: Next(b) 
Subscription: 2, event: Next(b) 
Subscription: 1, event: Next(c) 
Subscription: 2, event: Next(c) 
Subscription: 1, event: Completed 
Subscription: 2, event: Completed

转换(Transform)

我们可以对序列做一些转换，类似于 Swift 中 CollectionType 的各种转换。在以前的坑中曾经提到过，可以参考：函数式的函数。

map

map 就是对每个元素都用函数做一次转换，挨个映射一遍。

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example("map") { 
    let originalSequence = sequenceOf(1,2,3) 
 
    originalSequence 
        .map { $0 * 2 } 
        .subscribe { print($0) } 
} 
 
--- map example --- 
Next(2) 
Next(4) 
Next(6) 
Completed

flatMap

map 在做转换的时候很容易出现『升维』的情况，即：转变之后，从一个序列变成了一个序列的序列。

什么是『升维』？在集合中我们可以举这样一个例子，我有一个好友列表 [p1, p2, p3]，那么如果要获取我好友的好友的列表，可以这样做：

myFriends.map { $0.getFriends() }
结果就成了 [[p1-1, p1-2, p1-3], [p2-1], [p3-1, p3-2]] ，这就成了好友的好友列表的列表了。这就是一个『升维』的例子。

在 Swift 中，我们可以用 flatMap 过滤掉 map 之后的 nil 结果。
在 Rx 中， flatMap 可以把一个序列转换成一组序列，然后再把这一组序列『拍扁』成一个序列。

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example("flatMap") { 
    let sequenceInt = sequenceOf(1, 2, 3) 
    let sequenceString = sequenceOf("A", "B", "--") 
 
    sequenceInt 
        .flatMap { int in 
            sequenceString 
        } 
        .subscribe { 
            print($0) 
        } 
} 
 
--- flatMap example --- 
Next(A) 
Next(B) 
Next(--) 
Next(A) 
Next(B) 
Next(--) 
Next(A) 
Next(B) 
Next(--) 
Completed

scan

scan 有点像 reduce ，它会把每次的运算结果累积起来，作为下一次运算的输入值。 

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example("scan") { 
    let sequenceToSum = sequenceOf(0, 1, 2, 3, 4, 5) 
 
    sequenceToSum 
        .scan(0) { acum, elem in 
            acum + elem 
        } 
        .subscribe { 
            print($0) 
        } 
} 
 
--- scan example --- 
Next(0) 
Next(1) 
Next(3) 
Next(6) 
Next(10) 
Next(15) 
Completed

过滤(Filtering)

除了上面的各种转换，我们还可以对序列进行过滤。

filter

filter 只会让符合条件的元素通过。

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example("filter") { 
    let subscription = sequenceOf(0, 1, 2, 3, 4, 5, 6, 7, 8, 9) 
        .filter { 
            $0 % 2 == 0 
        } 
        .subscribe { 
            print($0) 
        } 
} 
 
--- filter example --- 
Next(0) 
Next(2) 
Next(4) 
Next(6) 
Next(8) 
Completed

distinctUntilChanged

distinctUntilChanged 会废弃掉重复的事件。 

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example("distinctUntilChanged") { 
    let subscription = sequenceOf(1, 2, 3, 1, 1, 4) 
        .distinctUntilChanged() 
        .subscribe { 
            print($0) 
        } 
} 
 
--- distinctUntilChanged example --- 
Next(1) 
Next(2) 
Next(3) 
Next(1) 
Next(4) 
Completed

take

take 只获取序列中的前 n 个事件，在满足数量之后会自动 .Completed 。

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example("take") { 
    let subscription = sequenceOf(1, 2, 3, 4, 5, 6) 
        .take(3) 
        .subscribe { 
            print($0) 
        } 
} 
 
--- take example --- 
Next(1) 
Next(2) 
Next(3) 
Completed

运算(Combining)

这部分是关于序列的运算，可以将多个序列源进行组合拼装成一个新的事件序列。

startWith

startWith 会在队列开始之前插入一个事件元素。 

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example("startWith") { 
    let subscription = sequenceOf(4, 5, 6) 
        .startWith(3) 
        .subscribe { 
            print($0) 
        } 
} 
 
--- startWith example --- 
Next(3) 
Next(4) 
Next(5) 
Next(6) 
Completed

combineLatest

如果存在两条事件队列，需要同时监听，那么每当有新的事件发生的时候，combineLatest 会将每个队列的最新的一个元素进行合并。

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example("combineLatest 1") { 
    let intOb1 = PublishSubject<String>() 
    let intOb2 = PublishSubject<Int>() 
 
    combineLatest(intOb1, intOb2) { 
        "\($0) \($1)" 
        } 
        .subscribe { 
            print($0) 
        } 
 
    intOb1.on(.Next("A")) 
    intOb2.on(.Next(1)) 
    intOb1.on(.Next("B")) 
    intOb2.on(.Next(2)) 
} 
 
--- combineLatest 1 example --- 
Next(A 1) 
Next(B 1) 
Next(B 2)

zip

zip 人如其名，就是压缩两条队列用的，不过它会等到两个队列的元素一一对应地凑齐了之后再合并。 

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example("zip 1") { 
    let intOb1 = PublishSubject<String>() 
    let intOb2 = PublishSubject<Int>() 
    zip(intOb1, intOb2) { 
        "\($0) \($1)" 
        } 
        .subscribe { 
            print($0) 
        } 
    intOb1.on(.Next("A")) 
    intOb2.on(.Next(1)) 
    intOb1.on(.Next("B")) 
    intOb1.on(.Next("C")) 
    intOb2.on(.Next(2)) 
} 
 
--- zip 1 example --- 
Next(A 1) 
Next(B 2)

marge

merge 就是把两个队列按照顺序组合在一起。

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example("merge 1") { 
    let subject1 = PublishSubject<Int>() 
    let subject2 = PublishSubject<Int>() 
 
    sequenceOf(subject1, subject2) 
        .merge() 
        .subscribeNext { int in 
            print(int) 
        } 
 
    subject1.on(.Next(1)) 
    subject1.on(.Next(2)) 
    subject2.on(.Next(3)) 
    subject1.on(.Next(4)) 
    subject2.on(.Next(5)) 
} 
 
--- merge 1 example --- 
1 
2 
3 
4 
5

switch

当你的事件序列是一个事件序列的序列 (Observable<Observable>) 的时候，（可以理解成二维序列？），可以使用 switch 将序列的序列平铺成一维，并且在出现新的序列的时候，自动切换到最新的那个序列上。
和 merge 相似的是，它也是起到了将多个序列『拍平』成一条序列的作用。

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example("switchLatest") { 
    let var1 = Variable(0) 
 
    let var2 = Variable(200) 
 
    // var3 is like an Observable<Observable<Int>> 
    let var3 = Variable(var1) 
 
    let d = var3 
        .switchLatest() 
        .subscribe { 
            print($0) 
        } 
 
    var1.value = 1 
    var1.value = 2 
    var1.value = 3 
    var1.value = 4 
 
    var3.value = var2 
    var2.value = 201 
    var1.value = 5 
 
    var3.value = var1 
    var2.value = 202 
    var1.value = 6 
} 
 
--- switchLatest example --- 
Next(0) 
Next(1) 
Next(2) 
Next(3) 
Next(4) 
Next(200) 
Next(201) 
Next(5) 
Next(6)

注意，虽然都是『拍平』，但是和 flatmap 是不同的， flatmap 是将一条序列变成另一条序列，而这变换过程会让维度变高，所以需要『拍平』，而 switch 是将本来二维的序列（序列的序列）拍平成了一维的序列。

异常处理(Error Handling)

在事件序列中，遇到异常也是很正常的事情，有以下几种处理异常的手段。

catchError

catchError 可以捕获异常事件，并且在后面无缝接上另一段事件序列，丝毫没有异常的痕迹。

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example("catchError 1") { 
    let sequenceThatFails = PublishSubject<Int>() 
    let recoverySequence = sequenceOf(100, 200) 
 
    sequenceThatFails 
        .catchError { error in 
            return recoverySequence 
        } 
        .subscribe { 
            print($0) 
        } 
 
    sequenceThatFails.on(.Next(1)) 
    sequenceThatFails.on(.Next(2)) 
    sequenceThatFails.on(.Error(NSError(domain: "Test", code: 0, userInfo: nil))) 
} 
 
--- catchError 1 example --- 
Next(1) 
Next(2) 
Next(100) 
Next(200) 
Completed

retry

retry 顾名思义，就是在出现异常的时候会再去从头订阅事件序列，妄图通过『从头再来』解决异常。

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example("retry") { 
    var count = 1 // bad practice, only for example purposes 
    let funnyLookingSequence: Observable<Int> = create { observer in 
        let error = NSError(domain: "Test", code: 0, userInfo: nil) 
        observer.on(.Next(0)) 
        observer.on(.Next(1)) 
        if count < 2 { 
            observer.on(.Error(error)) 
            count++ 
        } 
        observer.on(.Next(2)) 
        observer.on(.Completed) 
 
        return NopDisposable.instance 
    } 
 
    funnyLookingSequence 
        .retry() 
        .subscribe { 
            print($0) 
        } 
} 
 
--- retry example --- 
Next(0) 
Next(1) 
Next(0) 
Next(1) 
Next(2) 
Completed

事件订阅(Utility)

这里列举了针对事件序列的一些方法。

subscribe

subscribe 在前面已经接触过了，有新的事件就会触发。

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example"subscribe") { 
    let sequenceOfInts = PublishSubject<Int>() 
 
    sequenceOfInts 
        .subscribe { 
            print($0) 
        } 
 
    sequenceOfInts.on(.Next(1)) 
    sequenceOfInts.on(.Completed) 
} 
 
--- subscribe example --- 
Next(1) 
Completed

subscribeNext

subscribeNext 也是订阅，但是只订阅 .Next 事件。 

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example("subscribeNext") { 
    let sequenceOfInts = PublishSubject<Int>() 
 
    sequenceOfInts 
        .subscribeNext { 
            print($0) 
        } 
 
    sequenceOfInts.on(.Next(1)) 
    sequenceOfInts.on(.Completed) 
} 
 
--- subscribeNext example --- 
1

subscribeCompleted

subscribeCompleted 是只订阅 .Completed 完成事件。 

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example("subscribeCompleted") { 
    let sequenceOfInts = PublishSubject<Int>() 
 
    sequenceOfInts 
        .subscribeCompleted { 
            print("It's completed") 
        } 
 
    sequenceOfInts.on(.Next(1)) 
    sequenceOfInts.on(.Completed) 
} 
 
--- subscribeCompleted example --- 
It's completed

subscribeError

subscribeError 只订阅 .Error 失败事件。 

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example("subscribeError") { 
    let sequenceOfInts = PublishSubject<Int>() 
 
    sequenceOfInts 
        .subscribeError { error in 
            print(error) 
        } 
 
    sequenceOfInts.on(.Next(1)) 
    sequenceOfInts.on(.Error(NSError(domain: "Examples", code: -1, userInfo: nil))) 
} 
 
--- subscribeError example --- 
Error Domain=Examples Code=-1 "The operation couldn’t be completed. (Examples error -1.)"

doOn

doOn 可以监听事件，并且在事件发生之前调用。 

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example("doOn") { 
    let sequenceOfInts = PublishSubject<Int>() 
 
    sequenceOfInts 
        .doOn { 
            print("Intercepted event \($0)") 
        } 
        .subscribe { 
            print($0) 
        } 
 
    sequenceOfInts.on(.Next(1)) 
    sequenceOfInts.on(.Completed) 
} 
 
--- doOn example --- 
Intercepted event Next(1) 
Next(1) 
Intercepted event Completed 
Completed

逻辑判断(Conditional)

我们可以对多个事件序列做一些复杂的逻辑判断。

takeUntil

takeUntil 其实就是 take ，它会在终于等到那个事件之后触发 .Completed 事件。

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example("takeUntil") { 
    let originalSequence = PublishSubject<Int>() 
    let whenThisSendsNextWorldStops = PublishSubject<Int>() 
 
    originalSequence 
        .takeUntil(whenThisSendsNextWorldStops) 
        .subscribe { 
            print($0) 
        } 
 
    originalSequence.on(.Next(1)) 
    originalSequence.on(.Next(2)) 
 
    whenThisSendsNextWorldStops.on(.Next(1)) 
 
    originalSequence.on(.Next(3)) 
} 
 
--- takeUntil example --- 
Next(1) 
Next(2) 
Completed

takeWhile

takeWhile 则是可以通过状态语句判断是否继续 take 。

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example("takeWhile") { 
    let sequence = PublishSubject<Int>() 
    sequence 
        .takeWhile { int in 
            int < 2 
        } 
        .subscribe { 
            print($0) 
        } 
    sequence.on(.Next(1)) 
    sequence.on(.Next(2)) 
    sequence.on(.Next(3)) 
} 
 
--- takeWhile example --- 
Next(1) 
Completed

集合运算(Aggregate)

我们可以对事件序列做一些集合运算。

concat

concat 可以把多个事件序列合并起来。

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example("concat") { 
    let var1 = BehaviorSubject(value: 0) 
    let var2 = BehaviorSubject(value: 200) 
 
    // var3 is like an Observable<Observable<Int>> 
    let var3 = BehaviorSubject(value: var1) 
 
    let d = var3 
        .concat() 
        .subscribe { 
            print($0) 
        } 
 
    var1.on(.Next(1)) 
    var1.on(.Next(2)) 
 
    var3.on(.Next(var2)) 
 
    var2.on(.Next(201)) 
 
    var1.on(.Next(3)) 
    var1.on(.Completed) 
 
    var2.on(.Next(202)) 
} 
 
--- concat example --- 
Next(0) 
Next(1) 
Next(2) 
Next(3) 
Next(201) 
Next(202)

reduce

这里的 reduce 和 CollectionType 中的 reduce 是一个意思，都是指通过对一系列数据的运算最后生成一个结果。

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example("reduce") { 
    sequenceOf(0, 1, 2, 3, 4, 5, 6, 7, 8, 9) 
        .reduce(0, +) 
        .subscribe { 
            print($0) 
        } 
} 
 
--- reduce example --- 
Next(45) 
Completed

转载自 大神都在看的RxSwift的完全入坑手册

另外附上我在实际应用中的示例