RxSwift语法笔记

Posted on Edited on In

记录RxSwift在使用过程中的一些常用语法。

说明

按照RxSwift 入坑手册 Part0 - 基础概念格式进行补充。

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)
}

Introduction

主要介绍了 Rx 的基础： Observable 。 Observable<Element> 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。
    （打个岔：协议命名，想起来上午汤哥在微博说的一段话：

另外，我觉得 protocol 名字用形容词会更加语义分明，比如 Swift : Flyable, Killable, Visible。全用名词的话显得比较生硬，比如 Swift : Head, Wings, Ass。

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
from

from 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

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

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：

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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。

（以上内容文档中依旧没有，依旧是我瞎掰的，依旧欢迎有错误当面打脸(￣ε(#￣)☆╰╮(￣▽￣///))

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

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

throttle VS. debounce

throttle 和 debounce 目的都是控制某个时间段内事件序列的速度。但是又有差别：

  • debounce : 空闲控制 空闲时间必须 >= 固定的时间，事件序列才会被放行
  • throttle : 频率控制 事件序列满足 1/delay 才会被放行

一般的按钮点击、用户输入、等都属于频率控制，应该使用 throttle 。

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let searchResults = searchBar.rx.text.orEmpty
    .throttle(0.3, scheduler: MainScheduler.instance)
    .distinctUntilChanged()
    .flatMapLatest { query -> Observable<[Repository]> in
        if query.isEmpty {
            return .just([])
        }
        return searchGitHub(query)
            .catchErrorJustReturn([])
    }
    .observeOn(MainScheduler.instance)

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，就是合并两条队列用的，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，正如百折不撓的米斯特菜所提醒的， zip 就像是拉链一样，两根拉链拉着拉着合并到了一根上：

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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)

merge

merge 就是 merge 啦，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 ---
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switch

当你的事件序列是一个事件序列的序列 (Observable<Observable<T>>) 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。

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

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

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

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

Connectable

坑待填，Xcode 里这个操场跑不起来了。