RxSwift学习之旅-Operators

Operators是RxSwift的基本组成部分,它的重要性就不言而喻了,前面只是提到在我们开发中,当收到observable发出.next的事件时,如果其携带的value并不是我们想要直接拿来使用的那个value,那我们就需要在我们拿到最终的value之前,对数据进行相应的处理

Filtering Operators

Ignoring operators

ignoreElements

ignoreElements能够忽略.next的事件,当我们只关心observable sequence是否结束时使用

example(of: "ignoreElements") {
    let strikes = PublishSubject()
    
    strikes.ignoreElements().subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
    
    strikes.onNext("X")
    strikes.onNext("X")
    strikes.onCompleted()
//    strikes.onError(MyError.myError)
}
--- Example of: ignoreElements ---
completed

上述事例中,我们使用ignoreElements忽略了两次.onNext("X")事件,只有当stries发出onCompleted或onError事件时,我们的订阅才能接收到响应,整个过程如下图所示


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elementAt

有些时候你只关注第n个元素发出的事件,而忽略其它事件elementAt就派上用场了

example(of: "elementAt") { 
    let strikes = PublishSubject()
    
    strikes.elementAt(2).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
    
    strikes.onNext("1")
    strikes.onNext("2")
    strikes.onNext("3")
    strikes.onNext("4")
}
--- Example of: elementAt ---
next(3)
completed

需要注意的是,上述事例中,我们并没有发出completed事件,但是当observable sequence响应到指定下下标元素的.onNext事件后,observable sequence会主动结束。提醒一下,这个下标是从0开始的哦~

那如果我们提前发送了completed事件会怎样呢

example(of: "elementAt") { 
    let strikes = PublishSubject()
    
    strikes.elementAt(2).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
    
    strikes.onNext("1")
    strikes.onNext("2")
    strikes.onCompleted()
    strikes.onNext("3")
    strikes.onNext("4")
}
--- Example of: elementAt ---
error(Argument out of range.)

当订阅还没有响应到指定下标的事件而提前发出.onCompleted()的话,这时候响应的将是一个error。整个过程如下图所示


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filter

filter只允许满足条件的value通过

example(of: "filter") { 
    Observable.of(1,2,3,4,5,6,7,8,9,10).filter({ (value) -> Bool in
        value % 2 == 0
    }).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
}
--- Example of: filter ---
next(2)
next(4)
next(6)
next(8)
next(10)
completed

过程如下图


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Skipping operators

skip

skip跳过前几个

example(of: "skip") { 
    Observable.of("A", "B", "C", "D", "E", "F", "G").skip(3).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
}
--- Example of: skip ---
next(D)
next(E)
next(F)
next(G)
completed

如图


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skipWhile

skipWhile一旦有元素让通过,后面的元素都被允许通过

example(of: "skipWhile") { 
    Observable.of(1,2,3,4,5,6,7,8,9,10,2,3,4).skipWhile({ (value) -> Bool in
        return value < 5
    }).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
}
--- Example of: skipWhile ---
next(5)
next(6)
next(7)
next(8)
next(9)
next(10)
next(2)
next(3)
next(4)
completed

如图


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skipUntil

skipUntil(Observable)直到Observable发出.next事件,才允许通过

example(of: "skipUntil") {
    let subject = PublishSubject()
    let trigger = PublishSubject()
    
    subject.skipUntil(trigger).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
    
    subject.onNext("A")
    subject.onNext("B")
    trigger.onNext("aaa")
    subject.onNext("C")
    subject.onNext("D")
}
--- Example of: skipUntil ---
next(C)
next(D)

直到trigger发出.onNext("aaa")事件,subject才能响应事件,如图


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Taking operators

take

take(n)只取前n个值。取完后自己completed

example(of: "take") { 
    Observable.of(1,2,3,4,5,6,7,8,9).take(3).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
}
--- Example of: take ---
next(1)
next(2)
next(3)
completed

如果n>elememt的个数,则全部取完后结束,如图


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takeWhileWithIndex

takeWhileWithIndex从第一个元素开始取,takeWhileWithIndex闭包的两个参数分别代表序列中的元素和下标,takeWhileWithIndex闭包中的条件只要不成立则终止取值

example(of: "takeWhileWithIndex") { 
    Observable.of(1,2,3,4,3,2,7,8,9).takeWhileWithIndex({ (element, index) -> Bool in
        return element < 5 && index < 3
    }).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
}
--- Example of: takeWhileWithIndex ---
next(1)
next(2)
next(3)
completed
takeUntil

takeUntil(Observable)只要Observable发出.next事件,则序列终止

example(of: "takeUntil") {
    let subject = PublishSubject()
    let trigger = PublishSubject()
    
    subject.takeUntil(trigger).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
    
    subject.onNext("1")
    subject.onNext("2")
    trigger.onNext("X")
    subject.onNext("3")
}
--- Example of: takeUntil ---
next(1)
next(2)
completed

trigger发出.onNext("X")事件,subject终止响应事件,如图


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takeUntil在RxCocoa中有一个很好的应用场景

 someObservable.takeUntil(self.rx.deallocated) .subscribe(onNext: {
      print($0)
 })

Distinct operators

distinctUntilChanged

distinctUntilChanged如果连续的value相同,则被忽略,直到发生改变

example(of: "distinctUntilChanged") {
    Observable.of("A", "A", "B", "B", "A").distinctUntilChanged()
        .subscribe(onNext: {
        print($0)
    }).addDisposableTo(dispose)
}
--- Example of: distinctUntilChanged ---
A
B
A

如图


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distinctUntilChanged(comparer: @escaping (Self.E, Self.E) throws -> Bool)

自定义一个比较器,只有符合比较器条件的值才能通过

example(of: "distinctUntilChanged(comparer: @escaping (Self.E, Self.E) throws -> Bool)") {
    Observable.of(1,2,3,10,10,5,6,70,8,9).distinctUntilChanged({ (element1, element2) -> Bool in
        print("element1:", element1, "element2:", element2)
        return element1 > element2
    }).subscribe({ (event) in
        print(event)
    }).disposed(by: dispose)
}
--- Example of: distinctUntilChanged(comparer: @escaping (Self.E, Self.E) throws -> Bool) ---
next(1)
element1: 1 element2: 2
next(2)
element1: 2 element2: 3
next(3)
element1: 3 element2: 10
next(10)
element1: 10 element2: 10
next(10)
element1: 10 element2: 5
element1: 10 element2: 6
element1: 10 element2: 70
next(70)
element1: 70 element2: 8
element1: 70 element2: 9
completed

上面例子中,自定义比较器element1 > element2,条件不成立,则element1取较大的值,并发射,element2依次往后取与element1进行比较。原理如图

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Transforming Operators

toArray

将序列中的若干元素转换为一个数组元素

example(of: "toArray") { 
    let disposeBag = DisposeBag()
    
    Observable.of("A", "B", "C").toArray().subscribe({ (event) in
        print(event)
    }).disposed(by: disposeBag)
}
--- Example of: toArray ---
next(["A", "B", "C"])
completed
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map

将sequence中的值进行自定义转换

example(of: "map") { 
    let disposeBag = DisposeBag()
    
    Observable.of(1,2,3).map({
        $0 * 2
    }).subscribe(onNext: { (value) in
        print(value)
    }).disposed(by: disposeBag)
}
--- Example of: map ---
2
4
6
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mapWithIndex
example(of: "mapWithIndex") {
    let disposeBag = DisposeBag()
    
    Observable.of(1,2,3,4,5,6).mapWithIndex({ (integer, index) in
        index > 2 ? integer * 2 : integer
    }).subscribe({
        print($0)
    }).disposed(by: disposeBag)
}
--- Example of: mapWithIndex ---
next(1)
next(2)
next(3)
next(8)
next(10)
next(12)
completed
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flatMap

将一个序列发射的值转换成序列,然后将他们压平到一个序列。这也类似于 SequenceType 中的 flatMap。比如说我想将Observable 的序列转换为 Observable 序列

example(of: "flatMap") { 
    struct Student {
        var score: Variable
    }
    
    let disposeBag = DisposeBag()
    
    let xiaoMing = Student(score: Variable(80))
    let xiaoHong = Student(score: Variable(95))
    
    let student = PublishSubject()
    
    student.asObservable().flatMap({
        $0.score.asObservable()
    }).subscribe({
        print($0)
    }).disposed(by: disposeBag)
    
    student.onNext(xiaoHong)
    student.onNext(xiaoMing)
    
    xiaoMing.score.value = 100
    
}
--- Example of: flatMap ---
next(95)
next(80)
next(100)
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flatMapLatest

跟flatMap相比较,flatMapLatest只能响应最后一个元素值的变化

example(of: "flatMapLatest") {
    struct Student {
        var score: Variable
    }
    
    let disposeBag = DisposeBag()
    
    let xiaoMing = Student(score: Variable(80))
    let xiaoHong = Student(score: Variable(95))
    
    let student = PublishSubject()
    
    student.asObservable().flatMapLatest({
        $0.score.asObservable()
    }).subscribe({
        print($0)
    }).disposed(by: disposeBag)
    
    student.onNext(xiaoMing)
    xiaoMing.score.value = 65
    
    student.onNext(xiaoHong)
    xiaoMing.score.value = 88
    xiaoHong.score.value = 100
    
}
--- Example of: flatMapLatest ---
next(80)
next(65)
next(95)
next(100)
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Combining Operators

startWith

指定队列起始元素

example(of: "startWith") { 
    let numbers = Observable.of(2,3,4)
    
    let observable = numbers.startWith(1)
    observable.subscribe(onNext: { (element) in
        print(element)
    }).dispose()
}
--- Example of: startWith ---
1
2
3
4
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Observable.concat

合并两个队列,concat合并的两个sequence元素必须是相同类型

example(of: "Observable.concat") {
    let first = Observable.of(1,2,3)
    let second = Observable.of(4,5,6)
    
    let observable = Observable.concat([first, second])
    observable.subscribe(onNext: { (element) in
        print(element)
    }).dispose()
}
--- Example of: Observable.concat ---
1
2
3
4
5
6
concat
example(of: "concat") { 
    let germanCities = Observable.of("Berlin", "Münich", "Frankfurt")
    let spanishCities = Observable.of("Madrid", "Barcelona", "Valencia")
    let observable = germanCities.concat(spanishCities)
    observable.subscribe(onNext: { value in
        print(value)
    }).dispose()
}
--- Example of: concat ---
Berlin
Münich
Frankfurt
Madrid
Barcelona
Valencia
concat one element
example(of: "concat one element") {  
    let numbers = Observable.of(2,3,4)
    
    let observable = Observable.just(1).concat(numbers)
    observable.subscribe(onNext: { (element) in
        print(element)
    }).dispose()
}
--- Example of: concat one element ---
1
2
3
4

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merge

把两个sequence合并成一个

example(of: "merge") { 
    let left = PublishSubject()
    let right = PublishSubject()
    
    let source = Observable.of(left.asObservable(), right.asObservable())
    let observable = source.merge()
    let disposable = observable.subscribe(onNext: { value in
        print(value)
    })
    
    var leftValues = ["Berlin", "Munich", "Frankfurt"]
    var rightValues = ["Madrid", "Barcelona", "Valencia"]
    repeat {
        if arc4random_uniform(2) == 0 {
            if !leftValues.isEmpty {
                left.onNext("Left:  " + leftValues.removeFirst())
            }
        } else if !rightValues.isEmpty {
            right.onNext("Right: " + rightValues.removeFirst())
        }
    } while !leftValues.isEmpty || !rightValues.isEmpty
    
    disposable.dispose()
}
--- Example of: merge ---
Right: Madrid
Right: Barcelona
Left:  Berlin
Right: Valencia
Left:  Munich
Left:  Frankfurt
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combineLatest

将两个observable绑定,只有当它们都发出事件的时候才能subscribe到它们的最新发送的值

example(of: "combineLatest") { 
    let left = PublishSubject()
    let right = PublishSubject()
    
    let observable = Observable.combineLatest(left,right)
    let disposable = observable.subscribe(onNext: { (left,right) in
        print(left,right)
    })
    
    print("> Sending a value to Left")
    left.onNext("Hello,")
    print("> Sending a value to Right")
    right.onNext("world")
    print("> Sending another value to Right")
    right.onNext("RxSwift")
    print("> Sending another value to Left")
    left.onNext("Have a good day,")
    
    disposable.dispose()
}
--- Example of: combineLatest ---
> Sending a value to Left
> Sending a value to Right
Hello, world
> Sending another value to Right
Hello, RxSwift
> Sending another value to Left
Have a good day, RxSwift
example(of: "combine user choice and value") {
    let choice : Observable =
        Observable.of(.short, .long)
    let dates = Observable.of(Date())
    let observable = Observable.combineLatest(choice, dates) {
        (format, when) -> String in
        let formatter = DateFormatter()
        formatter.dateStyle = format
        return formatter.string(from: when)
    }
    observable.subscribe(onNext: { value in
        print(value)
    })
}
--- Example of: combine user choice and value ---
6/1/17
June 1, 2017
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zip

把多个sequence合并为一个sequence,当所有的observable都产生element的时候,selector function会生成一个对应索引的element

example(of: "zip") {
    enum Weather {
        case cloudy
        case sunny
    }
    let bag = DisposeBag()
    
    let left = Observable.of(.sunny, .cloudy, .cloudy, .sunny)
    let right = Observable.of("Lisbon", "Copenhagen", "London", "Madrid", "Vienna")
    
    let observable = Observable.zip(left, right){ (weather, city) -> String in
        "it's \(weather) in \(city)"
    }
    observable.subscribe(onNext: { (result) in
        print(result)
    }).addDisposableTo(bag)
}
--- Example of: zip ---
it's sunny in Lisbon
it's cloudy in Copenhagen
it's cloudy in London
it's sunny in Madrid
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Triggers

withLatestFrom

将两个 observable sequence 合并到一个 observable sequence ,每当self发出element将获取到第二个 observable sequence 的最新element

example(of: "withLatestFrom") {
    let bag = DisposeBag()
    
    let button = PublishSubject()
    let textField = PublishSubject()
    
    let observable = button.withLatestFrom(textField)
    observable.subscribe(onNext: { (text) in
        print(text)
    }).addDisposableTo(bag)
    
    textField.onNext("Par")
    textField.onNext("Pari")
    textField.onNext("Paris")
    
    button.onNext()
    button.onNext()
}
--- Example of: withLatestFrom ---
Paris
Paris
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sample

每当sampler产生一个.next()事件,将从源sequence中获取最新element,如果在sampler两次事件间隔中源sequence没有新element产生,则获取不到element

example(of: "sample") {
    let bag = DisposeBag()
    
    let button = PublishSubject()
    let textField = PublishSubject()
    
    let observable = textField.sample(button)
    observable.subscribe(onNext: { (text) in
        print(text)
    }).addDisposableTo(bag)
    
    textField.onNext("Par")
    textField.onNext("Pari")
    textField.onNext("Paris")
    
    button.onNext()
    button.onNext()
}
--- Example of: sample ---
Paris
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Switches

amb

两个observable sequence谁先发送element谁就是第一响应者。

example(of: "amb") {
    let left = PublishSubject()
    let right = PublishSubject()

    let observable = left.amb(right)
    let disposable = observable.subscribe(onNext: { value in
        print(value)
    })

    right.onNext("Copenhagen")
    left.onNext("Lisbon")
    left.onNext("London")
    left.onNext("Madrid")
    right.onNext("Vienna")
    disposable.dispose()
}
--- Example of: amb ---
Copenhagen
Vienna
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switchLatest

响应最新的observable sequence中的element

example(of: "switchLatest") {
    let one = PublishSubject()
    let two = PublishSubject()
    let three = PublishSubject()
    
    let source = PublishSubject>()
    let observable = source.switchLatest()
    let dispose = observable.subscribe(onNext: { (value) in
        print(value)
    })
    
    source.onNext(one)
    one.onNext("one first")
    two.onNext("two first")
    
    source.onNext(two)
    one.onNext("one second")
    two.onNext("two second")
    
    source.onNext(three)
    three.onNext("three first")
    one.onNext("one third")
    three.onNext("three second")
    
    source.onNext(two)
    two.onNext("two third")
    three.onNext("three third")
}
--- Example of: switchLatest ---
one first
two second
three first
three second
two third
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reduce

将一个observable sequence中的元素做一个累加,返回一个单一的结果。第一个参数为最初的累加值

example(of: "reduce") { 
    let source = Observable.of(1, 3, 5, 7, 9)
    
    let observable = source.reduce(0, accumulator: { summary, newValue in
        return summary + newValue
    })
   // 可简写为    
   // let observable = source.reduce(0, accumulator: +)
    
    observable.subscribe(onNext: { value in
        print(value)
    })
}
--- Example of: reduce ---
25
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scan

将一个observable sequence中的元素做一个累加,返回每个累加结果。第一个参数为最初的累加值

example(of: "scan") { 
    let source = Observable.of(1,3,5,7,9)
    
    let observable = source.scan(0, accumulator: { (summary, newValue) -> Int in
        return summary + newValue
    }) 
    // 可简写为  
    // let observable = source.scan(0, accumulator: +)
    observable.subscribe(onNext: { (value) in
        print(value)
    })
}
--- Example of: scan ---
1
4
9
16
25
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小结

最后提一下RxSwift所有的Operators都是定义在ObservableType的协议延展里面的,返回值都是 -> RxSwift.Observable,以满足开发者在日常开发中的链式调用,所以在日常开发中,你可以这样写代码

 input.skipWhile { $0 == 0 }
        .filter { $0 < 10 }
        .take(10)
        .toArray()
        .subscribe(onNext: {
            print($0)
        })
        .addDisposableTo(disposeBag)

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