RxSwift官方使用示例之<過濾及條件操作符>
本文是示例主要來自於官方的例項程式碼，可從這裡下載 ，例項程式碼稍有改動。其中的圖片來自於reactivex.io/
本文主要的作用是檢視例項程式碼並檢視對應例項程式碼的效果，不需要執行專案
操作符目錄
- RxSwift官方使用示例之<建立和訂閱Observables>
- RxSwift官方使用示例之<使用Subjects>
- RxSwift官方使用示例之<組合操作符>
- RxSwift官方使用示例之<轉換操作符>
- RxSwift官方使用示例之<過濾及條件操作符>
- RxSwift官方使用示例之<數學和聚合操作符>
- RxSwift官方使用示例之<可連線操作符>
- RxSwift官方使用示例之<錯誤處理操作符>
debug 操作符會列印所有的 subscriptions, events, disposals
filter
只發出 Observable 序列中滿足指定條件的元素
``` let disposeBag = DisposeBag()
Observable.of(2, 30, 22, 5, 60, 1) .filter { $0 > 10 } .subscribe(onNext: { print($0) }) .disposed(by: disposeBag) ``` 列印
30
22
60
distinctUntilChanged
抑制由 Observable 序列發出的連續重複元素
```
let disposeBag = DisposeBag()
Observable.of(1, 2, 2, 1, 3)
.distinctUntilChanged()
.subscribe(onNext: { print($0) })
.disposed(by: disposeBag)
列印
1
2
1
3
```
elementAt
只發射 Observable 序列指定索引處的元素
``` let disposeBag = DisposeBag()
Observable.of(1, 2, 3, 4)
.element(at: 2)
.subscribe(onNext: { print($0) })
.disposed(by: disposeBag)
列印
3
```
single
僅發射 Observable 序列發射的第一個元素（或第一個滿足條件的元素）。 如果 Observable 序列沒有發出恰好一個元素，則會丟擲錯誤
``` let disposeBag = DisposeBag()
Observable.of("🐱", "🐰", "🐶", "🐸", "🐷", "🐵") .single() .subscribe(onNext: { print($0) }) .disposed(by: disposeBag) ``` 列印
🐱
Unhandled error happened: Sequence contains more than one element.
此時會有錯誤資訊列印，如果想處理該錯誤資訊，可以加上 onError:，如下
``` let disposeBag = DisposeBag()
Observable.of("🐱", "🐰", "🐶", "🐸", "🐷", "🐵") .single() .subscribe(onNext: { print($0) }, onError: { print("出錯啦：", $0) } ) .disposed(by: disposeBag) ``` 列印
🐱
出錯啦： Sequence contains more than one element.
此外，single 還可以加上條件判斷
``` let disposeBag = DisposeBag()
Observable.of("🐱", "🐰", "🐶", "🐸", "🐷", "🐵") .single { $0 == "🐸" } .subscribe { print($0) } .disposed(by: disposeBag)
Observable.of("🐱", "🐰", "🐶", "🐱", "🐰", "🐶") .single { $0 == "🐰" } .subscribe { print($0) } .disposed(by: disposeBag)
Observable.of("🐱", "🐰", "🐶", "🐸", "🐷", "🐵") .single { $0 == "🔵" } .subscribe { print($0) } .disposed(by: disposeBag) ``` 列印
next(🐸)
completed
next(🐰)
error(Sequence contains more than one element.)
error(Sequence doesn't contain any elements.)
從列印可以看出，第一個列印正常，輸出一個結果之後立即completed；第二個列印異常，因為有2個🐰，所以丟擲異常 error(Sequence contains more than one element.)；第三個也異常，因為序列中沒有🔵，丟擲異常error(Sequence doesn't contain any elements.)
take
僅從 Observable 序列的開頭髮出指定數量的元素
```
let disposeBag = DisposeBag()
Observable.of(1, 2, 3, 4) .take(2) .subscribe(onNext: { print($0) }) .disposed(by: disposeBag) ``` 列印
1
2
takeLast
僅從 Observable 序列的末尾發出指定數量的元素
``` let disposeBag = DisposeBag()
Observable.of(1, 2, 3, 4) .take(1) .subscribe(onNext: { print($0) }) .disposed(by: disposeBag) ``` 列印
4
takeWhile
從 Observable 序列的開頭髮出滿足條件的元素
```
let disposeBag = DisposeBag()
Observable.of(1, 2, 3, 4, 5, 6) .take(while: { $0 < 4 } ) .subscribe(onNext: { print($0) }) .disposed(by: disposeBag) ``` 列印
1
2
3
takeUntil
從源 Observable 序列中發出元素，直到引用 Observable 序列發出元素
``` let disposeBag = DisposeBag()
let sourceSequence = PublishSubject
sourceSequence .take(until: referenceSequence) .subscribe { print($0) } .disposed(by: disposeBag)
sourceSequence.onNext("1") sourceSequence.onNext("2") sourceSequence.onNext("3") sourceSequence.onNext("3") sourceSequence.onNext("4") sourceSequence.onNext("5")
referenceSequence.onNext("0")
sourceSequence.onNext("6") sourceSequence.onNext("7") sourceSequence.onNext("8")
referenceSequence.onNext("0")
sourceSequence.onNext("9") ``` 列印
next(1)
next(2)
next(3)
next(3)
next(4)
next(5)
completed
skip
從 Observable 序列的開頭跳過發射指定數量的元素
``` let disposeBag = DisposeBag()
Observable.of(1, 2, 3, 4) .skip(2) .subscribe { print($0) } .disposed(by: disposeBag) ``` 列印
next(3)
next(4)
completed
skipWhile
從 Observable 序列的開頭跳過發射滿足條件的元素
```
let disposeBag = DisposeBag()
Observable.of(1, 2, 3, 4, 5, 6)
.skip(while: { $0 < 4 } )
.subscribe(onNext: { print($0) })
.disposed(by: disposeBag)
列印
4
5
6
```
skipWhileWithIndex
從 Observable 序列的開頭跳過發射滿足條件的元素
這不就是 skipWhile 操作符嗎，不過是帶索引的skip ``` let disposeBag = DisposeBag()
Observable.of("🐱", "🐰", "🐶", "🐸", "🐷", "🐵")
.enumerated()
.skip(while: { $0.index < 3 } )
.subscribe(onNext: { print($0) } )
.disposed(by: disposeBag)
列印
(index: 3, element: "🐸")
(index: 4, element: "🐷")
(index: 5, element: "🐵")
```
skipUntil
跳過從源 Observable 序列中發射元素，直到引用 Observable 序列發射一個元素。
``` let disposeBag = DisposeBag()
let sourceSequence = PublishSubject
sourceSequence .skip(until: referenceSequence) .subscribe(onNext: { print($0) }) .disposed(by: disposeBag)
sourceSequence.onNext("1") sourceSequence.onNext("2") sourceSequence.onNext("3") sourceSequence.onNext("4") sourceSequence.onNext("5")
referenceSequence.onNext("0")
sourceSequence.onNext("6") sourceSequence.onNext("7") sourceSequence.onNext("8")
referenceSequence.onNext("0")
sourceSequence.onNext("9") ``` 列印
6
7
8
9