Swift Combine — Operators（常用Filtering类操作符介绍）

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Combine中对 Publisher的值进行操作的方法称为 Operator（操作符）。 Combine中的 Operator通常会生成一个 Publisher，该 Publisher处理传入事件，对其进行转换，然后将更改后的事件发送给 Subscriber。

本篇文章主要介绍一下过滤这一类的操作符。

filter(_: )

filter操作符主要用户过滤数据，比如下面的数据中，将大于5的数输出。

func filterSample() {let intArray = [1, 2, 3, 4, 5, 6, 7, 8, 9]_ = intArray.publisher.filter {$0 > 5}.sink { value inprint("value is : \(value)")}
}

输出为：

value is : 6
value is : 7
value is : 8
value is : 9

tryFilter(_: )

使用tryFilter(_:)来过滤在抛出错误的闭包中求值的元素。如果闭包抛出错误，则Publisher将因该错误而失败终止。

func tryFilterSample() {struct ZeroError: Error {}let intArray = [1, 2, 3, 4, 5, 6, 0, 8, 9]_ = intArray.publisher.tryFilter {if $0 == 0 {throw ZeroError()} else {return $0 > 5}}.sink(receiveCompletion: { completion inprint("Received completion: \(completion)")}, receiveValue: { value inprint("Received value: \(value)")})
}

当tryFilter闭包接收到0时，抛出错误，整个Publisher链结束。上面输出结果为：

Received value: 6
Received completion: failure((extension in CombineLearning_PreviewReplacement_OperatorDemo_1):CombineLearning.OperatorViewModel.(unknown context at $1055a3cdc).(unknown context at $1055a3ce8).ZeroError())

compactMap(_: )

Combine的compactMap(_:)操作符的功能与Swift标准库中的compactMap(_:)类似。
Combine中的compactMap(_:)操作符移除Publisher流中的nil元素，并将非nil元素重新发布给下游订阅者。

func compactMapSample() {let numbers = (0...5)let romanNumeralDict: [Int : String] = [1: "one", 2: "two", 3: "three", 5: "five"]_ = numbers.publisher.compactMap { romanNumeralDict[$0] }.sink { print("\($0)", terminator: " ") }
}

当通过key为4的值的时候为nil了，所以这里将nil去除了。
输出结果为：

one two three five 

tryCompactMap(_: )

tryCompactMap(_: )相比compactMap(_:)除了都能去除nil外，前者还能在闭包内抛出错误。
如果闭包抛出错误，则Publisher流将因该错误而失败终止。

  func tryCompactMapSample() {struct ParseError: Error {}func romanNumeral(from: Int) throws -> String? {let romanNumeralDict: [Int : String] =[1: "one", 2: "two", 3: "three", 4: "four", 5: "five"]guard from != 0 else { throw ParseError() }return romanNumeralDict[from]}let numbers = [6, 5, 4, 3, 2, 1, 0]_ = numbers.publisher.tryCompactMap { try romanNumeral(from: $0) }.sink(receiveCompletion: { print ("\($0)") },receiveValue: { print ("\($0)", terminator: " ") })}

在上面代码中，当取key为6的元素时为nil，这个nil没有继续往下发送。当取key为0的元素时，抛出了错误，随后Publisher流结束。

输出结果为：

five four three two one failure((extension in CombineLearning_PreviewReplacement_OperatorDemo_1):CombineLearning.OperatorViewModel.(unknown context at $104a0bbec).(unknown context at $104a0bbf8).ParseError())

removeDuplicates()

有些时候下游的订阅者不希望收到重复的数据，那么用removeDuplicates方法可以去除重复数据。

func removeDuplicatesSample() {let intArray = [1, 1, 3, 5, 5, 6, 7, 8, 9]_ = intArray.publisher.removeDuplicates().sink { value inprint("value is : \(value)")}
}

上面代码中将数组中重复的数据去除，然后输出。
输出为：

value is : 1
value is : 3
value is : 5
value is : 6
value is : 7
value is : 8
value is : 9

另外需要注意removeDuplicates操作符不会将Publisher作为一个集合去排重，而是随着时间根据上下接收到的数据排重，如果相同的数据不挨着，那么不会认为是重复的。

func removeDuplicatesSample() {let intArray = [1, 1, 3, 5, 5, 6, 7, 1, 5]_ = intArray.publisher.removeDuplicates().sink { value inprint("value is : \(value)")}
}

上面数组中在末尾又添加了1和5，看看输出结果：

value is : 1
value is : 3
value is : 5
value is : 6
value is : 7
value is : 1
value is : 5

first(where:)

first(where:)操作符和filter操作符很相似，filter是找出所有满足条件的数据，而first(where:)操作符是找出第一个满足条件的数据。

func firstWhereSample() {let intArray = [1, 2, 3, 3, 5, 6, 7, 8, 9]_ = intArray.publisher.first {$0 > 5}.sink(receiveCompletion: { completion inprint("Received completion")}, receiveValue: { value inprint("Received value: \(value)")})
}

比如上面代码找出第一个大于5的数据，结果如下：

Received value: 6
Received completion

last(where:)

last(where:)和first(where:)操作符正好相反， last(where:)操作符查找符合条件的最后一个数据。

func lastWhereSample() {let intArray = [1, 5, 3, 7, 2, 6, 4, 8, 9]_ = intArray.publisher.last {$0 < 6}.sink(receiveCompletion: { completion inprint("Received completion")}, receiveValue: { value inprint("Received value: \(value)")})
}

上面代码找出小于6的最后一个元素，输出结果为：

Received value: 4
Received completion

如果是找出大于6的最后一个元素，输出结果为：

Received value: 9
Received completion

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