filter

filter 只會讓滿足條件的值通過，filter接受一個閉包作為引數，該閉包返回一個Bool型別的值，只有結果為true的資料才會被儲存下來

[1, 6, 15, 22].publisher
    .filter { value in
        value > 10
    }
    .sink(receiveCompletion: { _ in
        print("finished")
    }, receiveValue: { value in
        print(value)
    })
/*印出 
15
22
finished
*/

compactMap

compactMap和map的功能類似，只是會自動過濾掉空的元素，它的引數中的閉包支援返回一個可選型別的引數，因此，我們可以把傳進來的資料直接返回，就這麼簡單的一個操作就實現了空值過濾

["1", nil, "2", nil, "3"].publisher
    .compactMap { value -> Int? in
        guard let v = value else {
            return nil
        }
        return Int(v)
    }
    .sink(receiveCompletion: { _ in
        print("finished")
    }, receiveValue: { value in
        print(value)
    })
/*印出 
1
2
3
finished
*/

removeDuplicates

removeDuplicates 過濾掉連續的重複值

[1, 1, 1, 2, 2, 1, 2, 3].publisher
    .removeDuplicates()
    .sink(receiveCompletion: { _ in
        print("finished")
    }, receiveValue: { value in
        print(value)
    })
/*印出 
1
2
1
2
3
finished
*/

replaceEmpty/replaceNil/replaceError

• 如果上游Publisher 是個空的數據流，replaceEmpty會發送指定的值

  Empty<Int, Never>()
      .replaceEmpty(with: 1)
      .sink(receiveCompletion: { _ in
          print("finished")
      }, receiveValue: { value in
          print(value)
      })
  /*印出 
  1
  finished
  */
  

  空的數據流：在收到finish事件前，從來沒有收到任何資料

  Empty：一個只發出完成事件的Publisher

• 將收到的nil轉換為給定的值

  [1, nil, 3].publisher
  		.replaceNil(with: -1)
  		.map { $0! }
  		.sink(receiveCompletion: { _ in
  				print("finished")
  		}, receiveValue: { value in
  				print(value)
   		})
  /*印出 
  1
  -1
  3
  finished
  */
  

• 如果上游Publisher 因錯誤而終止，replaceError會發送指定的值

  enum MyError: Error {
    case failed
  }
  
  var subject = PassthroughSubject<Int, Error>()
  
  subject
    .replaceError(with: 0)
    .sink(receiveCompletion: { completion in
      print(completion)
    }, receiveValue: { int in
      print(int)
    })
  
  subject.send(1)
  subject.send(2)
  subject.send(completion: .failure(MyError.failed))