Day19 Combine 06 - Operators 类型介绍 : 过滤操作符

filter

filter 只会让满足条件的值通过，filter接受一个闭包作为引数，该闭包返回一个Bool型别的值，只有结果为true的资料才会被储存下来

[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的功能类似，只是会自动过滤掉空的元素，它的引数中的闭包支援返回一个可选型别的引数，因此，我们可以把传进来的资料直接返回，就这麽简单的一个操作就实现了空值过滤

["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 过滤掉连续的重复值

[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 是个空的数据流，replaceEmpty会发送指定的值

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

  空的数据流：在收到finish事件前，从来没有收到任何资料

  Empty：一个只发出完成事件的Publisher

• 将收到的nil转换为给定的值

  [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 因错误而终止，replaceError会发送指定的值

  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))