map compactmap flatmap操作符的区别

之前已经使用过了map操作符：

[1, 2, 3]
    .publisher
    .map { int in "Number: \(int)" }
    .sink(receiveValue: { int in
        print(int)
    })

输出：

Number: 1
Number: 2
Number: 3

compactmap可用于过滤nil

[1, 2, nil, 3, nil, 4, nil, 5].publisher
    .compactMap { $0 }
    .sink(receiveValue: { int in
        print("Received \(int)")
    })

输出：

Received 1
Received 2
Received 3
Received 4
Received 5

flatmap可用于将数组降维，例如 [[1, 2], [3, 4]] -> [1, 2, 3, 4]

[1, 2, 3].publisher.flatMap({ int in
  return (0..<int).publisher
  }).sink(receiveCompletion: { _ in }, receiveValue: { value in
    print("value: \(value)")
  })

输出

value: 0
value: 0
value: 1
value: 0
value: 1
value: 2

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