flatMap/compactMap （swift 5.3）的使用

- 不返回nil

flatMap/compactMap处理返回后的数组不存在nil，同时它会把Optional解包

看下map和flatMap/compactMap实现方式如下

let colors = ["red", "yellow", "green", ""]
let colorsOfMap = colors.map { item -> Int? in
    let length = item.count
    guard length > 0 else {
        return nil
    }
    
    return length
}

print(colorsOfMap)

结果是

[Optional(3), Optional(6), Optional(5), nil]

flatMap/compactMap实现方式如下

- flatMap

let colorsOfFlatMap = colors.flatMap { item ->Int? in
    let length = item.count
    guard length > 0 else {
        return nil
    }

    return length
}

print(colorsOfFlatMap)

- compactMap

let colorsOfFlatMap = colors.compactMap { item ->Int? in
    let length = item.count
    guard length > 0 else {
        return nil
    }
    
    return length
}

print(colorsOfFlatMap)

这里之所以用compactMap，是因为'flatMap' is deprecated Please use compactMap(_:)

结果是

[3, 6, 5]

- 打开数组

compactMap能把（二维、N维）数组一同打开变成一个新的数组

let array = [[1,2,3],[4,5,6],[7,8,9]]

// 对比
let arr1 = array.map { $0 }
print(arr1)

let arr2 = array.flatMap { $0 }
print(arr2)

结果分别是

[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

[1, 2, 3, 4, 5, 6, 7, 8, 9]

- 合并数组

compactMap能把不同的数组合并为一个数组，合并后的数组的个数是要合并两个数组个数的乘积

let animals = ["cat", "dog", "pig"]
let counts = [1,2,3]

let newArray = counts.flatMap { count in
    animals.map({ animal in
        return animal + "\(count)"
    })
}

print(newArray)

结果是

["cat1", "dog1", "pig1", "cat2", "dog2", "pig2", "cat3", "dog3", "pig3"]