オブジェクト配列からプロパティ値の配列を取得する

というクラスがありEmployeeます。

class Employee {

    var id: Int
    var firstName: String
    var lastName: String
    var dateOfBirth: NSDate?

    init(id: Int, firstName: String, lastName: String) {
        self.id = id
        self.firstName = firstName
        self.lastName = lastName
    }
}

そして、私はEmployeeオブジェクトの配列を持っています。今必要なのはid、その配列内のすべてのオブジェクトのsを新しい配列に抽出することです。

私もこの同様の質問を見つけました。しかし、それはObjective-Cにあるため、valueForKeyPathこれを達成するために使用しています。

Swiftでこれを行うにはどうすればよいですか？

map特定のタイプの配列を別のタイプの配列に変換するメソッドを使用できます-この場合、配列のEmployee配列からInt：

var array = [Employee]()
array.append(Employee(id: 4, firstName: "", lastName: ""))
array.append(Employee(id: 2, firstName: "", lastName: ""))

let ids = array.map { $0.id }

Swift 5は、類似したオブジェクトの配列からプロパティ値の配列を取得する多くの方法を提供します。必要に応じて、次の6つのPlaygroundコード例のいずれかを選択して問題を解決できます。

1. mapメソッドの使用

Swiftでは、Sequenceプロトコルに準拠する型にmap(_:)メソッドがあります。次のサンプルコードは、その使用方法を示しています。

class Employee {
    
    let id: Int, firstName: String, lastName: String
    
    init(id: Int, firstName: String, lastName: String) {
        self.id = id
        self.firstName = firstName
        self.lastName = lastName
    }

}

let employeeArray = [
    Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
    Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
    Employee(id: 4, firstName: "Hans", lastName: "Passant")
]

let idArray = employeeArray.map({ (employee: Employee) -> Int in
    employee.id
})
// let idArray = employeeArray.map { $0.id } // also works
print(idArray) // prints [1, 2, 4]

2. forループの使用

class Employee {
    
    let id: Int, firstName: String, lastName: String

    init(id: Int, firstName: String, lastName: String) {
        self.id = id
        self.firstName = firstName
        self.lastName = lastName
    }

}

let employeeArray = [
    Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
    Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
    Employee(id: 4, firstName: "Hans", lastName: "Passant")
]

var idArray = [Int]()    
for employee in employeeArray {
    idArray.append(employee.id)
}
print(idArray) // prints [1, 2, 4]

3. whileループの使用

Swiftでは、舞台裏では、forループはのイテレータwhileに対するループにすぎませんsequence（詳細については、IteratorProtocolを参照してください）。

class Employee {
    
    let id: Int, firstName: String, lastName: String
    
    init(id: Int, firstName: String, lastName: String) {
        self.id = id
        self.firstName = firstName
        self.lastName = lastName
    }

}

let employeeArray = [
    Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
    Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
    Employee(id: 4, firstName: "Hans", lastName: "Passant")
]

var idArray = [Int]()
var iterator = employeeArray.makeIterator()    
while let employee = iterator.next() {
    idArray.append(employee.id)
}
print(idArray) // prints [1, 2, 4]

4. とプロトコルにstruct準拠したの使用IteratorProtocolSequence

class Employee {
    
    let id: Int, firstName: String, lastName: String
    
    init(id: Int, firstName: String, lastName: String) {
        self.id = id
        self.firstName = firstName
        self.lastName = lastName
    }
    
}

struct EmployeeSequence: Sequence, IteratorProtocol {
    
    let employeeArray: [Employee]
    private var index = 0
    
    init(employeeArray: [Employee]) {
        self.employeeArray = employeeArray
    }
    
    mutating func next() -> Int? {
        guard index < employeeArray.count else { return nil }
        defer { index += 1 }
        return employeeArray[index].id
    }
    
}

let employeeArray = [
    Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
    Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
    Employee(id: 4, firstName: "Hans", lastName: "Passant")
]
let employeeSequence = EmployeeSequence(employeeArray: employeeArray)
let idArray = Array(employeeSequence)
print(idArray) // prints [1, 2, 4]

5. Collectionプロトコル拡張とAnyIterator

class Employee {
    
    let id: Int, firstName: String, lastName: String
    
    init(id: Int, firstName: String, lastName: String) {
        self.id = id
        self.firstName = firstName
        self.lastName = lastName
    }

}

extension Collection where Iterator.Element: Employee {
    
    func getIDs() -> Array<Int> {
        var index = startIndex
        let iterator: AnyIterator<Int> = AnyIterator {
            defer { index = self.index(index, offsetBy: 1) }
            return index != self.endIndex ? self[index].id : nil
        }
        return Array(iterator)
    }
    
}

let employeeArray = [
    Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
    Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
    Employee(id: 4, firstName: "Hans", lastName: "Passant")
]

let idArray = employeeArray.getIDs()
print(idArray) // prints [1, 2, 4]

6. KVCとNSArrayのvalue(forKeyPath:)メソッドを使用する

この例では、class Employeeから継承する必要があることに注意してくださいNSObject。

import Foundation

class Employee: NSObject {

    @objc let id: Int, firstName: String, lastName: String

    init(id: Int, firstName: String, lastName: String) {
        self.id = id
        self.firstName = firstName
        self.lastName = lastName
    }

}

let employeeArray = [
    Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
    Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
    Employee(id: 4, firstName: "Hans", lastName: "Passant")
]

let employeeNSArray = employeeArray as NSArray
if let idArray = employeeNSArray.value(forKeyPath: #keyPath(Employee.id)) as? [Int] {
    print(idArray) // prints [1, 2, 4]
}