从对象数组中获取属性值数组

Question

有一个类叫Employee。

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
    }
}

我有一个Employee 对象数组。我现在需要的是将该数组中所有这些对象的ids 提取到一个新数组中。

我也发现了类似的question。但它在 Objective-C 中，所以它使用 valueForKeyPath 来完成此操作。

如何在 Swift 中做到这一点？

Answer 1

您可以使用map 方法，它将某种类型的数组转换为另一种类型的数组 - 在您的情况下，从Employee 的数组到Int 的数组：

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

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

Answer 2

Swift 5 提供了许多方法来从类似对象的数组中获取属性值数组。根据您的需要，您可以选择以下六个 Playground 代码示例之一来解决您的问题。

---

1。使用map方法

使用 Swift，符合Sequence 协议的类型有一个map(_:) 方法。下面的示例代码展示了如何使用它：

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。使用for循环

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。使用while循环

请注意，对于 Swift，在幕后，for 循环只是在 sequence 的迭代器上的 while 循环（有关详细信息，请参阅 IteratorProtocol）。

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。使用符合IteratorProtocol 和Sequence 协议的struct

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。使用Collection 协议扩展和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。使用KVC和NSArray的value(forKeyPath:)方法

请注意，此示例要求 class Employee 继承自 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]
}