从对象数组中获取属性值的数组。

There's a class called Employee.

有一个叫Employee的类。

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
    }
}

And I have an array of Employee objects. What I now need is to extract the ids of all those objects in that array into a new array.

我有一个Employee对象数组。我现在需要的是将数组中所有这些对象的id提取到一个新的数组中。

I also found this similar question. But it's in Objective-C so it's using valueForKeyPath to accomplish this.

我也发现了类似的问题。但它在Objective-C中，所以它使用valueForKeyPath来完成这个。

How can I do this in Swift?

我怎么能在Swift中做到这一点呢?

2 个解决方案

#1

You can use the map method, which transform an array of a certain type to an array of another type - in your case, from array of Employee to array of Int:

您可以使用map方法，它将一个特定类型的数组转换为另一个类型的数组——在您的例子中，从雇员数组转换为Int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 }

#2

Swift 3 offers many ways to get an array of property values from an array of similar objects. According to your needs and tastes, you may choose one of the six following Playground code examples to solve your problem.

Swift 3提供了从相似对象数组中获取属性值数组的多种方法。根据您的需要和品味，您可以选择以下六种游戏中的一种来解决您的问题。

---

1. Using map method

Swift provides a map(_:) method for types that conform to Sequence protocol (including Array). (see also Transforming an Array)

Swift为符合序列协议(包括数组)的类型提供一个map(_:)方法。(参见转换数组)

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. Using for loop

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. Using while loop

Note that with Swift, behind the scenes, a for loop is just a while loop over a sequence's iterator (see IteratorProtocol for more details).

注意，对于Swift，在幕后，for循环只是对序列的迭代器的一个while循环(有关更多细节，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. Using a struct that conforms to IteratorProtocol and Sequence protocols

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. Using Collection protocol extension and 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. Using KVC and NSArray's value(forKeyPath:) method

Note that this example requires class Employee to inherit from NSObject.

注意，这个示例要求类Employee从NSObject继承。

import Foundation

class Employee: NSObject {

    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]
}

---

See GitHub's Iterating-over-arrays repo for more examples.

有关更多示例，请参见GitHub的遍历数组repo。