编辑/更新:Xcode 11.4•Swift 5.2

请检查代码中的注释

IntegerField.swift文件内容:

import UIKit

class IntegerField: UITextField {

    // returns the textfield contents, removes non digit characters and converts the result to an integer value
    var value: Int { string.digits.integer ?? 0 }

    var maxValue: Int = 999_999_999
    private var lastValue: Int = 0

    override func willMove(toSuperview newSuperview: UIView?) {
        // adds a target to the textfield to monitor when the text changes
        addTarget(self, action: #selector(editingChanged), for: .editingChanged)
        // sets the keyboard type to digits only
        keyboardType = .numberPad
        // set the text alignment to right
        textAlignment = .right
        // sends an editingChanged action to force the textfield to be updated
        sendActions(for: .editingChanged)
    }
    // deletes the last digit of the text field
    override func deleteBackward() {
        // note that the field text property default value is an empty string so force unwrap its value is safe
        // note also that collection remove at requires a non empty collection which is true as well in this case so no need to check if the collection is not empty.
        text!.remove(at: text!.index(before: text!.endIndex))
        // sends an editingChanged action to force the textfield to be updated
        sendActions(for: .editingChanged)
    }
    @objc func editingChanged() {
        guard value <= maxValue else {
            text = Formatter.decimal.string(for: lastValue)
            return
        }
        // This will format the textfield respecting the user device locale and settings
        text = Formatter.decimal.string(for: value)
        print("Value:", value)
        lastValue = value
    }
}

您还需要将这些扩展添加到您的项目中:

扩展UITextField.swift文件内容:

import UIKit
extension UITextField {
    var string: String { text ?? "" }
}

Formatter.swift文件内容:

import Foundation
extension Formatter {
    static let decimal = NumberFormatter(numberStyle: .decimal)
}

扩展NumberFormatter.swift文件内容:

import Foundation
extension NumberFormatter {
    convenience init(numberStyle: Style) {
        self.init()
        self.numberStyle = numberStyle
    }
}

StringProtocol.swift文件内容:

extension StringProtocol where Self: RangeReplaceableCollection {
    var digits: Self { filter(\.isWholeNumber) }
    var integer: Int? { Int(self) }
}

示例项目

转换字符串值为整数在Swift 4

let strValue:String = "100"
let intValue = strValue as! Int
var intValueFromString:Int = strValue as! Int
or
var intValueFromString = Int(strValue)!

这对我很有用

var a:Int? = Int(userInput.text!)

在Swift 4中:

extension String {            
    var numberValue:NSNumber? {
        let formatter = NumberFormatter()
        formatter.numberStyle = .decimal
        return formatter.number(from: self)
    }
}
let someFloat = "12".numberValue

Swift 2.0+的更新答案:

toInt()方法给出了一个错误，因为它是从Swift 2.x中的String中删除的。相反，Int类型现在有一个接受String的初始化式:

let a: Int? = Int(firstTextField.text)
let b: Int? = Int(secondTextField.text)

对于Swift3.x

extension String {
    func toInt(defaultValue: Int) -> Int {
        if let n = Int(self.trimmingCharacters(in: CharacterSet.whitespacesAndNewlines)) {
            return n
        } else {
            return defaultValue
        }
    }
}