//  To convert user input (i.e string) to int for calculation.I did this , and it works.


    let num:Int? = Int(firstTextField.text!);

    let sum:Int = num!-2

    print(sum);

用这个:

// get the values from text boxes
    let a:Double = firstText.text.bridgeToObjectiveC().doubleValue
    let b:Double = secondText.text.bridgeToObjectiveC().doubleValue

//  we checking against 0.0, because above function return 0.0 if it gets failed to convert
    if (a != 0.0) && (b != 0.0) {
        var ans = a + b
        answerLabel.text = "Answer is \(ans)"
    } else {
        answerLabel.text = "Input values are not numberic"
    }

OR

使你的UITextField KeyboardType为DecimalTab从你的XIB或故事板，并删除任何if条件做任何计算，即。

var ans = a + b
answerLabel.text = "Answer is \(ans)"

因为键盘类型是DecimalPad，没有机会输入其他0-9或。

希望这对你有帮助!!

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)

因为字符串可能包含非数字字符，所以应该使用保护来保护操作。例子:

guard let labelInt:Int = Int(labelString) else {
    return
}

useLabelInt()

@IBAction func calculateAclr(_ sender: Any) {
    if let addition = addition(arrayString: [txtBox1.text, txtBox2.text, txtBox3.text]) {
      print("Answer = \(addition)")
      lblAnswer.text = "\(addition)"
    }
}

func addition(arrayString: [Any?]) -> Int? {

    var answer:Int?
    for arrayElement in arrayString {
        if let stringValue = arrayElement, let intValue = Int(stringValue)  {
            answer = (answer ?? 0) + intValue
        }
    }

    return answer
}

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