swift - Swift 中的条件语句

Question

我正在快速构建一个计算空间面积的简单应用程序。如果用户未在文本框中输入宽度或高度，条件语句将返回一条消息，我遇到了问题。

//
//  ViewController.swift
//  areaCalculator
//
//  
//  Copyright (c) 2014 Dandre Ealy. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var widthTxt: UITextField!
    @IBOutlet weak var heightTxt: UITextField!

    @IBOutlet weak var area: UILabel!

    @IBAction func buttonPressed(sender: AnyObject) {

        var width = widthTxt.text.toInt()
        var height = heightTxt.text.toInt()

        var areaPressed = width! * height!

        if ((width) && (height) != nil){
            area.text = "The area is \(areaPressed)"
        } else {
            area.text = "Enter the width and the height"
        }
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

Answer 1

这个说法是错误的:

if ((width) && (height) != nil)

你必须单独明确地检查 not nil:

if width != nil && height != nil

不过还有另一个错误，它会生成运行时异常:

var areaPressed = width! * height!

如果 width 或 height 为 nil。您应该将其移动到 if 正文中:

if width != nil && height != nil {
    var areaPressed = width! * height!
    area.text = "The area is \(areaPressed)"
} else {
    area.text = "Enter the width and the height"
}

原因是强制解包运算符 ! 要求应用它的可选变量包含非 nil 值 - 解包 nil 会导致运行时异常。