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为什么我在展开 Optional 值时得到“意外发现 nil?我检查了 timerSeconds 的值,它被正确地分配给了我想要分配的值。但是,当我调用函数 StartTimer 时,我的应用程序崩溃了。

300 EggTimer/ViewController.swift:30: 致命错误:在展开可选值时意外发现 nil 2021-06-02 19:17:04.380375+1000 EggTimer[27674:932041] EggTimer/ViewController.swift:30: 致命错误:意外在展开可选值 (lldb) 时发现 nil

import UIKit

class ViewController: UIViewController {
    
let eggTimes : [String : Int] = ["Soft": 300, "Medium": 420, "Hard": 720]
var secondsRemaining: Int?
@IBAction func hardnessSelected(_ sender: UIButton) {
    let hardness = sender.currentTitle!
    let timerSeconds = eggTimes[hardness]!

    print(timerSeconds)
    //until here the code seems to work fine
    
    
    startTimer(secondsRemaining: timerSeconds)
    //call the function start timer and give the secondRemaining argument the value of timerSeconds
    
}
func startTimer (secondsRemaining: Int?){
//create a function called startTimer which accepts an interger as argument called secondsremaining
    Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { (Timer) in
        if self.secondsRemaining! > 0 {
            //if the secondsRemaining >
            print ("\(self.secondsRemaining ?? 0) seconds")
            self.secondsRemaining! -= 1
        }else {
            Timer.invalidate()
          }
        }
     
    }

}

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2 回答 2

2

请注意,在startTimer,self.secondsRemaining中与参数 不是指同一事物secondsRemaining

var secondsRemaining: Int? // self.secondsRemaining

@IBAction func hardnessSelected(_ sender: UIButton) {
   ...
}
func startTimer (secondsRemaining: Int?){ // you never use this parameter
//create a function called startTimer which accepts an interger as argument called secondsremaining
    Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { (Timer) in

        // here you are referring to the var declared outside of the methods
        // which you never assign anything to.
        // this does not refer to the parameter
        if self.secondsRemaining! > 0 {

一个简单的解决方法是在开始时设置self.secondsRemaining参数:secondsRemainingstartTimer

func startTimer (secondsRemaining: Int?){ // you never use this parameter
    self.secondsRemaining = secondsRemaining
    Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { (Timer) in
        // same as before...
于 2021-06-05T03:40:14.197 回答
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嗨@dumanji,您还使用 ! 强制展开两个常量。如果您尝试使用 nil 值,这可能会崩溃。尤其是如果您计划将此应用程序投入生产,这可能会导致意外的运行时错误和应用程序崩溃。

例子:

  • 让硬度 = sender.currentTitle!
  • 让 timerSeconds = eggTimes[硬度]!

考虑使用?? (nil 合并运算符)在常量右侧提供默认值,以防可选项返回 nil。

可能的方法:

  • 让 timerSeconds = eggTimes[硬度] ?? 420

让我知道这是否有帮助。

于 2021-06-05T07:40:38.953 回答