iOS真机后台定时器不运行但模拟器正常问题求助

我正在开发一款倒计时定时器应用，要求倒计时完成前每10分钟发出提示音。该定时器在模拟器中后台运行时可正常倒计时，但在真机上却无法正常工作，且两者均已开启后台刷新功能。我原本期望定时器可在后台倒计时，但实际并非如此，我是否应该使用日期相关函数？

我的ViewController.swift代码 #

//
//  ViewController.swift
//  timer
//
//  Created by Bharath Reddy on 6/5/23.
//

import UIKit
import AVFoundation

@available(iOS 16.0, *)
class ViewController: UIViewController {
    
    @IBOutlet var timer: UITextField!
    @IBOutlet var start: UIButton!
    @IBOutlet var reset: UIButton!
    
    var player: AVAudioPlayer!
    
    var sec = 0
    var min = 0
    
    var time1:Timer = Timer()
    var time2:Timer = Timer()

    override func viewDidLoad() {
        super.viewDidLoad()
        timer.becomeFirstResponder()
    }
    
    @IBAction func startTimer() {
        timer.resignFirstResponder()
        start.isEnabled = false
        
        min = Int(timer.text ?? "") ?? 0
        
        sec = min * 60
        
        time1 = Timer.scheduledTimer(timeInterval: 600, target: self, selector: #selector(timer1Counter), userInfo: nil, repeats: true)
        
        let url = Bundle.main.url(forResource: "notification-sound-7062", withExtension: "mp3")
        player = try! AVAudioPlayer(contentsOf: url!)
        player.play()
        player.play()
        player.play()
        
    }
    
    @IBAction func stopTimer() {
        time1.invalidate()
        time2.invalidate()
        start.isEnabled = true
    }
    
    @objc func timer2Counter() -> Void {
        let url = Bundle.main.url(forResource: "notification-sound-7062", withExtension: "mp3")
        player = try! AVAudioPlayer(contentsOf: url!)
        player.play()
        sec = sec-60
    }
    
    @objc func timer1Counter() -> Void {
        let url = Bundle.main.url(forResource: "notification-sound-7062", withExtension: "mp3")
        player = try! AVAudioPlayer(contentsOf: url!)
        player.play()
        sec = sec-600
        if (sec<=0) {
            time1.invalidate()
        }
        else {
            time2 = Timer.scheduledTimer(timeInterval: 60, target: self, selector: #selector(timer2Counter), userInfo: nil, repeats: false)
        }
    }

}

问题原因与解决方法 #

1. 为什么Timer在真机后台失效？ #

iOS后台机制对普通Timer限制严格：应用进入后台后，主线程会被挂起，依赖主线程运行的scheduledTimer会暂停甚至失效。模拟器后台机制没有真机严格，所以能正常运行，但真机上不行。

2. 正确实现方式：日期计算+本地通知+后台任务 #

放弃依赖实时Timer的方式，改用记录目标结束时间，结合本地通知实现后台提示：

步骤一：记录目标结束时间

启动定时器时，计算倒计时结束的具体日期，而非维护剩余秒数：

import UserNotifications

var endDate: Date?
var player: AVAudioPlayer!

@IBAction func startTimer() {
    timer.resignFirstResponder()
    start.isEnabled = false
    
    let minutes = Int(timer.text ?? "") ?? 0
    endDate = Date().addingTimeInterval(TimeInterval(minutes * 60))
    
    // 立即播放提示音
    playNotificationSound()
    // 安排第一个10分钟提示
    scheduleNextNotification()
}

步骤二：用本地通知实现后台提示

iOS后台无法持续运行代码，但可提前安排本地通知，到时间系统会触发（即使应用在后台）：

override func viewDidLoad() {
    super.viewDidLoad()
    timer.becomeFirstResponder()
    
    // 请求通知权限
    UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound]) { granted, _ in
        if granted {
            print("通知权限已获取")
        }
    }
}

func scheduleNextNotification() {
    guard let endDate = endDate else { return }
    let now = Date()
    let remainingSeconds = endDate.timeIntervalSince(now)
    
    if remainingSeconds <= 0 {
        // 倒计时结束
        playNotificationSound()
        start.isEnabled = true
        return
    }
    
    // 计算下一个提示时间：10分钟后或到结束时间
    let nextTriggerTime = min(remainingSeconds, 600)
    let trigger = UNTimeIntervalNotificationTrigger(timeInterval: nextTriggerTime, repeats: false)
    
    let content = UNMutableNotificationContent()
    content.title = "倒计时提示"
    content.body = "距离结束还有\(Int(remainingSeconds/60))分钟"
    if let soundName = Bundle.main.url(forResource: "notification-sound-7062", withExtension: "mp3")?.lastPathComponent {
        content.sound = UNNotificationSound(named: UNNotificationSoundName(rawValue: soundName))
    }
    
    let request = UNNotificationRequest(identifier: UUID().uuidString, content: content, trigger: trigger)
    UNUserNotificationCenter.current().add(request) { [weak self] error in
        if error == nil {
            // 安排成功后，继续调度下一次提示
            DispatchQueue.main.asyncAfter(deadline: .now() + nextTriggerTime) {
                self?.scheduleNextNotification()
            }
        }
    }
}

func playNotificationSound() {
    guard let url = Bundle.main.url(forResource: "notification-sound-7062", withExtension: "mp3") else { return }
    do {
        player = try AVAudioPlayer(contentsOf: url)
        player.play()
    } catch {
        print("音效播放失败：\(error)")
    }
}

步骤三：可选后台任务（用于短暂后台更新）

如果需要在后台短暂运行代码，可申请后台任务，但无法长期占用资源：

var backgroundTaskID: UIBackgroundTaskIdentifier = .invalid

func startBackgroundTask() {
    backgroundTaskID = UIApplication.shared.beginBackgroundTask(withName: "TimerBackgroundTask") { [weak self] in
        UIApplication.shared.endBackgroundTask(self?.backgroundTaskID ?? .invalid)
        self?.backgroundTaskID = .invalid
    }
}

func endBackgroundTask() {
    if backgroundTaskID != .invalid {
        UIApplication.shared.endBackgroundTask(backgroundTaskID)
        backgroundTaskID = .invalid
    }
}

启动定时器时调用startBackgroundTask()，倒计时结束或停止时调用endBackgroundTask()。

3. 注意事项 #

• 需在Info.plist中添加UIBackgroundModes并勾选audio，确保后台可播放音效。
• 真机测试时，确认应用已获取通知权限，且设备未静音。
• 不要依赖Timer在后台持续运行，iOS不允许普通应用长期占用后台资源。

内容的提问来源于stack exchange，提问作者Bharath Reddy