NSTimers 在后台运行

func applicationWillResignActive(_ application: UIApplication) {
    let backgroundTask = UIApplication.shared.beginBackgroundTask {
        UIApplication.shared.endBackgroundTask(backgroundTask)
    }
    
    DispatchQueue.global().async {
        // 在这里执行 NSTimer 相关的代码
        // ...
        
        UIApplication.shared.endBackgroundTask(backgroundTask)
    }
}

func applicationDidEnterBackground(_ application: UIApplication) {
    let timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(timerFired(_:)), userInfo: nil, repeats: true)
    
    let runLoop = RunLoop.current
    runLoop.add(timer, forMode: .common)
    
    runLoop.run()
}
@objc func timerFired(_ timer: Timer) {
    // 在这里执行 NSTimer 相关的代码
    // ...
}

import UIKit
class ViewController: UIViewController {
    var timer: Timer?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        startTimer()
    }
    
    func startTimer() {
        timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(timerFired(_:)), userInfo: nil, repeats: true)
        
        let runLoop = RunLoop.current
        runLoop.add(timer!, forMode: .common)
        
        runLoop.run()
    }
    
    @objc func timerFired(_ timer: Timer) {
        print("Timer fired!")
    }
}