Swift 4 Xcode中切换容器视图时如何停止后台Timer以降低CPU占用

Hey, I’ve run into exactly this issue before! The problem here is that scheduledTimer adds the timer to the RunLoop’s common mode by default—even when your container view’s ViewController is hidden, the RunLoop keeps ticking, so the timer keeps running in the background and eats up CPU. Here are two solid solutions to fix this:

1. Use View Controller Lifecycle Methods #

When you hide/show a container view, the embedded ViewController’s view lifecycle methods should trigger. You can leverage these to start/stop your timers automatically:

For ContainerA's ViewController: #

var ViewATimer: Timer?

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    // Only start the timer if it doesn't already exist
    if ViewATimer == nil {
        ViewATimer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(update), userInfo: nil, repeats: true)
    }
}

override func viewDidDisappear(_ animated: Bool) {
    super.viewDidDisappear(animated)
    // Stop and clean up the timer
    ViewATimer?.invalidate()
    ViewATimer = nil // Critical to avoid retain cycles and re-use issues
}

@objc func update() {
    // Your timer logic here
}

For ContainerB's ViewController: #

var ViewBTimer: Timer?

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    if ViewBTimer == nil {
        ViewBTimer = Timer.scheduledTimer(timeInterval: 10, target: self, selector: #selector(secondUpdate), userInfo: nil, repeats: true)
    }
}

override func viewDidDisappear(_ animated: Bool) {
    super.viewDidDisappear(animated)
    ViewBTimer?.invalidate()
    ViewBTimer = nil
}

@objc func secondUpdate() {
    // Your timer logic here
}

Why this works: When the container view is shown, viewWillAppear fires and starts the timer. When hidden, viewDidDisappear stops and destroys the timer, ensuring it doesn’t run in the background.

2. Manually Control Timers from the Main ViewController #

If for some reason the lifecycle methods don’t trigger (e.g., you’re only toggling isHidden without modifying the view hierarchy), you can directly call methods on your child ViewControllers to start/stop timers:

Step 1: Keep references to child ViewControllers in your main VC #

First, get references to the embedded ViewControllers when the segues fire:

class MainViewController: UIViewController {
    var viewControllerA: ViewControllerA?
    var viewControllerB: ViewControllerB?
    
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "SegueToContainerA" { // Match your segue ID
            viewControllerA = segue.destination as? ViewControllerA
        } else if segue.identifier == "SegueToContainerB" {
            viewControllerB = segue.destination as? ViewControllerB
        }
    }
}

Step 2: Add timer control methods to child ViewControllers #

In ViewControllerA:

func startTimer() {
    if ViewATimer == nil {
        ViewATimer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(update), userInfo: nil, repeats: true)
    }
}

func stopTimer() {
    ViewATimer?.invalidate()
    ViewATimer = nil
}

In ViewControllerB:

func startTimer() {
    if ViewBTimer == nil {
        ViewBTimer = Timer.scheduledTimer(timeInterval: 10, target: self, selector: #selector(secondUpdate), userInfo: nil, repeats: true)
    }
}

func stopTimer() {
    ViewBTimer?.invalidate()
    ViewBTimer = nil
}

Step 3: Call these methods in your button tap handlers #

@IBAction func showContainerA(_ sender: UIButton) {
    containerA.isHidden = false
    containerB.isHidden = true
    
    viewControllerA?.startTimer()
    viewControllerB?.stopTimer()
}

@IBAction func showContainerB(_ sender: UIButton) {
    containerA.isHidden = true
    containerB.isHidden = false
    
    viewControllerB?.startTimer()
    viewControllerA?.stopTimer()
}

Key Notes to Avoid Issues #

• Always set your timer to nil after calling invalidate(): This breaks any retain cycles and ensures you don’t accidentally reuse an invalidated timer.
• If you switch to closure-based timers (recommended for Swift), use [weak self] to prevent memory leaks:

  ViewATimer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] timer in
      self?.update()
  }
  

• Never create multiple timers without checking if an existing one exists—this would cause overlapping timers and even higher CPU usage.

内容的提问来源于stack exchange，提问作者Shahid Miah